TOMOYO Linux Cross Reference
Linux/mm/gup.c

   // SPDX-License-Identifier: GPL-2.0-only
   #include <linux/kernel.h>
   #include <linux/errno.h>
   #include <linux/err.h>
   #include <linux/spinlock.h>
   
   #include <linux/mm.h>
   #include <linux/memfd.h>
   #include <linux/memremap.h>
  #include <linux/pagemap.h>
  #include <linux/rmap.h>
  #include <linux/swap.h>
  #include <linux/swapops.h>
  #include <linux/secretmem.h>
  
  #include <linux/sched/signal.h>
  #include <linux/rwsem.h>
  #include <linux/hugetlb.h>
  #include <linux/migrate.h>
  #include <linux/mm_inline.h>
  #include <linux/pagevec.h>
  #include <linux/sched/mm.h>
  #include <linux/shmem_fs.h>
  
  #include <asm/mmu_context.h>
  #include <asm/tlbflush.h>
  
  #include "internal.h"
  
  struct follow_page_context {
          struct dev_pagemap *pgmap;
          unsigned int page_mask;
  };
  
  static inline void sanity_check_pinned_pages(struct page **pages,
                                               unsigned long npages)
  {
          if (!IS_ENABLED(CONFIG_DEBUG_VM))
                  return;
  
          /*
           * We only pin anonymous pages if they are exclusive. Once pinned, we
           * can no longer turn them possibly shared and PageAnonExclusive() will
           * stick around until the page is freed.
           *
           * We'd like to verify that our pinned anonymous pages are still mapped
           * exclusively. The issue with anon THP is that we don't know how
           * they are/were mapped when pinning them. However, for anon
           * THP we can assume that either the given page (PTE-mapped THP) or
           * the head page (PMD-mapped THP) should be PageAnonExclusive(). If
           * neither is the case, there is certainly something wrong.
           */
          for (; npages; npages--, pages++) {
                  struct page *page = *pages;
                  struct folio *folio = page_folio(page);
  
                  if (is_zero_page(page) ||
                      !folio_test_anon(folio))
                          continue;
                  if (!folio_test_large(folio) || folio_test_hugetlb(folio))
                          VM_BUG_ON_PAGE(!PageAnonExclusive(&folio->page), page);
                  else
                          /* Either a PTE-mapped or a PMD-mapped THP. */
                          VM_BUG_ON_PAGE(!PageAnonExclusive(&folio->page) &&
                                         !PageAnonExclusive(page), page);
          }
  }
  
  /*
   * Return the folio with ref appropriately incremented,
   * or NULL if that failed.
   */
  static inline struct folio *try_get_folio(struct page *page, int refs)
  {
          struct folio *folio;
  
  retry:
          folio = page_folio(page);
          if (WARN_ON_ONCE(folio_ref_count(folio) < 0))
                  return NULL;
          if (unlikely(!folio_ref_try_add(folio, refs)))
                  return NULL;
  
          /*
           * At this point we have a stable reference to the folio; but it
           * could be that between calling page_folio() and the refcount
           * increment, the folio was split, in which case we'd end up
           * holding a reference on a folio that has nothing to do with the page
           * we were given anymore.
           * So now that the folio is stable, recheck that the page still
           * belongs to this folio.
           */
          if (unlikely(page_folio(page) != folio)) {
                  if (!put_devmap_managed_folio_refs(folio, refs))
                          folio_put_refs(folio, refs);
                  goto retry;
          }
  
          return folio;
 }
 
 static void gup_put_folio(struct folio *folio, int refs, unsigned int flags)
 {
         if (flags & FOLL_PIN) {
                 if (is_zero_folio(folio))
                         return;
                 node_stat_mod_folio(folio, NR_FOLL_PIN_RELEASED, refs);
                 if (folio_test_large(folio))
                         atomic_sub(refs, &folio->_pincount);
                 else
                         refs *= GUP_PIN_COUNTING_BIAS;
         }
 
         if (!put_devmap_managed_folio_refs(folio, refs))
                 folio_put_refs(folio, refs);
 }
 
 /**
  * try_grab_folio() - add a folio's refcount by a flag-dependent amount
  * @folio:    pointer to folio to be grabbed
  * @refs:     the value to (effectively) add to the folio's refcount
  * @flags:    gup flags: these are the FOLL_* flag values
  *
  * This might not do anything at all, depending on the flags argument.
  *
  * "grab" names in this file mean, "look at flags to decide whether to use
  * FOLL_PIN or FOLL_GET behavior, when incrementing the folio's refcount.
  *
  * Either FOLL_PIN or FOLL_GET (or neither) may be set, but not both at the same
  * time.
  *
  * Return: 0 for success, or if no action was required (if neither FOLL_PIN
  * nor FOLL_GET was set, nothing is done). A negative error code for failure:
  *
  *   -ENOMEM            FOLL_GET or FOLL_PIN was set, but the folio could not
  *                      be grabbed.
  *
  * It is called when we have a stable reference for the folio, typically in
  * GUP slow path.
  */
 int __must_check try_grab_folio(struct folio *folio, int refs,
                                 unsigned int flags)
 {
         if (WARN_ON_ONCE(folio_ref_count(folio) <= 0))
                 return -ENOMEM;
 
         if (unlikely(!(flags & FOLL_PCI_P2PDMA) && is_pci_p2pdma_page(&folio->page)))
                 return -EREMOTEIO;
 
         if (flags & FOLL_GET)
                 folio_ref_add(folio, refs);
         else if (flags & FOLL_PIN) {
                 /*
                  * Don't take a pin on the zero page - it's not going anywhere
                  * and it is used in a *lot* of places.
                  */
                 if (is_zero_folio(folio))
                         return 0;
 
                 /*
                  * Increment the normal page refcount field at least once,
                  * so that the page really is pinned.
                  */
                 if (folio_test_large(folio)) {
                         folio_ref_add(folio, refs);
                         atomic_add(refs, &folio->_pincount);
                 } else {
                         folio_ref_add(folio, refs * GUP_PIN_COUNTING_BIAS);
                 }
 
                 node_stat_mod_folio(folio, NR_FOLL_PIN_ACQUIRED, refs);
         }
 
         return 0;
 }
 
 /**
  * unpin_user_page() - release a dma-pinned page
  * @page:            pointer to page to be released
  *
  * Pages that were pinned via pin_user_pages*() must be released via either
  * unpin_user_page(), or one of the unpin_user_pages*() routines. This is so
  * that such pages can be separately tracked and uniquely handled. In
  * particular, interactions with RDMA and filesystems need special handling.
  */
 void unpin_user_page(struct page *page)
 {
         sanity_check_pinned_pages(&page, 1);
         gup_put_folio(page_folio(page), 1, FOLL_PIN);
 }
 EXPORT_SYMBOL(unpin_user_page);
 
 /**
  * unpin_folio() - release a dma-pinned folio
  * @folio:         pointer to folio to be released
  *
  * Folios that were pinned via memfd_pin_folios() or other similar routines
  * must be released either using unpin_folio() or unpin_folios().
  */
 void unpin_folio(struct folio *folio)
 {
         gup_put_folio(folio, 1, FOLL_PIN);
 }
 EXPORT_SYMBOL_GPL(unpin_folio);
 
 /**
  * folio_add_pin - Try to get an additional pin on a pinned folio
  * @folio: The folio to be pinned
  *
  * Get an additional pin on a folio we already have a pin on.  Makes no change
  * if the folio is a zero_page.
  */
 void folio_add_pin(struct folio *folio)
 {
         if (is_zero_folio(folio))
                 return;
 
         /*
          * Similar to try_grab_folio(): be sure to *also* increment the normal
          * page refcount field at least once, so that the page really is
          * pinned.
          */
         if (folio_test_large(folio)) {
                 WARN_ON_ONCE(atomic_read(&folio->_pincount) < 1);
                 folio_ref_inc(folio);
                 atomic_inc(&folio->_pincount);
         } else {
                 WARN_ON_ONCE(folio_ref_count(folio) < GUP_PIN_COUNTING_BIAS);
                 folio_ref_add(folio, GUP_PIN_COUNTING_BIAS);
         }
 }
 
 static inline struct folio *gup_folio_range_next(struct page *start,
                 unsigned long npages, unsigned long i, unsigned int *ntails)
 {
         struct page *next = nth_page(start, i);
         struct folio *folio = page_folio(next);
         unsigned int nr = 1;
 
         if (folio_test_large(folio))
                 nr = min_t(unsigned int, npages - i,
                            folio_nr_pages(folio) - folio_page_idx(folio, next));
 
         *ntails = nr;
         return folio;
 }
 
 static inline struct folio *gup_folio_next(struct page **list,
                 unsigned long npages, unsigned long i, unsigned int *ntails)
 {
         struct folio *folio = page_folio(list[i]);
         unsigned int nr;
 
         for (nr = i + 1; nr < npages; nr++) {
                 if (page_folio(list[nr]) != folio)
                         break;
         }
 
         *ntails = nr - i;
         return folio;
 }
 
 /**
  * unpin_user_pages_dirty_lock() - release and optionally dirty gup-pinned pages
  * @pages:  array of pages to be maybe marked dirty, and definitely released.
  * @npages: number of pages in the @pages array.
  * @make_dirty: whether to mark the pages dirty
  *
  * "gup-pinned page" refers to a page that has had one of the get_user_pages()
  * variants called on that page.
  *
  * For each page in the @pages array, make that page (or its head page, if a
  * compound page) dirty, if @make_dirty is true, and if the page was previously
  * listed as clean. In any case, releases all pages using unpin_user_page(),
  * possibly via unpin_user_pages(), for the non-dirty case.
  *
  * Please see the unpin_user_page() documentation for details.
  *
  * set_page_dirty_lock() is used internally. If instead, set_page_dirty() is
  * required, then the caller should a) verify that this is really correct,
  * because _lock() is usually required, and b) hand code it:
  * set_page_dirty_lock(), unpin_user_page().
  *
  */
 void unpin_user_pages_dirty_lock(struct page **pages, unsigned long npages,
                                  bool make_dirty)
 {
         unsigned long i;
         struct folio *folio;
         unsigned int nr;
 
         if (!make_dirty) {
                 unpin_user_pages(pages, npages);
                 return;
         }
 
         sanity_check_pinned_pages(pages, npages);
         for (i = 0; i < npages; i += nr) {
                 folio = gup_folio_next(pages, npages, i, &nr);
                 /*
                  * Checking PageDirty at this point may race with
                  * clear_page_dirty_for_io(), but that's OK. Two key
                  * cases:
                  *
                  * 1) This code sees the page as already dirty, so it
                  * skips the call to set_page_dirty(). That could happen
                  * because clear_page_dirty_for_io() called
                  * folio_mkclean(), followed by set_page_dirty().
                  * However, now the page is going to get written back,
                  * which meets the original intention of setting it
                  * dirty, so all is well: clear_page_dirty_for_io() goes
                  * on to call TestClearPageDirty(), and write the page
                  * back.
                  *
                  * 2) This code sees the page as clean, so it calls
                  * set_page_dirty(). The page stays dirty, despite being
                  * written back, so it gets written back again in the
                  * next writeback cycle. This is harmless.
                  */
                 if (!folio_test_dirty(folio)) {
                         folio_lock(folio);
                         folio_mark_dirty(folio);
                         folio_unlock(folio);
                 }
                 gup_put_folio(folio, nr, FOLL_PIN);
         }
 }
 EXPORT_SYMBOL(unpin_user_pages_dirty_lock);
 
 /**
  * unpin_user_page_range_dirty_lock() - release and optionally dirty
  * gup-pinned page range
  *
  * @page:  the starting page of a range maybe marked dirty, and definitely released.
  * @npages: number of consecutive pages to release.
  * @make_dirty: whether to mark the pages dirty
  *
  * "gup-pinned page range" refers to a range of pages that has had one of the
  * pin_user_pages() variants called on that page.
  *
  * For the page ranges defined by [page .. page+npages], make that range (or
  * its head pages, if a compound page) dirty, if @make_dirty is true, and if the
  * page range was previously listed as clean.
  *
  * set_page_dirty_lock() is used internally. If instead, set_page_dirty() is
  * required, then the caller should a) verify that this is really correct,
  * because _lock() is usually required, and b) hand code it:
  * set_page_dirty_lock(), unpin_user_page().
  *
  */
 void unpin_user_page_range_dirty_lock(struct page *page, unsigned long npages,
                                       bool make_dirty)
 {
         unsigned long i;
         struct folio *folio;
         unsigned int nr;
 
         for (i = 0; i < npages; i += nr) {
                 folio = gup_folio_range_next(page, npages, i, &nr);
                 if (make_dirty && !folio_test_dirty(folio)) {
                         folio_lock(folio);
                         folio_mark_dirty(folio);
                         folio_unlock(folio);
                 }
                 gup_put_folio(folio, nr, FOLL_PIN);
         }
 }
 EXPORT_SYMBOL(unpin_user_page_range_dirty_lock);
 
 static void gup_fast_unpin_user_pages(struct page **pages, unsigned long npages)
 {
         unsigned long i;
         struct folio *folio;
         unsigned int nr;
 
         /*
          * Don't perform any sanity checks because we might have raced with
          * fork() and some anonymous pages might now actually be shared --
          * which is why we're unpinning after all.
          */
         for (i = 0; i < npages; i += nr) {
                 folio = gup_folio_next(pages, npages, i, &nr);
                 gup_put_folio(folio, nr, FOLL_PIN);
         }
 }
 
 /**
  * unpin_user_pages() - release an array of gup-pinned pages.
  * @pages:  array of pages to be marked dirty and released.
  * @npages: number of pages in the @pages array.
  *
  * For each page in the @pages array, release the page using unpin_user_page().
  *
  * Please see the unpin_user_page() documentation for details.
  */
 void unpin_user_pages(struct page **pages, unsigned long npages)
 {
         unsigned long i;
         struct folio *folio;
         unsigned int nr;
 
         /*
          * If this WARN_ON() fires, then the system *might* be leaking pages (by
          * leaving them pinned), but probably not. More likely, gup/pup returned
          * a hard -ERRNO error to the caller, who erroneously passed it here.
          */
         if (WARN_ON(IS_ERR_VALUE(npages)))
                 return;
 
         sanity_check_pinned_pages(pages, npages);
         for (i = 0; i < npages; i += nr) {
                 folio = gup_folio_next(pages, npages, i, &nr);
                 gup_put_folio(folio, nr, FOLL_PIN);
         }
 }
 EXPORT_SYMBOL(unpin_user_pages);
 
 /**
  * unpin_folios() - release an array of gup-pinned folios.
  * @folios:  array of folios to be marked dirty and released.
  * @nfolios: number of folios in the @folios array.
  *
  * For each folio in the @folios array, release the folio using gup_put_folio.
  *
  * Please see the unpin_folio() documentation for details.
  */
 void unpin_folios(struct folio **folios, unsigned long nfolios)
 {
         unsigned long i = 0, j;
 
         /*
          * If this WARN_ON() fires, then the system *might* be leaking folios
          * (by leaving them pinned), but probably not. More likely, gup/pup
          * returned a hard -ERRNO error to the caller, who erroneously passed
          * it here.
          */
         if (WARN_ON(IS_ERR_VALUE(nfolios)))
                 return;
 
         while (i < nfolios) {
                 for (j = i + 1; j < nfolios; j++)
                         if (folios[i] != folios[j])
                                 break;
 
                 if (folios[i])
                         gup_put_folio(folios[i], j - i, FOLL_PIN);
                 i = j;
         }
 }
 EXPORT_SYMBOL_GPL(unpin_folios);
 
 /*
  * Set the MMF_HAS_PINNED if not set yet; after set it'll be there for the mm's
  * lifecycle.  Avoid setting the bit unless necessary, or it might cause write
  * cache bouncing on large SMP machines for concurrent pinned gups.
  */
 static inline void mm_set_has_pinned_flag(unsigned long *mm_flags)
 {
         if (!test_bit(MMF_HAS_PINNED, mm_flags))
                 set_bit(MMF_HAS_PINNED, mm_flags);
 }
 
 #ifdef CONFIG_MMU
 
 #ifdef CONFIG_HAVE_GUP_FAST
 static int record_subpages(struct page *page, unsigned long sz,
                            unsigned long addr, unsigned long end,
                            struct page **pages)
 {
         struct page *start_page;
         int nr;
 
         start_page = nth_page(page, (addr & (sz - 1)) >> PAGE_SHIFT);
         for (nr = 0; addr != end; nr++, addr += PAGE_SIZE)
                 pages[nr] = nth_page(start_page, nr);
 
         return nr;
 }
 
 /**
  * try_grab_folio_fast() - Attempt to get or pin a folio in fast path.
  * @page:  pointer to page to be grabbed
  * @refs:  the value to (effectively) add to the folio's refcount
  * @flags: gup flags: these are the FOLL_* flag values.
  *
  * "grab" names in this file mean, "look at flags to decide whether to use
  * FOLL_PIN or FOLL_GET behavior, when incrementing the folio's refcount.
  *
  * Either FOLL_PIN or FOLL_GET (or neither) must be set, but not both at the
  * same time. (That's true throughout the get_user_pages*() and
  * pin_user_pages*() APIs.) Cases:
  *
  *    FOLL_GET: folio's refcount will be incremented by @refs.
  *
  *    FOLL_PIN on large folios: folio's refcount will be incremented by
  *    @refs, and its pincount will be incremented by @refs.
  *
  *    FOLL_PIN on single-page folios: folio's refcount will be incremented by
  *    @refs * GUP_PIN_COUNTING_BIAS.
  *
  * Return: The folio containing @page (with refcount appropriately
  * incremented) for success, or NULL upon failure. If neither FOLL_GET
  * nor FOLL_PIN was set, that's considered failure, and furthermore,
  * a likely bug in the caller, so a warning is also emitted.
  *
  * It uses add ref unless zero to elevate the folio refcount and must be called
  * in fast path only.
  */
 static struct folio *try_grab_folio_fast(struct page *page, int refs,
                                          unsigned int flags)
 {
         struct folio *folio;
 
         /* Raise warn if it is not called in fast GUP */
         VM_WARN_ON_ONCE(!irqs_disabled());
 
         if (WARN_ON_ONCE((flags & (FOLL_GET | FOLL_PIN)) == 0))
                 return NULL;
 
         if (unlikely(!(flags & FOLL_PCI_P2PDMA) && is_pci_p2pdma_page(page)))
                 return NULL;
 
         if (flags & FOLL_GET)
                 return try_get_folio(page, refs);
 
         /* FOLL_PIN is set */
 
         /*
          * Don't take a pin on the zero page - it's not going anywhere
          * and it is used in a *lot* of places.
          */
         if (is_zero_page(page))
                 return page_folio(page);
 
         folio = try_get_folio(page, refs);
         if (!folio)
                 return NULL;
 
         /*
          * Can't do FOLL_LONGTERM + FOLL_PIN gup fast path if not in a
          * right zone, so fail and let the caller fall back to the slow
          * path.
          */
         if (unlikely((flags & FOLL_LONGTERM) &&
                      !folio_is_longterm_pinnable(folio))) {
                 if (!put_devmap_managed_folio_refs(folio, refs))
                         folio_put_refs(folio, refs);
                 return NULL;
         }
 
         /*
          * When pinning a large folio, use an exact count to track it.
          *
          * However, be sure to *also* increment the normal folio
          * refcount field at least once, so that the folio really
          * is pinned.  That's why the refcount from the earlier
          * try_get_folio() is left intact.
          */
         if (folio_test_large(folio))
                 atomic_add(refs, &folio->_pincount);
         else
                 folio_ref_add(folio,
                                 refs * (GUP_PIN_COUNTING_BIAS - 1));
         /*
          * Adjust the pincount before re-checking the PTE for changes.
          * This is essentially a smp_mb() and is paired with a memory
          * barrier in folio_try_share_anon_rmap_*().
          */
         smp_mb__after_atomic();
 
         node_stat_mod_folio(folio, NR_FOLL_PIN_ACQUIRED, refs);
 
         return folio;
 }
 #endif  /* CONFIG_HAVE_GUP_FAST */
 
 static struct page *no_page_table(struct vm_area_struct *vma,
                                   unsigned int flags, unsigned long address)
 {
         if (!(flags & FOLL_DUMP))
                 return NULL;
 
         /*
          * When core dumping, we don't want to allocate unnecessary pages or
          * page tables.  Return error instead of NULL to skip handle_mm_fault,
          * then get_dump_page() will return NULL to leave a hole in the dump.
          * But we can only make this optimization where a hole would surely
          * be zero-filled if handle_mm_fault() actually did handle it.
          */
         if (is_vm_hugetlb_page(vma)) {
                 struct hstate *h = hstate_vma(vma);
 
                 if (!hugetlbfs_pagecache_present(h, vma, address))
                         return ERR_PTR(-EFAULT);
         } else if ((vma_is_anonymous(vma) || !vma->vm_ops->fault)) {
                 return ERR_PTR(-EFAULT);
         }
 
         return NULL;
 }
 
 #ifdef CONFIG_PGTABLE_HAS_HUGE_LEAVES
 static struct page *follow_huge_pud(struct vm_area_struct *vma,
                                     unsigned long addr, pud_t *pudp,
                                     int flags, struct follow_page_context *ctx)
 {
         struct mm_struct *mm = vma->vm_mm;
         struct page *page;
         pud_t pud = *pudp;
         unsigned long pfn = pud_pfn(pud);
         int ret;
 
         assert_spin_locked(pud_lockptr(mm, pudp));
 
         if ((flags & FOLL_WRITE) && !pud_write(pud))
                 return NULL;
 
         if (!pud_present(pud))
                 return NULL;
 
         pfn += (addr & ~PUD_MASK) >> PAGE_SHIFT;
 
         if (IS_ENABLED(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD) &&
             pud_devmap(pud)) {
                 /*
                  * device mapped pages can only be returned if the caller
                  * will manage the page reference count.
                  *
                  * At least one of FOLL_GET | FOLL_PIN must be set, so
                  * assert that here:
                  */
                 if (!(flags & (FOLL_GET | FOLL_PIN)))
                         return ERR_PTR(-EEXIST);
 
                 if (flags & FOLL_TOUCH)
                         touch_pud(vma, addr, pudp, flags & FOLL_WRITE);
 
                 ctx->pgmap = get_dev_pagemap(pfn, ctx->pgmap);
                 if (!ctx->pgmap)
                         return ERR_PTR(-EFAULT);
         }
 
         page = pfn_to_page(pfn);
 
         if (!pud_devmap(pud) && !pud_write(pud) &&
             gup_must_unshare(vma, flags, page))
                 return ERR_PTR(-EMLINK);
 
         ret = try_grab_folio(page_folio(page), 1, flags);
         if (ret)
                 page = ERR_PTR(ret);
         else
                 ctx->page_mask = HPAGE_PUD_NR - 1;
 
         return page;
 }
 
 /* FOLL_FORCE can write to even unwritable PMDs in COW mappings. */
 static inline bool can_follow_write_pmd(pmd_t pmd, struct page *page,
                                         struct vm_area_struct *vma,
                                         unsigned int flags)
 {
         /* If the pmd is writable, we can write to the page. */
         if (pmd_write(pmd))
                 return true;
 
         /* Maybe FOLL_FORCE is set to override it? */
         if (!(flags & FOLL_FORCE))
                 return false;
 
         /* But FOLL_FORCE has no effect on shared mappings */
         if (vma->vm_flags & (VM_MAYSHARE | VM_SHARED))
                 return false;
 
         /* ... or read-only private ones */
         if (!(vma->vm_flags & VM_MAYWRITE))
                 return false;
 
         /* ... or already writable ones that just need to take a write fault */
         if (vma->vm_flags & VM_WRITE)
                 return false;
 
         /*
          * See can_change_pte_writable(): we broke COW and could map the page
          * writable if we have an exclusive anonymous page ...
          */
         if (!page || !PageAnon(page) || !PageAnonExclusive(page))
                 return false;
 
         /* ... and a write-fault isn't required for other reasons. */
         if (pmd_needs_soft_dirty_wp(vma, pmd))
                 return false;
         return !userfaultfd_huge_pmd_wp(vma, pmd);
 }
 
 static struct page *follow_huge_pmd(struct vm_area_struct *vma,
                                     unsigned long addr, pmd_t *pmd,
                                     unsigned int flags,
                                     struct follow_page_context *ctx)
 {
         struct mm_struct *mm = vma->vm_mm;
         pmd_t pmdval = *pmd;
         struct page *page;
         int ret;
 
         assert_spin_locked(pmd_lockptr(mm, pmd));
 
         page = pmd_page(pmdval);
         if ((flags & FOLL_WRITE) &&
             !can_follow_write_pmd(pmdval, page, vma, flags))
                 return NULL;
 
         /* Avoid dumping huge zero page */
         if ((flags & FOLL_DUMP) && is_huge_zero_pmd(pmdval))
                 return ERR_PTR(-EFAULT);
 
         if (pmd_protnone(*pmd) && !gup_can_follow_protnone(vma, flags))
                 return NULL;
 
         if (!pmd_write(pmdval) && gup_must_unshare(vma, flags, page))
                 return ERR_PTR(-EMLINK);
 
         VM_BUG_ON_PAGE((flags & FOLL_PIN) && PageAnon(page) &&
                         !PageAnonExclusive(page), page);
 
         ret = try_grab_folio(page_folio(page), 1, flags);
         if (ret)
                 return ERR_PTR(ret);
 
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
         if (pmd_trans_huge(pmdval) && (flags & FOLL_TOUCH))
                 touch_pmd(vma, addr, pmd, flags & FOLL_WRITE);
 #endif  /* CONFIG_TRANSPARENT_HUGEPAGE */
 
         page += (addr & ~HPAGE_PMD_MASK) >> PAGE_SHIFT;
         ctx->page_mask = HPAGE_PMD_NR - 1;
 
         return page;
 }
 
 #else  /* CONFIG_PGTABLE_HAS_HUGE_LEAVES */
 static struct page *follow_huge_pud(struct vm_area_struct *vma,
                                     unsigned long addr, pud_t *pudp,
                                     int flags, struct follow_page_context *ctx)
 {
         return NULL;
 }
 
 static struct page *follow_huge_pmd(struct vm_area_struct *vma,
                                     unsigned long addr, pmd_t *pmd,
                                     unsigned int flags,
                                     struct follow_page_context *ctx)
 {
         return NULL;
 }
 #endif  /* CONFIG_PGTABLE_HAS_HUGE_LEAVES */
 
 static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
                 pte_t *pte, unsigned int flags)
 {
         if (flags & FOLL_TOUCH) {
                 pte_t orig_entry = ptep_get(pte);
                 pte_t entry = orig_entry;
 
                 if (flags & FOLL_WRITE)
                         entry = pte_mkdirty(entry);
                 entry = pte_mkyoung(entry);
 
                 if (!pte_same(orig_entry, entry)) {
                         set_pte_at(vma->vm_mm, address, pte, entry);
                         update_mmu_cache(vma, address, pte);
                 }
         }
 
         /* Proper page table entry exists, but no corresponding struct page */
         return -EEXIST;
 }
 
 /* FOLL_FORCE can write to even unwritable PTEs in COW mappings. */
 static inline bool can_follow_write_pte(pte_t pte, struct page *page,
                                         struct vm_area_struct *vma,
                                         unsigned int flags)
 {
         /* If the pte is writable, we can write to the page. */
         if (pte_write(pte))
                 return true;
 
         /* Maybe FOLL_FORCE is set to override it? */
         if (!(flags & FOLL_FORCE))
                 return false;
 
         /* But FOLL_FORCE has no effect on shared mappings */
         if (vma->vm_flags & (VM_MAYSHARE | VM_SHARED))
                 return false;
 
         /* ... or read-only private ones */
         if (!(vma->vm_flags & VM_MAYWRITE))
                 return false;
 
         /* ... or already writable ones that just need to take a write fault */
         if (vma->vm_flags & VM_WRITE)
                 return false;
 
         /*
          * See can_change_pte_writable(): we broke COW and could map the page
          * writable if we have an exclusive anonymous page ...
          */
         if (!page || !PageAnon(page) || !PageAnonExclusive(page))
                 return false;
 
         /* ... and a write-fault isn't required for other reasons. */
         if (pte_needs_soft_dirty_wp(vma, pte))
                 return false;
         return !userfaultfd_pte_wp(vma, pte);
 }
 
 static struct page *follow_page_pte(struct vm_area_struct *vma,
                 unsigned long address, pmd_t *pmd, unsigned int flags,
                 struct dev_pagemap **pgmap)
 {
         struct mm_struct *mm = vma->vm_mm;
         struct page *page;
         spinlock_t *ptl;
         pte_t *ptep, pte;
         int ret;
 
         /* FOLL_GET and FOLL_PIN are mutually exclusive. */
         if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
                          (FOLL_PIN | FOLL_GET)))
                 return ERR_PTR(-EINVAL);
 
         ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
         if (!ptep)
                 return no_page_table(vma, flags, address);
         pte = ptep_get(ptep);
         if (!pte_present(pte))
                 goto no_page;
         if (pte_protnone(pte) && !gup_can_follow_protnone(vma, flags))
                 goto no_page;
 
         page = vm_normal_page(vma, address, pte);
 
         /*
          * We only care about anon pages in can_follow_write_pte() and don't
          * have to worry about pte_devmap() because they are never anon.
          */
         if ((flags & FOLL_WRITE) &&
             !can_follow_write_pte(pte, page, vma, flags)) {
                 page = NULL;
                 goto out;
         }
 
         if (!page && pte_devmap(pte) && (flags & (FOLL_GET | FOLL_PIN))) {
                 /*
                  * Only return device mapping pages in the FOLL_GET or FOLL_PIN
                  * case since they are only valid while holding the pgmap
                  * reference.
                  */
                 *pgmap = get_dev_pagemap(pte_pfn(pte), *pgmap);
                 if (*pgmap)
                         page = pte_page(pte);
                 else
                         goto no_page;
         } else if (unlikely(!page)) {
                 if (flags & FOLL_DUMP) {
                         /* Avoid special (like zero) pages in core dumps */
                         page = ERR_PTR(-EFAULT);
                         goto out;
                 }
 
                 if (is_zero_pfn(pte_pfn(pte))) {
                         page = pte_page(pte);
                 } else {
                         ret = follow_pfn_pte(vma, address, ptep, flags);
                         page = ERR_PTR(ret);
                         goto out;
                 }
         }
 
         if (!pte_write(pte) && gup_must_unshare(vma, flags, page)) {
                 page = ERR_PTR(-EMLINK);
                 goto out;
         }
 
         VM_BUG_ON_PAGE((flags & FOLL_PIN) && PageAnon(page) &&
                        !PageAnonExclusive(page), page);
 
         /* try_grab_folio() does nothing unless FOLL_GET or FOLL_PIN is set. */
         ret = try_grab_folio(page_folio(page), 1, flags);
         if (unlikely(ret)) {
                 page = ERR_PTR(ret);
                 goto out;
         }
 
         /*
          * We need to make the page accessible if and only if we are going
          * to access its content (the FOLL_PIN case).  Please see
          * Documentation/core-api/pin_user_pages.rst for details.
          */
         if (flags & FOLL_PIN) {
                 ret = arch_make_page_accessible(page);
                 if (ret) {
                         unpin_user_page(page);
                         page = ERR_PTR(ret);
                         goto out;
                 }
         }
         if (flags & FOLL_TOUCH) {
                 if ((flags & FOLL_WRITE) &&
                     !pte_dirty(pte) && !PageDirty(page))
                         set_page_dirty(page);
                 /*
                  * pte_mkyoung() would be more correct here, but atomic care
                  * is needed to avoid losing the dirty bit: it is easier to use
                  * mark_page_accessed().
                  */
                 mark_page_accessed(page);
         }
 out:
         pte_unmap_unlock(ptep, ptl);
         return page;
 no_page:
         pte_unmap_unlock(ptep, ptl);
         if (!pte_none(pte))
                 return NULL;
         return no_page_table(vma, flags, address);
 }
 
 static struct page *follow_pmd_mask(struct vm_area_struct *vma,
                                     unsigned long address, pud_t *pudp,
                                     unsigned int flags,
                                     struct follow_page_context *ctx)
 {
         pmd_t *pmd, pmdval;
         spinlock_t *ptl;
         struct page *page;
         struct mm_struct *mm = vma->vm_mm;
 
         pmd = pmd_offset(pudp, address);
         pmdval = pmdp_get_lockless(pmd);
         if (pmd_none(pmdval))
                 return no_page_table(vma, flags, address);
         if (!pmd_present(pmdval))
                 return no_page_table(vma, flags, address);
         if (pmd_devmap(pmdval)) {
                 ptl = pmd_lock(mm, pmd);
                 page = follow_devmap_pmd(vma, address, pmd, flags, &ctx->pgmap);
                 spin_unlock(ptl);
                 if (page)
                         return page;
                 return no_page_table(vma, flags, address);
         }
         if (likely(!pmd_leaf(pmdval)))
                 return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
 
         if (pmd_protnone(pmdval) && !gup_can_follow_protnone(vma, flags))
                 return no_page_table(vma, flags, address);
 
         ptl = pmd_lock(mm, pmd);
         pmdval = *pmd;
         if (unlikely(!pmd_present(pmdval))) {
                 spin_unlock(ptl);
                 return no_page_table(vma, flags, address);
         }
         if (unlikely(!pmd_leaf(pmdval))) {
                 spin_unlock(ptl);
                 return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
         }
         if (pmd_trans_huge(pmdval) && (flags & FOLL_SPLIT_PMD)) {
                 spin_unlock(ptl);
                 split_huge_pmd(vma, pmd, address);
                 /* If pmd was left empty, stuff a page table in there quickly */
                 return pte_alloc(mm, pmd) ? ERR_PTR(-ENOMEM) :
                         follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
         }
         page = follow_huge_pmd(vma, address, pmd, flags, ctx);
         spin_unlock(ptl);
         return page;
 }
 
 static struct page *follow_pud_mask(struct vm_area_struct *vma,
                                     unsigned long address, p4d_t *p4dp,
                                     unsigned int flags,
                                     struct follow_page_context *ctx)
 {
         pud_t *pudp, pud;
         spinlock_t *ptl;
         struct page *page;
         struct mm_struct *mm = vma->vm_mm;
 
         pudp = pud_offset(p4dp, address);
         pud = READ_ONCE(*pudp);
         if (!pud_present(pud))
                 return no_page_table(vma, flags, address);
         if (pud_leaf(pud)) {
                 ptl = pud_lock(mm, pudp);
                 page = follow_huge_pud(vma, address, pudp, flags, ctx);
                 spin_unlock(ptl);
                 if (page)
                         return page;
                 return no_page_table(vma, flags, address);
         }
         if (unlikely(pud_bad(pud)))
                 return no_page_table(vma, flags, address);
 
         return follow_pmd_mask(vma, address, pudp, flags, ctx);
 }
 
 static struct page *follow_p4d_mask(struct vm_area_struct *vma,
                                     unsigned long address, pgd_t *pgdp,
                                     unsigned int flags,
                                     struct follow_page_context *ctx)
 {
         p4d_t *p4dp, p4d;
 
         p4dp = p4d_offset(pgdp, address);
         p4d = READ_ONCE(*p4dp);
         BUILD_BUG_ON(p4d_leaf(p4d));
 
         if (!p4d_present(p4d) || p4d_bad(p4d))
                 return no_page_table(vma, flags, address);
 
         return follow_pud_mask(vma, address, p4dp, flags, ctx);
 }
 
 /**
  * follow_page_mask - look up a page descriptor from a user-virtual address
  * @vma: vm_area_struct mapping @address
  * @address: virtual address to look up
  * @flags: flags modifying lookup behaviour
  * @ctx: contains dev_pagemap for %ZONE_DEVICE memory pinning and a
  *       pointer to output page_mask
  *
  * @flags can have FOLL_ flags set, defined in <linux/mm.h>
  *
  * When getting pages from ZONE_DEVICE memory, the @ctx->pgmap caches
  * the device's dev_pagemap metadata to avoid repeating expensive lookups.
  *
  * When getting an anonymous page and the caller has to trigger unsharing
  * of a shared anonymous page first, -EMLINK is returned. The caller should
  * trigger a fault with FAULT_FLAG_UNSHARE set. Note that unsharing is only
  * relevant with FOLL_PIN and !FOLL_WRITE.
  *
  * On output, the @ctx->page_mask is set according to the size of the page.
  *
  * Return: the mapped (struct page *), %NULL if no mapping exists, or
  * an error pointer if there is a mapping to something not represented
  * by a page descriptor (see also vm_normal_page()).
  */
 static struct page *follow_page_mask(struct vm_area_struct *vma,
                               unsigned long address, unsigned int flags,
                               struct follow_page_context *ctx)
 {
         pgd_t *pgd;
         struct mm_struct *mm = vma->vm_mm;
         struct page *page;
 
         vma_pgtable_walk_begin(vma);
 
         ctx->page_mask = 0;
         pgd = pgd_offset(mm, address);
 
         if (pgd_none(*pgd) || unlikely(pgd_bad(*pgd)))
                 page = no_page_table(vma, flags, address);
         else
                 page = follow_p4d_mask(vma, address, pgd, flags, ctx);
 
         vma_pgtable_walk_end(vma);
 
         return page;
 }
 
 struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
                          unsigned int foll_flags)
 {
         struct follow_page_context ctx = { NULL };
         struct page *page;
 
         if (vma_is_secretmem(vma))
                 return NULL;
 
         if (WARN_ON_ONCE(foll_flags & FOLL_PIN))
                 return NULL;
 
         /*
          * We never set FOLL_HONOR_NUMA_FAULT because callers don't expect
          * to fail on PROT_NONE-mapped pages.
          */
         page = follow_page_mask(vma, address, foll_flags, &ctx);
         if (ctx.pgmap)
                 put_dev_pagemap(ctx.pgmap);
         return page;
 }
 
 static int get_gate_page(struct mm_struct *mm, unsigned long address,
                 unsigned int gup_flags, struct vm_area_struct **vma,
                 struct page **page)
 {
         pgd_t *pgd;
         p4d_t *p4d;
         pud_t *pud;
         pmd_t *pmd;
         pte_t *pte;
         pte_t entry;
         int ret = -EFAULT;
 
         /* user gate pages are read-only */
         if (gup_flags & FOLL_WRITE)
                 return -EFAULT;
         if (address > TASK_SIZE)
                 pgd = pgd_offset_k(address);
         else
                 pgd = pgd_offset_gate(mm, address);
         if (pgd_none(*pgd))
                 return -EFAULT;
         p4d = p4d_offset(pgd, address);
         if (p4d_none(*p4d))
                 return -EFAULT;
         pud = pud_offset(p4d, address);
         if (pud_none(*pud))
                 return -EFAULT;
         pmd = pmd_offset(pud, address);
         if (!pmd_present(*pmd))
                 return -EFAULT;
         pte = pte_offset_map(pmd, address);
         if (!pte)
                 return -EFAULT;
         entry = ptep_get(pte);
         if (pte_none(entry))
                 goto unmap;
         *vma = get_gate_vma(mm);
         if (!page)
                 goto out;
         *page = vm_normal_page(*vma, address, entry);
         if (!*page) {
                 if ((gup_flags & FOLL_DUMP) || !is_zero_pfn(pte_pfn(entry)))
                         goto unmap;
                 *page = pte_page(entry);
         }
         ret = try_grab_folio(page_folio(*page), 1, gup_flags);
         if (unlikely(ret))
                 goto unmap;
 out:
         ret = 0;
 unmap:
         pte_unmap(pte);
         return ret;
 }
 
 /*
  * mmap_lock must be held on entry.  If @flags has FOLL_UNLOCKABLE but not
  * FOLL_NOWAIT, the mmap_lock may be released.  If it is, *@locked will be set
  * to 0 and -EBUSY returned.
  */
 static int faultin_page(struct vm_area_struct *vma,
                 unsigned long address, unsigned int *flags, bool unshare,
                 int *locked)
 {
         unsigned int fault_flags = 0;
         vm_fault_t ret;
 
         if (*flags & FOLL_NOFAULT)
                 return -EFAULT;
         if (*flags & FOLL_WRITE)
                 fault_flags |= FAULT_FLAG_WRITE;
         if (*flags & FOLL_REMOTE)
                 fault_flags |= FAULT_FLAG_REMOTE;
         if (*flags & FOLL_UNLOCKABLE) {
                 fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
                 /*
                  * FAULT_FLAG_INTERRUPTIBLE is opt-in. GUP callers must set
                  * FOLL_INTERRUPTIBLE to enable FAULT_FLAG_INTERRUPTIBLE.
                  * That's because some callers may not be prepared to
                  * handle early exits caused by non-fatal signals.
                  */
                 if (*flags & FOLL_INTERRUPTIBLE)
                         fault_flags |= FAULT_FLAG_INTERRUPTIBLE;
         }
         if (*flags & FOLL_NOWAIT)
                 fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT;
         if (*flags & FOLL_TRIED) {
                 /*
                  * Note: FAULT_FLAG_ALLOW_RETRY and FAULT_FLAG_TRIED
                  * can co-exist
                  */
                 fault_flags |= FAULT_FLAG_TRIED;
         }
         if (unshare) {
                 fault_flags |= FAULT_FLAG_UNSHARE;
                 /* FAULT_FLAG_WRITE and FAULT_FLAG_UNSHARE are incompatible */
                 VM_BUG_ON(fault_flags & FAULT_FLAG_WRITE);
         }
 
         ret = handle_mm_fault(vma, address, fault_flags, NULL);
 
         if (ret & VM_FAULT_COMPLETED) {
                 /*
                  * With FAULT_FLAG_RETRY_NOWAIT we'll never release the
                  * mmap lock in the page fault handler. Sanity check this.
                  */
                 WARN_ON_ONCE(fault_flags & FAULT_FLAG_RETRY_NOWAIT);
                 *locked = 0;
 
                 /*
                  * We should do the same as VM_FAULT_RETRY, but let's not
                  * return -EBUSY since that's not reflecting the reality of
                  * what has happened - we've just fully completed a page
                  * fault, with the mmap lock released.  Use -EAGAIN to show
                  * that we want to take the mmap lock _again_.
                  */
                 return -EAGAIN;
         }
 
         if (ret & VM_FAULT_ERROR) {
                 int err = vm_fault_to_errno(ret, *flags);
 
                 if (err)
                         return err;
                 BUG();
         }
 
         if (ret & VM_FAULT_RETRY) {
                 if (!(fault_flags & FAULT_FLAG_RETRY_NOWAIT))
                         *locked = 0;
                 return -EBUSY;
         }
 
         return 0;
 }
 
 /*
  * Writing to file-backed mappings which require folio dirty tracking using GUP
  * is a fundamentally broken operation, as kernel write access to GUP mappings
  * do not adhere to the semantics expected by a file system.
  *
  * Consider the following scenario:-
  *
  * 1. A folio is written to via GUP which write-faults the memory, notifying
  *    the file system and dirtying the folio.
  * 2. Later, writeback is triggered, resulting in the folio being cleaned and
  *    the PTE being marked read-only.
  * 3. The GUP caller writes to the folio, as it is mapped read/write via the
  *    direct mapping.
  * 4. The GUP caller, now done with the page, unpins it and sets it dirty
  *    (though it does not have to).
  *
  * This results in both data being written to a folio without writenotify, and
  * the folio being dirtied unexpectedly (if the caller decides to do so).
  */
 static bool writable_file_mapping_allowed(struct vm_area_struct *vma,
                                           unsigned long gup_flags)
 {
         /*
          * If we aren't pinning then no problematic write can occur. A long term
          * pin is the most egregious case so this is the case we disallow.
          */
         if ((gup_flags & (FOLL_PIN | FOLL_LONGTERM)) !=
             (FOLL_PIN | FOLL_LONGTERM))
                 return true;
 
         /*
          * If the VMA does not require dirty tracking then no problematic write
          * can occur either.
          */
         return !vma_needs_dirty_tracking(vma);
 }
 
 static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
 {
         vm_flags_t vm_flags = vma->vm_flags;
         int write = (gup_flags & FOLL_WRITE);
         int foreign = (gup_flags & FOLL_REMOTE);
         bool vma_anon = vma_is_anonymous(vma);
 
         if (vm_flags & (VM_IO | VM_PFNMAP))
                 return -EFAULT;
 
         if ((gup_flags & FOLL_ANON) && !vma_anon)
                 return -EFAULT;
 
         if ((gup_flags & FOLL_LONGTERM) && vma_is_fsdax(vma))
                 return -EOPNOTSUPP;
 
         if (vma_is_secretmem(vma))
                 return -EFAULT;
 
         if (write) {
                 if (!vma_anon &&
                     !writable_file_mapping_allowed(vma, gup_flags))
                         return -EFAULT;
 
                 if (!(vm_flags & VM_WRITE) || (vm_flags & VM_SHADOW_STACK)) {
                         if (!(gup_flags & FOLL_FORCE))
                                 return -EFAULT;
                         /* hugetlb does not support FOLL_FORCE|FOLL_WRITE. */
                         if (is_vm_hugetlb_page(vma))
                                 return -EFAULT;
                         /*
                          * We used to let the write,force case do COW in a
                          * VM_MAYWRITE VM_SHARED !VM_WRITE vma, so ptrace could
                          * set a breakpoint in a read-only mapping of an
                          * executable, without corrupting the file (yet only
                          * when that file had been opened for writing!).
                          * Anon pages in shared mappings are surprising: now
                          * just reject it.
                          */
                         if (!is_cow_mapping(vm_flags))
                                 return -EFAULT;
                 }
         } else if (!(vm_flags & VM_READ)) {
                 if (!(gup_flags & FOLL_FORCE))
                         return -EFAULT;
                 /*
                  * Is there actually any vma we can reach here which does not
                  * have VM_MAYREAD set?
                  */
                 if (!(vm_flags & VM_MAYREAD))
                         return -EFAULT;
         }
         /*
          * gups are always data accesses, not instruction
          * fetches, so execute=false here
          */
         if (!arch_vma_access_permitted(vma, write, false, foreign))
                 return -EFAULT;
         return 0;
 }
 
 /*
  * This is "vma_lookup()", but with a warning if we would have
  * historically expanded the stack in the GUP code.
  */
 static struct vm_area_struct *gup_vma_lookup(struct mm_struct *mm,
          unsigned long addr)
 {
 #ifdef CONFIG_STACK_GROWSUP
         return vma_lookup(mm, addr);
 #else
         static volatile unsigned long next_warn;
         struct vm_area_struct *vma;
         unsigned long now, next;
 
         vma = find_vma(mm, addr);
         if (!vma || (addr >= vma->vm_start))
                 return vma;
 
         /* Only warn for half-way relevant accesses */
         if (!(vma->vm_flags & VM_GROWSDOWN))
                 return NULL;
         if (vma->vm_start - addr > 65536)
                 return NULL;
 
         /* Let's not warn more than once an hour.. */
         now = jiffies; next = next_warn;
         if (next && time_before(now, next))
                 return NULL;
         next_warn = now + 60*60*HZ;
 
         /* Let people know things may have changed. */
         pr_warn("GUP no longer grows the stack in %s (%d): %lx-%lx (%lx)\n",
                 current->comm, task_pid_nr(current),
                 vma->vm_start, vma->vm_end, addr);
         dump_stack();
         return NULL;
 #endif
 }
 
 /**
  * __get_user_pages() - pin user pages in memory
  * @mm:         mm_struct of target mm
  * @start:      starting user address
  * @nr_pages:   number of pages from start to pin
  * @gup_flags:  flags modifying pin behaviour
  * @pages:      array that receives pointers to the pages pinned.
  *              Should be at least nr_pages long. Or NULL, if caller
  *              only intends to ensure the pages are faulted in.
  * @locked:     whether we're still with the mmap_lock held
  *
  * Returns either number of pages pinned (which may be less than the
  * number requested), or an error. Details about the return value:
  *
  * -- If nr_pages is 0, returns 0.
  * -- If nr_pages is >0, but no pages were pinned, returns -errno.
  * -- If nr_pages is >0, and some pages were pinned, returns the number of
  *    pages pinned. Again, this may be less than nr_pages.
  * -- 0 return value is possible when the fault would need to be retried.
  *
  * The caller is responsible for releasing returned @pages, via put_page().
  *
  * Must be called with mmap_lock held.  It may be released.  See below.
  *
  * __get_user_pages walks a process's page tables and takes a reference to
  * each struct page that each user address corresponds to at a given
  * instant. That is, it takes the page that would be accessed if a user
  * thread accesses the given user virtual address at that instant.
  *
  * This does not guarantee that the page exists in the user mappings when
  * __get_user_pages returns, and there may even be a completely different
  * page there in some cases (eg. if mmapped pagecache has been invalidated
  * and subsequently re-faulted). However it does guarantee that the page
  * won't be freed completely. And mostly callers simply care that the page
  * contains data that was valid *at some point in time*. Typically, an IO
  * or similar operation cannot guarantee anything stronger anyway because
  * locks can't be held over the syscall boundary.
  *
  * If @gup_flags & FOLL_WRITE == 0, the page must not be written to. If
  * the page is written to, set_page_dirty (or set_page_dirty_lock, as
  * appropriate) must be called after the page is finished with, and
  * before put_page is called.
  *
  * If FOLL_UNLOCKABLE is set without FOLL_NOWAIT then the mmap_lock may
  * be released. If this happens *@locked will be set to 0 on return.
  *
  * A caller using such a combination of @gup_flags must therefore hold the
  * mmap_lock for reading only, and recognize when it's been released. Otherwise,
  * it must be held for either reading or writing and will not be released.
  *
  * In most cases, get_user_pages or get_user_pages_fast should be used
  * instead of __get_user_pages. __get_user_pages should be used only if
  * you need some special @gup_flags.
  */
 static long __get_user_pages(struct mm_struct *mm,
                 unsigned long start, unsigned long nr_pages,
                 unsigned int gup_flags, struct page **pages,
                 int *locked)
 {
         long ret = 0, i = 0;
         struct vm_area_struct *vma = NULL;
         struct follow_page_context ctx = { NULL };
 
         if (!nr_pages)
                 return 0;
 
         start = untagged_addr_remote(mm, start);
 
         VM_BUG_ON(!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN)));
 
         do {
                 struct page *page;
                 unsigned int foll_flags = gup_flags;
                 unsigned int page_increm;
 
                 /* first iteration or cross vma bound */
                 if (!vma || start >= vma->vm_end) {
                         /*
                          * MADV_POPULATE_(READ|WRITE) wants to handle VMA
                          * lookups+error reporting differently.
                          */
                         if (gup_flags & FOLL_MADV_POPULATE) {
                                 vma = vma_lookup(mm, start);
                                 if (!vma) {
                                         ret = -ENOMEM;
                                         goto out;
                                 }
                                 if (check_vma_flags(vma, gup_flags)) {
                                         ret = -EINVAL;
                                         goto out;
                                 }
                                 goto retry;
                         }
                         vma = gup_vma_lookup(mm, start);
                         if (!vma && in_gate_area(mm, start)) {
                                 ret = get_gate_page(mm, start & PAGE_MASK,
                                                 gup_flags, &vma,
                                                 pages ? &page : NULL);
                                 if (ret)
                                         goto out;
                                 ctx.page_mask = 0;
                                 goto next_page;
                         }
 
                         if (!vma) {
                                 ret = -EFAULT;
                                 goto out;
                         }
                         ret = check_vma_flags(vma, gup_flags);
                         if (ret)
                                 goto out;
                 }
 retry:
                 /*
                  * If we have a pending SIGKILL, don't keep faulting pages and
                  * potentially allocating memory.
                  */
                 if (fatal_signal_pending(current)) {
                         ret = -EINTR;
                         goto out;
                 }
                 cond_resched();
 
                 page = follow_page_mask(vma, start, foll_flags, &ctx);
                 if (!page || PTR_ERR(page) == -EMLINK) {
                         ret = faultin_page(vma, start, &foll_flags,
                                            PTR_ERR(page) == -EMLINK, locked);
                         switch (ret) {
                         case 0:
                                 goto retry;
                         case -EBUSY:
                         case -EAGAIN:
                                 ret = 0;
                                 fallthrough;
                         case -EFAULT:
                         case -ENOMEM:
                         case -EHWPOISON:
                                 goto out;
                         }
                         BUG();
                 } else if (PTR_ERR(page) == -EEXIST) {
                         /*
                          * Proper page table entry exists, but no corresponding
                          * struct page. If the caller expects **pages to be
                          * filled in, bail out now, because that can't be done
                          * for this page.
                          */
                         if (pages) {
                                 ret = PTR_ERR(page);
                                 goto out;
                         }
                 } else if (IS_ERR(page)) {
                         ret = PTR_ERR(page);
                         goto out;
                 }
 next_page:
                 page_increm = 1 + (~(start >> PAGE_SHIFT) & ctx.page_mask);
                 if (page_increm > nr_pages)
                         page_increm = nr_pages;
 
                 if (pages) {
                         struct page *subpage;
                         unsigned int j;
 
                         /*
                          * This must be a large folio (and doesn't need to
                          * be the whole folio; it can be part of it), do
                          * the refcount work for all the subpages too.
                          *
                          * NOTE: here the page may not be the head page
                          * e.g. when start addr is not thp-size aligned.
                          * try_grab_folio() should have taken care of tail
                          * pages.
                          */
                         if (page_increm > 1) {
                                 struct folio *folio = page_folio(page);
 
                                 /*
                                  * Since we already hold refcount on the
                                  * large folio, this should never fail.
                                  */
                                 if (try_grab_folio(folio, page_increm - 1,
                                                    foll_flags)) {
                                         /*
                                          * Release the 1st page ref if the
                                          * folio is problematic, fail hard.
                                          */
                                         gup_put_folio(folio, 1,
                                                       foll_flags);
                                         ret = -EFAULT;
                                         goto out;
                                 }
                         }
 
                         for (j = 0; j < page_increm; j++) {
                                 subpage = nth_page(page, j);
                                 pages[i + j] = subpage;
                                 flush_anon_page(vma, subpage, start + j * PAGE_SIZE);
                                 flush_dcache_page(subpage);
                         }
                 }
 
                 i += page_increm;
                 start += page_increm * PAGE_SIZE;
                 nr_pages -= page_increm;
         } while (nr_pages);
 out:
         if (ctx.pgmap)
                 put_dev_pagemap(ctx.pgmap);
         return i ? i : ret;
 }
 
 static bool vma_permits_fault(struct vm_area_struct *vma,
                               unsigned int fault_flags)
 {
         bool write   = !!(fault_flags & FAULT_FLAG_WRITE);
         bool foreign = !!(fault_flags & FAULT_FLAG_REMOTE);
         vm_flags_t vm_flags = write ? VM_WRITE : VM_READ;
 
         if (!(vm_flags & vma->vm_flags))
                 return false;
 
         /*
          * The architecture might have a hardware protection
          * mechanism other than read/write that can deny access.
          *
          * gup always represents data access, not instruction
          * fetches, so execute=false here:
          */
         if (!arch_vma_access_permitted(vma, write, false, foreign))
                 return false;
 
         return true;
 }
 
 /**
  * fixup_user_fault() - manually resolve a user page fault
  * @mm:         mm_struct of target mm
  * @address:    user address
  * @fault_flags:flags to pass down to handle_mm_fault()
  * @unlocked:   did we unlock the mmap_lock while retrying, maybe NULL if caller
  *              does not allow retry. If NULL, the caller must guarantee
  *              that fault_flags does not contain FAULT_FLAG_ALLOW_RETRY.
  *
  * This is meant to be called in the specific scenario where for locking reasons
  * we try to access user memory in atomic context (within a pagefault_disable()
  * section), this returns -EFAULT, and we want to resolve the user fault before
  * trying again.
  *
  * Typically this is meant to be used by the futex code.
  *
  * The main difference with get_user_pages() is that this function will
  * unconditionally call handle_mm_fault() which will in turn perform all the
  * necessary SW fixup of the dirty and young bits in the PTE, while
  * get_user_pages() only guarantees to update these in the struct page.
  *
  * This is important for some architectures where those bits also gate the
  * access permission to the page because they are maintained in software.  On
  * such architectures, gup() will not be enough to make a subsequent access
  * succeed.
  *
  * This function will not return with an unlocked mmap_lock. So it has not the
  * same semantics wrt the @mm->mmap_lock as does filemap_fault().
  */
 int fixup_user_fault(struct mm_struct *mm,
                      unsigned long address, unsigned int fault_flags,
                      bool *unlocked)
 {
         struct vm_area_struct *vma;
         vm_fault_t ret;
 
         address = untagged_addr_remote(mm, address);
 
         if (unlocked)
                 fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
 
 retry:
         vma = gup_vma_lookup(mm, address);
         if (!vma)
                 return -EFAULT;
 
         if (!vma_permits_fault(vma, fault_flags))
                 return -EFAULT;
 
         if ((fault_flags & FAULT_FLAG_KILLABLE) &&
             fatal_signal_pending(current))
                 return -EINTR;
 
         ret = handle_mm_fault(vma, address, fault_flags, NULL);
 
         if (ret & VM_FAULT_COMPLETED) {
                 /*
                  * NOTE: it's a pity that we need to retake the lock here
                  * to pair with the unlock() in the callers. Ideally we
                  * could tell the callers so they do not need to unlock.
                  */
                 mmap_read_lock(mm);
                 *unlocked = true;
                 return 0;
         }
 
         if (ret & VM_FAULT_ERROR) {
                 int err = vm_fault_to_errno(ret, 0);
 
                 if (err)
                         return err;
                 BUG();
         }
 
         if (ret & VM_FAULT_RETRY) {
                 mmap_read_lock(mm);
                 *unlocked = true;
                 fault_flags |= FAULT_FLAG_TRIED;
                 goto retry;
         }
 
         return 0;
 }
 EXPORT_SYMBOL_GPL(fixup_user_fault);
 
 /*
  * GUP always responds to fatal signals.  When FOLL_INTERRUPTIBLE is
  * specified, it'll also respond to generic signals.  The caller of GUP
  * that has FOLL_INTERRUPTIBLE should take care of the GUP interruption.
  */
 static bool gup_signal_pending(unsigned int flags)
 {
         if (fatal_signal_pending(current))
                 return true;
 
         if (!(flags & FOLL_INTERRUPTIBLE))
                 return false;
 
         return signal_pending(current);
 }
 
 /*
  * Locking: (*locked == 1) means that the mmap_lock has already been acquired by
  * the caller. This function may drop the mmap_lock. If it does so, then it will
  * set (*locked = 0).
  *
  * (*locked == 0) means that the caller expects this function to acquire and
  * drop the mmap_lock. Therefore, the value of *locked will still be zero when
  * the function returns, even though it may have changed temporarily during
  * function execution.
  *
  * Please note that this function, unlike __get_user_pages(), will not return 0
  * for nr_pages > 0, unless FOLL_NOWAIT is used.
  */
 static __always_inline long __get_user_pages_locked(struct mm_struct *mm,
                                                 unsigned long start,
                                                 unsigned long nr_pages,
                                                 struct page **pages,
                                                 int *locked,
                                                 unsigned int flags)
 {
         long ret, pages_done;
         bool must_unlock = false;
 
         if (!nr_pages)
                 return 0;
 
         /*
          * The internal caller expects GUP to manage the lock internally and the
          * lock must be released when this returns.
          */
         if (!*locked) {
                 if (mmap_read_lock_killable(mm))
                         return -EAGAIN;
                 must_unlock = true;
                 *locked = 1;
         }
         else
                 mmap_assert_locked(mm);
 
         if (flags & FOLL_PIN)
                 mm_set_has_pinned_flag(&mm->flags);
 
         /*
          * FOLL_PIN and FOLL_GET are mutually exclusive. Traditional behavior
          * is to set FOLL_GET if the caller wants pages[] filled in (but has
          * carelessly failed to specify FOLL_GET), so keep doing that, but only
          * for FOLL_GET, not for the newer FOLL_PIN.
          *
          * FOLL_PIN always expects pages to be non-null, but no need to assert
          * that here, as any failures will be obvious enough.
          */
         if (pages && !(flags & FOLL_PIN))
                 flags |= FOLL_GET;
 
         pages_done = 0;
         for (;;) {
                 ret = __get_user_pages(mm, start, nr_pages, flags, pages,
                                        locked);
                 if (!(flags & FOLL_UNLOCKABLE)) {
                         /* VM_FAULT_RETRY couldn't trigger, bypass */
                         pages_done = ret;
                         break;
                 }
 
                 /* VM_FAULT_RETRY or VM_FAULT_COMPLETED cannot return errors */
                 if (!*locked) {
                         BUG_ON(ret < 0);
                         BUG_ON(ret >= nr_pages);
                 }
 
                 if (ret > 0) {
                         nr_pages -= ret;
                         pages_done += ret;
                         if (!nr_pages)
                                 break;
                 }
                 if (*locked) {
                         /*
                          * VM_FAULT_RETRY didn't trigger or it was a
                          * FOLL_NOWAIT.
                          */
                         if (!pages_done)
                                 pages_done = ret;
                         break;
                 }
                 /*
                  * VM_FAULT_RETRY triggered, so seek to the faulting offset.
                  * For the prefault case (!pages) we only update counts.
                  */
                 if (likely(pages))
                         pages += ret;
                 start += ret << PAGE_SHIFT;
 
                 /* The lock was temporarily dropped, so we must unlock later */
                 must_unlock = true;
 
 retry:
                 /*
                  * Repeat on the address that fired VM_FAULT_RETRY
                  * with both FAULT_FLAG_ALLOW_RETRY and
                  * FAULT_FLAG_TRIED.  Note that GUP can be interrupted
                  * by fatal signals of even common signals, depending on
                  * the caller's request. So we need to check it before we
                  * start trying again otherwise it can loop forever.
                  */
                 if (gup_signal_pending(flags)) {
                         if (!pages_done)
                                 pages_done = -EINTR;
                         break;
                 }
 
                 ret = mmap_read_lock_killable(mm);
                 if (ret) {
                         BUG_ON(ret > 0);
                         if (!pages_done)
                                 pages_done = ret;
                         break;
                 }
 
                 *locked = 1;
                 ret = __get_user_pages(mm, start, 1, flags | FOLL_TRIED,
                                        pages, locked);
                 if (!*locked) {
                         /* Continue to retry until we succeeded */
                         BUG_ON(ret != 0);
                         goto retry;
                 }
                 if (ret != 1) {
                         BUG_ON(ret > 1);
                         if (!pages_done)
                                 pages_done = ret;
                         break;
                 }
                 nr_pages--;
                 pages_done++;
                 if (!nr_pages)
                         break;
                 if (likely(pages))
                         pages++;
                 start += PAGE_SIZE;
         }
         if (must_unlock && *locked) {
                 /*
                  * We either temporarily dropped the lock, or the caller
                  * requested that we both acquire and drop the lock. Either way,
                  * we must now unlock, and notify the caller of that state.
                  */
                 mmap_read_unlock(mm);
                 *locked = 0;
         }
 
         /*
          * Failing to pin anything implies something has gone wrong (except when
          * FOLL_NOWAIT is specified).
          */
         if (WARN_ON_ONCE(pages_done == 0 && !(flags & FOLL_NOWAIT)))
                 return -EFAULT;
 
         return pages_done;
 }
 
 /**
  * populate_vma_page_range() -  populate a range of pages in the vma.
  * @vma:   target vma
  * @start: start address
  * @end:   end address
  * @locked: whether the mmap_lock is still held
  *
  * This takes care of mlocking the pages too if VM_LOCKED is set.
  *
  * Return either number of pages pinned in the vma, or a negative error
  * code on error.
  *
  * vma->vm_mm->mmap_lock must be held.
  *
  * If @locked is NULL, it may be held for read or write and will
  * be unperturbed.
  *
  * If @locked is non-NULL, it must held for read only and may be
  * released.  If it's released, *@locked will be set to 0.
  */
 long populate_vma_page_range(struct vm_area_struct *vma,
                 unsigned long start, unsigned long end, int *locked)
 {
         struct mm_struct *mm = vma->vm_mm;
         unsigned long nr_pages = (end - start) / PAGE_SIZE;
         int local_locked = 1;
         int gup_flags;
         long ret;
 
         VM_BUG_ON(!PAGE_ALIGNED(start));
         VM_BUG_ON(!PAGE_ALIGNED(end));
         VM_BUG_ON_VMA(start < vma->vm_start, vma);
         VM_BUG_ON_VMA(end   > vma->vm_end, vma);
         mmap_assert_locked(mm);
 
         /*
          * Rightly or wrongly, the VM_LOCKONFAULT case has never used
          * faultin_page() to break COW, so it has no work to do here.
          */
         if (vma->vm_flags & VM_LOCKONFAULT)
                 return nr_pages;
 
         /* ... similarly, we've never faulted in PROT_NONE pages */
         if (!vma_is_accessible(vma))
                 return -EFAULT;
 
         gup_flags = FOLL_TOUCH;
         /*
          * We want to touch writable mappings with a write fault in order
          * to break COW, except for shared mappings because these don't COW
          * and we would not want to dirty them for nothing.
          *
          * Otherwise, do a read fault, and use FOLL_FORCE in case it's not
          * readable (ie write-only or executable).
          */
         if ((vma->vm_flags & (VM_WRITE | VM_SHARED)) == VM_WRITE)
                 gup_flags |= FOLL_WRITE;
         else
                 gup_flags |= FOLL_FORCE;
 
         if (locked)
                 gup_flags |= FOLL_UNLOCKABLE;
 
         /*
          * We made sure addr is within a VMA, so the following will
          * not result in a stack expansion that recurses back here.
          */
         ret = __get_user_pages(mm, start, nr_pages, gup_flags,
                                NULL, locked ? locked : &local_locked);
         lru_add_drain();
         return ret;
 }
 
 /*
  * faultin_page_range() - populate (prefault) page tables inside the
  *                        given range readable/writable
  *
  * This takes care of mlocking the pages, too, if VM_LOCKED is set.
  *
  * @mm: the mm to populate page tables in
  * @start: start address
  * @end: end address
  * @write: whether to prefault readable or writable
  * @locked: whether the mmap_lock is still held
  *
  * Returns either number of processed pages in the MM, or a negative error
  * code on error (see __get_user_pages()). Note that this function reports
  * errors related to VMAs, such as incompatible mappings, as expected by
  * MADV_POPULATE_(READ|WRITE).
  *
  * The range must be page-aligned.
  *
  * mm->mmap_lock must be held. If it's released, *@locked will be set to 0.
  */
 long faultin_page_range(struct mm_struct *mm, unsigned long start,
                         unsigned long end, bool write, int *locked)
 {
         unsigned long nr_pages = (end - start) / PAGE_SIZE;
         int gup_flags;
         long ret;
 
         VM_BUG_ON(!PAGE_ALIGNED(start));
         VM_BUG_ON(!PAGE_ALIGNED(end));
         mmap_assert_locked(mm);
 
         /*
          * FOLL_TOUCH: Mark page accessed and thereby young; will also mark
          *             the page dirty with FOLL_WRITE -- which doesn't make a
          *             difference with !FOLL_FORCE, because the page is writable
          *             in the page table.
          * FOLL_HWPOISON: Return -EHWPOISON instead of -EFAULT when we hit
          *                a poisoned page.
          * !FOLL_FORCE: Require proper access permissions.
          */
         gup_flags = FOLL_TOUCH | FOLL_HWPOISON | FOLL_UNLOCKABLE |
                     FOLL_MADV_POPULATE;
         if (write)
                 gup_flags |= FOLL_WRITE;
 
         ret = __get_user_pages_locked(mm, start, nr_pages, NULL, locked,
                                       gup_flags);
         lru_add_drain();
         return ret;
 }
 
 /*
  * __mm_populate - populate and/or mlock pages within a range of address space.
  *
  * This is used to implement mlock() and the MAP_POPULATE / MAP_LOCKED mmap
  * flags. VMAs must be already marked with the desired vm_flags, and
  * mmap_lock must not be held.
  */
 int __mm_populate(unsigned long start, unsigned long len, int ignore_errors)
 {
         struct mm_struct *mm = current->mm;
         unsigned long end, nstart, nend;
         struct vm_area_struct *vma = NULL;
         int locked = 0;
         long ret = 0;
 
         end = start + len;
 
         for (nstart = start; nstart < end; nstart = nend) {
                 /*
                  * We want to fault in pages for [nstart; end) address range.
                  * Find first corresponding VMA.
                  */
                 if (!locked) {
                         locked = 1;
                         mmap_read_lock(mm);
                         vma = find_vma_intersection(mm, nstart, end);
                 } else if (nstart >= vma->vm_end)
                         vma = find_vma_intersection(mm, vma->vm_end, end);
 
                 if (!vma)
                         break;
                 /*
                  * Set [nstart; nend) to intersection of desired address
                  * range with the first VMA. Also, skip undesirable VMA types.
                  */
                 nend = min(end, vma->vm_end);
                 if (vma->vm_flags & (VM_IO | VM_PFNMAP))
                         continue;
                 if (nstart < vma->vm_start)
                         nstart = vma->vm_start;
                 /*
                  * Now fault in a range of pages. populate_vma_page_range()
                  * double checks the vma flags, so that it won't mlock pages
                  * if the vma was already munlocked.
                  */
                 ret = populate_vma_page_range(vma, nstart, nend, &locked);
                 if (ret < 0) {
                         if (ignore_errors) {
                                 ret = 0;
                                 continue;       /* continue at next VMA */
                         }
                         break;
                 }
                 nend = nstart + ret * PAGE_SIZE;
                 ret = 0;
         }
         if (locked)
                 mmap_read_unlock(mm);
         return ret;     /* 0 or negative error code */
 }
 #else /* CONFIG_MMU */
 static long __get_user_pages_locked(struct mm_struct *mm, unsigned long start,
                 unsigned long nr_pages, struct page **pages,
                 int *locked, unsigned int foll_flags)
 {
         struct vm_area_struct *vma;
         bool must_unlock = false;
         unsigned long vm_flags;
         long i;
 
         if (!nr_pages)
                 return 0;
 
         /*
          * The internal caller expects GUP to manage the lock internally and the
          * lock must be released when this returns.
          */
         if (!*locked) {
                 if (mmap_read_lock_killable(mm))
                         return -EAGAIN;
                 must_unlock = true;
                 *locked = 1;
         }
 
         /* calculate required read or write permissions.
          * If FOLL_FORCE is set, we only require the "MAY" flags.
          */
         vm_flags  = (foll_flags & FOLL_WRITE) ?
                         (VM_WRITE | VM_MAYWRITE) : (VM_READ | VM_MAYREAD);
         vm_flags &= (foll_flags & FOLL_FORCE) ?
                         (VM_MAYREAD | VM_MAYWRITE) : (VM_READ | VM_WRITE);
 
         for (i = 0; i < nr_pages; i++) {
                 vma = find_vma(mm, start);
                 if (!vma)
                         break;
 
                 /* protect what we can, including chardevs */
                 if ((vma->vm_flags & (VM_IO | VM_PFNMAP)) ||
                     !(vm_flags & vma->vm_flags))
                         break;
 
                 if (pages) {
                         pages[i] = virt_to_page((void *)start);
                         if (pages[i])
                                 get_page(pages[i]);
                 }
 
                 start = (start + PAGE_SIZE) & PAGE_MASK;
         }
 
         if (must_unlock && *locked) {
                 mmap_read_unlock(mm);
                 *locked = 0;
         }
 
         return i ? : -EFAULT;
 }
 #endif /* !CONFIG_MMU */
 
 /**
  * fault_in_writeable - fault in userspace address range for writing
  * @uaddr: start of address range
  * @size: size of address range
  *
  * Returns the number of bytes not faulted in (like copy_to_user() and
  * copy_from_user()).
  */
 size_t fault_in_writeable(char __user *uaddr, size_t size)
 {
         char __user *start = uaddr, *end;
 
         if (unlikely(size == 0))
                 return 0;
         if (!user_write_access_begin(uaddr, size))
                 return size;
         if (!PAGE_ALIGNED(uaddr)) {
                 unsafe_put_user(0, uaddr, out);
                 uaddr = (char __user *)PAGE_ALIGN((unsigned long)uaddr);
         }
         end = (char __user *)PAGE_ALIGN((unsigned long)start + size);
         if (unlikely(end < start))
                 end = NULL;
         while (uaddr != end) {
                 unsafe_put_user(0, uaddr, out);
                 uaddr += PAGE_SIZE;
         }
 
 out:
         user_write_access_end();
         if (size > uaddr - start)
                 return size - (uaddr - start);
         return 0;
 }
 EXPORT_SYMBOL(fault_in_writeable);
 
 /**
  * fault_in_subpage_writeable - fault in an address range for writing
  * @uaddr: start of address range
  * @size: size of address range
  *
  * Fault in a user address range for writing while checking for permissions at
  * sub-page granularity (e.g. arm64 MTE). This function should be used when
  * the caller cannot guarantee forward progress of a copy_to_user() loop.
  *
  * Returns the number of bytes not faulted in (like copy_to_user() and
  * copy_from_user()).
  */
 size_t fault_in_subpage_writeable(char __user *uaddr, size_t size)
 {
         size_t faulted_in;
 
         /*
          * Attempt faulting in at page granularity first for page table
          * permission checking. The arch-specific probe_subpage_writeable()
          * functions may not check for this.
          */
         faulted_in = size - fault_in_writeable(uaddr, size);
         if (faulted_in)
                 faulted_in -= probe_subpage_writeable(uaddr, faulted_in);
 
         return size - faulted_in;
 }
 EXPORT_SYMBOL(fault_in_subpage_writeable);
 
 /*
  * fault_in_safe_writeable - fault in an address range for writing
  * @uaddr: start of address range
  * @size: length of address range
  *
  * Faults in an address range for writing.  This is primarily useful when we
  * already know that some or all of the pages in the address range aren't in
  * memory.
  *
  * Unlike fault_in_writeable(), this function is non-destructive.
  *
  * Note that we don't pin or otherwise hold the pages referenced that we fault
  * in.  There's no guarantee that they'll stay in memory for any duration of
  * time.
  *
  * Returns the number of bytes not faulted in, like copy_to_user() and
  * copy_from_user().
  */
 size_t fault_in_safe_writeable(const char __user *uaddr, size_t size)
 {
         unsigned long start = (unsigned long)uaddr, end;
         struct mm_struct *mm = current->mm;
         bool unlocked = false;
 
         if (unlikely(size == 0))
                 return 0;
         end = PAGE_ALIGN(start + size);
         if (end < start)
                 end = 0;
 
         mmap_read_lock(mm);
         do {
                 if (fixup_user_fault(mm, start, FAULT_FLAG_WRITE, &unlocked))
                         break;
                 start = (start + PAGE_SIZE) & PAGE_MASK;
         } while (start != end);
         mmap_read_unlock(mm);
 
         if (size > (unsigned long)uaddr - start)
                 return size - ((unsigned long)uaddr - start);
         return 0;
 }
 EXPORT_SYMBOL(fault_in_safe_writeable);
 
 /**
  * fault_in_readable - fault in userspace address range for reading
  * @uaddr: start of user address range
  * @size: size of user address range
  *
  * Returns the number of bytes not faulted in (like copy_to_user() and
  * copy_from_user()).
  */
 size_t fault_in_readable(const char __user *uaddr, size_t size)
 {
         const char __user *start = uaddr, *end;
         volatile char c;
 
         if (unlikely(size == 0))
                 return 0;
         if (!user_read_access_begin(uaddr, size))
                 return size;
         if (!PAGE_ALIGNED(uaddr)) {
                 unsafe_get_user(c, uaddr, out);
                 uaddr = (const char __user *)PAGE_ALIGN((unsigned long)uaddr);
         }
         end = (const char __user *)PAGE_ALIGN((unsigned long)start + size);
         if (unlikely(end < start))
                 end = NULL;
         while (uaddr != end) {
                 unsafe_get_user(c, uaddr, out);
                 uaddr += PAGE_SIZE;
         }
 
 out:
         user_read_access_end();
         (void)c;
         if (size > uaddr - start)
                 return size - (uaddr - start);
         return 0;
 }
 EXPORT_SYMBOL(fault_in_readable);
 
 /**
  * get_dump_page() - pin user page in memory while writing it to core dump
  * @addr: user address
  *
  * Returns struct page pointer of user page pinned for dump,
  * to be freed afterwards by put_page().
  *
  * Returns NULL on any kind of failure - a hole must then be inserted into
  * the corefile, to preserve alignment with its headers; and also returns
  * NULL wherever the ZERO_PAGE, or an anonymous pte_none, has been found -
  * allowing a hole to be left in the corefile to save disk space.
  *
  * Called without mmap_lock (takes and releases the mmap_lock by itself).
  */
 #ifdef CONFIG_ELF_CORE
 struct page *get_dump_page(unsigned long addr)
 {
         struct page *page;
         int locked = 0;
         int ret;
 
         ret = __get_user_pages_locked(current->mm, addr, 1, &page, &locked,
                                       FOLL_FORCE | FOLL_DUMP | FOLL_GET);
         return (ret == 1) ? page : NULL;
 }
 #endif /* CONFIG_ELF_CORE */
 
 #ifdef CONFIG_MIGRATION
 /*
  * Returns the number of collected folios. Return value is always >= 0.
  */
 static unsigned long collect_longterm_unpinnable_folios(
                                         struct list_head *movable_folio_list,
                                         unsigned long nr_folios,
                                         struct folio **folios)
 {
         unsigned long i, collected = 0;
         struct folio *prev_folio = NULL;
         bool drain_allow = true;
 
         for (i = 0; i < nr_folios; i++) {
                 struct folio *folio = folios[i];
 
                 if (folio == prev_folio)
                         continue;
                 prev_folio = folio;
 
                 if (folio_is_longterm_pinnable(folio))
                         continue;
 
                 collected++;
 
                 if (folio_is_device_coherent(folio))
                         continue;
 
                 if (folio_test_hugetlb(folio)) {
                         isolate_hugetlb(folio, movable_folio_list);
                         continue;
                 }
 
                 if (!folio_test_lru(folio) && drain_allow) {
                         lru_add_drain_all();
                         drain_allow = false;
                 }
 
                 if (!folio_isolate_lru(folio))
                         continue;
 
                 list_add_tail(&folio->lru, movable_folio_list);
                 node_stat_mod_folio(folio,
                                     NR_ISOLATED_ANON + folio_is_file_lru(folio),
                                     folio_nr_pages(folio));
         }
 
         return collected;
 }
 
 /*
  * Unpins all folios and migrates device coherent folios and movable_folio_list.
  * Returns -EAGAIN if all folios were successfully migrated or -errno for
  * failure (or partial success).
  */
 static int migrate_longterm_unpinnable_folios(
                                         struct list_head *movable_folio_list,
                                         unsigned long nr_folios,
                                         struct folio **folios)
 {
         int ret;
         unsigned long i;
 
         for (i = 0; i < nr_folios; i++) {
                 struct folio *folio = folios[i];
 
                 if (folio_is_device_coherent(folio)) {
                         /*
                          * Migration will fail if the folio is pinned, so
                          * convert the pin on the source folio to a normal
                          * reference.
                          */
                         folios[i] = NULL;
                         folio_get(folio);
                         gup_put_folio(folio, 1, FOLL_PIN);
 
                         if (migrate_device_coherent_page(&folio->page)) {
                                 ret = -EBUSY;
                                 goto err;
                         }
 
                         continue;
                 }
 
                 /*
                  * We can't migrate folios with unexpected references, so drop
                  * the reference obtained by __get_user_pages_locked().
                  * Migrating folios have been added to movable_folio_list after
                  * calling folio_isolate_lru() which takes a reference so the
                  * folio won't be freed if it's migrating.
                  */
                 unpin_folio(folios[i]);
                 folios[i] = NULL;
         }
 
         if (!list_empty(movable_folio_list)) {
                 struct migration_target_control mtc = {
                         .nid = NUMA_NO_NODE,
                         .gfp_mask = GFP_USER | __GFP_NOWARN,
                         .reason = MR_LONGTERM_PIN,
                 };
 
                 if (migrate_pages(movable_folio_list, alloc_migration_target,
                                   NULL, (unsigned long)&mtc, MIGRATE_SYNC,
                                   MR_LONGTERM_PIN, NULL)) {
                         ret = -ENOMEM;
                         goto err;
                 }
         }
 
         putback_movable_pages(movable_folio_list);
 
         return -EAGAIN;
 
 err:
         unpin_folios(folios, nr_folios);
         putback_movable_pages(movable_folio_list);
 
         return ret;
 }
 
 /*
  * Check whether all folios are *allowed* to be pinned indefinitely (longterm).
  * Rather confusingly, all folios in the range are required to be pinned via
  * FOLL_PIN, before calling this routine.
  *
  * If any folios in the range are not allowed to be pinned, then this routine
  * will migrate those folios away, unpin all the folios in the range and return
  * -EAGAIN. The caller should re-pin the entire range with FOLL_PIN and then
  * call this routine again.
  *
  * If an error other than -EAGAIN occurs, this indicates a migration failure.
  * The caller should give up, and propagate the error back up the call stack.
  *
  * If everything is OK and all folios in the range are allowed to be pinned,
  * then this routine leaves all folios pinned and returns zero for success.
  */
 static long check_and_migrate_movable_folios(unsigned long nr_folios,
                                              struct folio **folios)
 {
         unsigned long collected;
         LIST_HEAD(movable_folio_list);
 
         collected = collect_longterm_unpinnable_folios(&movable_folio_list,
                                                        nr_folios, folios);
         if (!collected)
                 return 0;
 
         return migrate_longterm_unpinnable_folios(&movable_folio_list,
                                                   nr_folios, folios);
 }
 
 /*
  * This routine just converts all the pages in the @pages array to folios and
  * calls check_and_migrate_movable_folios() to do the heavy lifting.
  *
  * Please see the check_and_migrate_movable_folios() documentation for details.
  */
 static long check_and_migrate_movable_pages(unsigned long nr_pages,
                                             struct page **pages)
 {
         struct folio **folios;
         long i, ret;
 
         folios = kmalloc_array(nr_pages, sizeof(*folios), GFP_KERNEL);
         if (!folios)
                 return -ENOMEM;
 
         for (i = 0; i < nr_pages; i++)
                 folios[i] = page_folio(pages[i]);
 
         ret = check_and_migrate_movable_folios(nr_pages, folios);
 
         kfree(folios);
         return ret;
 }
 #else
 static long check_and_migrate_movable_pages(unsigned long nr_pages,
                                             struct page **pages)
 {
         return 0;
 }
 
 static long check_and_migrate_movable_folios(unsigned long nr_folios,
                                              struct folio **folios)
 {
         return 0;
 }
 #endif /* CONFIG_MIGRATION */
 
 /*
  * __gup_longterm_locked() is a wrapper for __get_user_pages_locked which
  * allows us to process the FOLL_LONGTERM flag.
  */
 static long __gup_longterm_locked(struct mm_struct *mm,
                                   unsigned long start,
                                   unsigned long nr_pages,
                                   struct page **pages,
                                   int *locked,
                                   unsigned int gup_flags)
 {
         unsigned int flags;
         long rc, nr_pinned_pages;
 
         if (!(gup_flags & FOLL_LONGTERM))
                 return __get_user_pages_locked(mm, start, nr_pages, pages,
                                                locked, gup_flags);
 
         flags = memalloc_pin_save();
         do {
                 nr_pinned_pages = __get_user_pages_locked(mm, start, nr_pages,
                                                           pages, locked,
                                                           gup_flags);
                 if (nr_pinned_pages <= 0) {
                         rc = nr_pinned_pages;
                         break;
                 }
 
                 /* FOLL_LONGTERM implies FOLL_PIN */
                 rc = check_and_migrate_movable_pages(nr_pinned_pages, pages);
         } while (rc == -EAGAIN);
         memalloc_pin_restore(flags);
         return rc ? rc : nr_pinned_pages;
 }
 
 /*
  * Check that the given flags are valid for the exported gup/pup interface, and
  * update them with the required flags that the caller must have set.
  */
 static bool is_valid_gup_args(struct page **pages, int *locked,
                               unsigned int *gup_flags_p, unsigned int to_set)
 {
         unsigned int gup_flags = *gup_flags_p;
 
         /*
          * These flags not allowed to be specified externally to the gup
          * interfaces:
          * - FOLL_TOUCH/FOLL_PIN/FOLL_TRIED/FOLL_FAST_ONLY are internal only
          * - FOLL_REMOTE is internal only and used on follow_page()
          * - FOLL_UNLOCKABLE is internal only and used if locked is !NULL
          */
         if (WARN_ON_ONCE(gup_flags & INTERNAL_GUP_FLAGS))
                 return false;
 
         gup_flags |= to_set;
         if (locked) {
                 /* At the external interface locked must be set */
                 if (WARN_ON_ONCE(*locked != 1))
                         return false;
 
                 gup_flags |= FOLL_UNLOCKABLE;
         }
 
         /* FOLL_GET and FOLL_PIN are mutually exclusive. */
         if (WARN_ON_ONCE((gup_flags & (FOLL_PIN | FOLL_GET)) ==
                          (FOLL_PIN | FOLL_GET)))
                 return false;
 
         /* LONGTERM can only be specified when pinning */
         if (WARN_ON_ONCE(!(gup_flags & FOLL_PIN) && (gup_flags & FOLL_LONGTERM)))
                 return false;
 
         /* Pages input must be given if using GET/PIN */
         if (WARN_ON_ONCE((gup_flags & (FOLL_GET | FOLL_PIN)) && !pages))
                 return false;
 
         /* We want to allow the pgmap to be hot-unplugged at all times */
         if (WARN_ON_ONCE((gup_flags & FOLL_LONGTERM) &&
                          (gup_flags & FOLL_PCI_P2PDMA)))
                 return false;
 
         *gup_flags_p = gup_flags;
         return true;
 }
 
 #ifdef CONFIG_MMU
 /**
  * get_user_pages_remote() - pin user pages in memory
  * @mm:         mm_struct of target mm
  * @start:      starting user address
  * @nr_pages:   number of pages from start to pin
  * @gup_flags:  flags modifying lookup behaviour
  * @pages:      array that receives pointers to the pages pinned.
  *              Should be at least nr_pages long. Or NULL, if caller
  *              only intends to ensure the pages are faulted in.
  * @locked:     pointer to lock flag indicating whether lock is held and
  *              subsequently whether VM_FAULT_RETRY functionality can be
  *              utilised. Lock must initially be held.
  *
  * Returns either number of pages pinned (which may be less than the
  * number requested), or an error. Details about the return value:
  *
  * -- If nr_pages is 0, returns 0.
  * -- If nr_pages is >0, but no pages were pinned, returns -errno.
  * -- If nr_pages is >0, and some pages were pinned, returns the number of
  *    pages pinned. Again, this may be less than nr_pages.
  *
  * The caller is responsible for releasing returned @pages, via put_page().
  *
  * Must be called with mmap_lock held for read or write.
  *
  * get_user_pages_remote walks a process's page tables and takes a reference
  * to each struct page that each user address corresponds to at a given
  * instant. That is, it takes the page that would be accessed if a user
  * thread accesses the given user virtual address at that instant.
  *
  * This does not guarantee that the page exists in the user mappings when
  * get_user_pages_remote returns, and there may even be a completely different
  * page there in some cases (eg. if mmapped pagecache has been invalidated
  * and subsequently re-faulted). However it does guarantee that the page
  * won't be freed completely. And mostly callers simply care that the page
  * contains data that was valid *at some point in time*. Typically, an IO
  * or similar operation cannot guarantee anything stronger anyway because
  * locks can't be held over the syscall boundary.
  *
  * If gup_flags & FOLL_WRITE == 0, the page must not be written to. If the page
  * is written to, set_page_dirty (or set_page_dirty_lock, as appropriate) must
  * be called after the page is finished with, and before put_page is called.
  *
  * get_user_pages_remote is typically used for fewer-copy IO operations,
  * to get a handle on the memory by some means other than accesses
  * via the user virtual addresses. The pages may be submitted for
  * DMA to devices or accessed via their kernel linear mapping (via the
  * kmap APIs). Care should be taken to use the correct cache flushing APIs.
  *
  * See also get_user_pages_fast, for performance critical applications.
  *
  * get_user_pages_remote should be phased out in favor of
  * get_user_pages_locked|unlocked or get_user_pages_fast. Nothing
  * should use get_user_pages_remote because it cannot pass
  * FAULT_FLAG_ALLOW_RETRY to handle_mm_fault.
  */
 long get_user_pages_remote(struct mm_struct *mm,
                 unsigned long start, unsigned long nr_pages,
                 unsigned int gup_flags, struct page **pages,
                 int *locked)
 {
         int local_locked = 1;
 
         if (!is_valid_gup_args(pages, locked, &gup_flags,
                                FOLL_TOUCH | FOLL_REMOTE))
                 return -EINVAL;
 
         return __get_user_pages_locked(mm, start, nr_pages, pages,
                                        locked ? locked : &local_locked,
                                        gup_flags);
 }
 EXPORT_SYMBOL(get_user_pages_remote);
 
 #else /* CONFIG_MMU */
 long get_user_pages_remote(struct mm_struct *mm,
                            unsigned long start, unsigned long nr_pages,
                            unsigned int gup_flags, struct page **pages,
                            int *locked)
 {
         return 0;
 }
 #endif /* !CONFIG_MMU */
 
 /**
  * get_user_pages() - pin user pages in memory
  * @start:      starting user address
  * @nr_pages:   number of pages from start to pin
  * @gup_flags:  flags modifying lookup behaviour
  * @pages:      array that receives pointers to the pages pinned.
  *              Should be at least nr_pages long. Or NULL, if caller
  *              only intends to ensure the pages are faulted in.
  *
  * This is the same as get_user_pages_remote(), just with a less-flexible
  * calling convention where we assume that the mm being operated on belongs to
  * the current task, and doesn't allow passing of a locked parameter.  We also
  * obviously don't pass FOLL_REMOTE in here.
  */
 long get_user_pages(unsigned long start, unsigned long nr_pages,
                     unsigned int gup_flags, struct page **pages)
 {
         int locked = 1;
 
         if (!is_valid_gup_args(pages, NULL, &gup_flags, FOLL_TOUCH))
                 return -EINVAL;
 
         return __get_user_pages_locked(current->mm, start, nr_pages, pages,
                                        &locked, gup_flags);
 }
 EXPORT_SYMBOL(get_user_pages);
 
 /*
  * get_user_pages_unlocked() is suitable to replace the form:
  *
  *      mmap_read_lock(mm);
  *      get_user_pages(mm, ..., pages, NULL);
  *      mmap_read_unlock(mm);
  *
  *  with:
  *
  *      get_user_pages_unlocked(mm, ..., pages);
  *
  * It is functionally equivalent to get_user_pages_fast so
  * get_user_pages_fast should be used instead if specific gup_flags
  * (e.g. FOLL_FORCE) are not required.
  */
 long get_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
                              struct page **pages, unsigned int gup_flags)
 {
         int locked = 0;
 
         if (!is_valid_gup_args(pages, NULL, &gup_flags,
                                FOLL_TOUCH | FOLL_UNLOCKABLE))
                 return -EINVAL;
 
         return __get_user_pages_locked(current->mm, start, nr_pages, pages,
                                        &locked, gup_flags);
 }
 EXPORT_SYMBOL(get_user_pages_unlocked);
 
 /*
  * GUP-fast
  *
  * get_user_pages_fast attempts to pin user pages by walking the page
  * tables directly and avoids taking locks. Thus the walker needs to be
  * protected from page table pages being freed from under it, and should
  * block any THP splits.
  *
  * One way to achieve this is to have the walker disable interrupts, and
  * rely on IPIs from the TLB flushing code blocking before the page table
  * pages are freed. This is unsuitable for architectures that do not need
  * to broadcast an IPI when invalidating TLBs.
  *
  * Another way to achieve this is to batch up page table containing pages
  * belonging to more than one mm_user, then rcu_sched a callback to free those
  * pages. Disabling interrupts will allow the gup_fast() walker to both block
  * the rcu_sched callback, and an IPI that we broadcast for splitting THPs
  * (which is a relatively rare event). The code below adopts this strategy.
  *
  * Before activating this code, please be aware that the following assumptions
  * are currently made:
  *
  *  *) Either MMU_GATHER_RCU_TABLE_FREE is enabled, and tlb_remove_table() is used to
  *  free pages containing page tables or TLB flushing requires IPI broadcast.
  *
  *  *) ptes can be read atomically by the architecture.
  *
  *  *) access_ok is sufficient to validate userspace address ranges.
  *
  * The last two assumptions can be relaxed by the addition of helper functions.
  *
  * This code is based heavily on the PowerPC implementation by Nick Piggin.
  */
 #ifdef CONFIG_HAVE_GUP_FAST
 /*
  * Used in the GUP-fast path to determine whether GUP is permitted to work on
  * a specific folio.
  *
  * This call assumes the caller has pinned the folio, that the lowest page table
  * level still points to this folio, and that interrupts have been disabled.
  *
  * GUP-fast must reject all secretmem folios.
  *
  * Writing to pinned file-backed dirty tracked folios is inherently problematic
  * (see comment describing the writable_file_mapping_allowed() function). We
  * therefore try to avoid the most egregious case of a long-term mapping doing
  * so.
  *
  * This function cannot be as thorough as that one as the VMA is not available
  * in the fast path, so instead we whitelist known good cases and if in doubt,
  * fall back to the slow path.
  */
 static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
 {
         bool reject_file_backed = false;
         struct address_space *mapping;
         bool check_secretmem = false;
         unsigned long mapping_flags;
 
         /*
          * If we aren't pinning then no problematic write can occur. A long term
          * pin is the most egregious case so this is the one we disallow.
          */
         if ((flags & (FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE)) ==
             (FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE))
                 reject_file_backed = true;
 
         /* We hold a folio reference, so we can safely access folio fields. */
 
         /* secretmem folios are always order-0 folios. */
         if (IS_ENABLED(CONFIG_SECRETMEM) && !folio_test_large(folio))
                 check_secretmem = true;
 
         if (!reject_file_backed && !check_secretmem)
                 return true;
 
         if (WARN_ON_ONCE(folio_test_slab(folio)))
                 return false;
 
         /* hugetlb neither requires dirty-tracking nor can be secretmem. */
         if (folio_test_hugetlb(folio))
                 return true;
 
         /*
          * GUP-fast disables IRQs. When IRQS are disabled, RCU grace periods
          * cannot proceed, which means no actions performed under RCU can
          * proceed either.
          *
          * inodes and thus their mappings are freed under RCU, which means the
          * mapping cannot be freed beneath us and thus we can safely dereference
          * it.
          */
         lockdep_assert_irqs_disabled();
 
         /*
          * However, there may be operations which _alter_ the mapping, so ensure
          * we read it once and only once.
          */
         mapping = READ_ONCE(folio->mapping);
 
         /*
          * The mapping may have been truncated, in any case we cannot determine
          * if this mapping is safe - fall back to slow path to determine how to
          * proceed.
          */
         if (!mapping)
                 return false;
 
         /* Anonymous folios pose no problem. */
         mapping_flags = (unsigned long)mapping & PAGE_MAPPING_FLAGS;
         if (mapping_flags)
                 return mapping_flags & PAGE_MAPPING_ANON;
 
         /*
          * At this point, we know the mapping is non-null and points to an
          * address_space object.
          */
         if (check_secretmem && secretmem_mapping(mapping))
                 return false;
         /* The only remaining allowed file system is shmem. */
         return !reject_file_backed || shmem_mapping(mapping);
 }
 
 static void __maybe_unused gup_fast_undo_dev_pagemap(int *nr, int nr_start,
                 unsigned int flags, struct page **pages)
 {
         while ((*nr) - nr_start) {
                 struct folio *folio = page_folio(pages[--(*nr)]);
 
                 folio_clear_referenced(folio);
                 gup_put_folio(folio, 1, flags);
         }
 }
 
 #ifdef CONFIG_ARCH_HAS_PTE_SPECIAL
 /*
  * GUP-fast relies on pte change detection to avoid concurrent pgtable
  * operations.
  *
  * To pin the page, GUP-fast needs to do below in order:
  * (1) pin the page (by prefetching pte), then (2) check pte not changed.
  *
  * For the rest of pgtable operations where pgtable updates can be racy
  * with GUP-fast, we need to do (1) clear pte, then (2) check whether page
  * is pinned.
  *
  * Above will work for all pte-level operations, including THP split.
  *
  * For THP collapse, it's a bit more complicated because GUP-fast may be
  * walking a pgtable page that is being freed (pte is still valid but pmd
  * can be cleared already).  To avoid race in such condition, we need to
  * also check pmd here to make sure pmd doesn't change (corresponds to
  * pmdp_collapse_flush() in the THP collapse code path).
  */
 static int gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr,
                 unsigned long end, unsigned int flags, struct page **pages,
                 int *nr)
 {
         struct dev_pagemap *pgmap = NULL;
         int nr_start = *nr, ret = 0;
         pte_t *ptep, *ptem;
 
         ptem = ptep = pte_offset_map(&pmd, addr);
         if (!ptep)
                 return 0;
         do {
                 pte_t pte = ptep_get_lockless(ptep);
                 struct page *page;
                 struct folio *folio;
 
                 /*
                  * Always fallback to ordinary GUP on PROT_NONE-mapped pages:
                  * pte_access_permitted() better should reject these pages
                  * either way: otherwise, GUP-fast might succeed in
                  * cases where ordinary GUP would fail due to VMA access
                  * permissions.
                  */
                 if (pte_protnone(pte))
                         goto pte_unmap;
 
                 if (!pte_access_permitted(pte, flags & FOLL_WRITE))
                         goto pte_unmap;
 
                 if (pte_devmap(pte)) {
                         if (unlikely(flags & FOLL_LONGTERM))
                                 goto pte_unmap;
 
                         pgmap = get_dev_pagemap(pte_pfn(pte), pgmap);
                         if (unlikely(!pgmap)) {
                                 gup_fast_undo_dev_pagemap(nr, nr_start, flags, pages);
                                 goto pte_unmap;
                         }
                 } else if (pte_special(pte))
                         goto pte_unmap;
 
                 VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
                 page = pte_page(pte);
 
                 folio = try_grab_folio_fast(page, 1, flags);
                 if (!folio)
                         goto pte_unmap;
 
                 if (unlikely(pmd_val(pmd) != pmd_val(*pmdp)) ||
                     unlikely(pte_val(pte) != pte_val(ptep_get(ptep)))) {
                         gup_put_folio(folio, 1, flags);
                         goto pte_unmap;
                 }
 
                 if (!gup_fast_folio_allowed(folio, flags)) {
                         gup_put_folio(folio, 1, flags);
                         goto pte_unmap;
                 }
 
                 if (!pte_write(pte) && gup_must_unshare(NULL, flags, page)) {
                         gup_put_folio(folio, 1, flags);
                         goto pte_unmap;
                 }
 
                 /*
                  * We need to make the page accessible if and only if we are
                  * going to access its content (the FOLL_PIN case).  Please
                  * see Documentation/core-api/pin_user_pages.rst for
                  * details.
                  */
                 if (flags & FOLL_PIN) {
                         ret = arch_make_page_accessible(page);
                         if (ret) {
                                 gup_put_folio(folio, 1, flags);
                                 goto pte_unmap;
                         }
                 }
                 folio_set_referenced(folio);
                 pages[*nr] = page;
                 (*nr)++;
         } while (ptep++, addr += PAGE_SIZE, addr != end);
 
         ret = 1;
 
 pte_unmap:
         if (pgmap)
                 put_dev_pagemap(pgmap);
         pte_unmap(ptem);
         return ret;
 }
 #else
 
 /*
  * If we can't determine whether or not a pte is special, then fail immediately
  * for ptes. Note, we can still pin HugeTLB and THP as these are guaranteed not
  * to be special.
  *
  * For a futex to be placed on a THP tail page, get_futex_key requires a
  * get_user_pages_fast_only implementation that can pin pages. Thus it's still
  * useful to have gup_fast_pmd_leaf even if we can't operate on ptes.
  */
 static int gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr,
                 unsigned long end, unsigned int flags, struct page **pages,
                 int *nr)
 {
         return 0;
 }
 #endif /* CONFIG_ARCH_HAS_PTE_SPECIAL */
 
 #if defined(CONFIG_ARCH_HAS_PTE_DEVMAP) && defined(CONFIG_TRANSPARENT_HUGEPAGE)
 static int gup_fast_devmap_leaf(unsigned long pfn, unsigned long addr,
         unsigned long end, unsigned int flags, struct page **pages, int *nr)
 {
         int nr_start = *nr;
         struct dev_pagemap *pgmap = NULL;
 
         do {
                 struct folio *folio;
                 struct page *page = pfn_to_page(pfn);
 
                 pgmap = get_dev_pagemap(pfn, pgmap);
                 if (unlikely(!pgmap)) {
                         gup_fast_undo_dev_pagemap(nr, nr_start, flags, pages);
                         break;
                 }
 
                 if (!(flags & FOLL_PCI_P2PDMA) && is_pci_p2pdma_page(page)) {
                         gup_fast_undo_dev_pagemap(nr, nr_start, flags, pages);
                         break;
                 }
 
                 folio = try_grab_folio_fast(page, 1, flags);
                 if (!folio) {
                         gup_fast_undo_dev_pagemap(nr, nr_start, flags, pages);
                         break;
                 }
                 folio_set_referenced(folio);
                 pages[*nr] = page;
                 (*nr)++;
                 pfn++;
         } while (addr += PAGE_SIZE, addr != end);
 
         put_dev_pagemap(pgmap);
         return addr == end;
 }
 
 static int gup_fast_devmap_pmd_leaf(pmd_t orig, pmd_t *pmdp, unsigned long addr,
                 unsigned long end, unsigned int flags, struct page **pages,
                 int *nr)
 {
         unsigned long fault_pfn;
         int nr_start = *nr;
 
         fault_pfn = pmd_pfn(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
         if (!gup_fast_devmap_leaf(fault_pfn, addr, end, flags, pages, nr))
                 return 0;
 
         if (unlikely(pmd_val(orig) != pmd_val(*pmdp))) {
                 gup_fast_undo_dev_pagemap(nr, nr_start, flags, pages);
                 return 0;
         }
         return 1;
 }
 
 static int gup_fast_devmap_pud_leaf(pud_t orig, pud_t *pudp, unsigned long addr,
                 unsigned long end, unsigned int flags, struct page **pages,
                 int *nr)
 {
         unsigned long fault_pfn;
         int nr_start = *nr;
 
         fault_pfn = pud_pfn(orig) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
         if (!gup_fast_devmap_leaf(fault_pfn, addr, end, flags, pages, nr))
                 return 0;
 
         if (unlikely(pud_val(orig) != pud_val(*pudp))) {
                 gup_fast_undo_dev_pagemap(nr, nr_start, flags, pages);
                 return 0;
         }
         return 1;
 }
 #else
 static int gup_fast_devmap_pmd_leaf(pmd_t orig, pmd_t *pmdp, unsigned long addr,
                 unsigned long end, unsigned int flags, struct page **pages,
                 int *nr)
 {
         BUILD_BUG();
         return 0;
 }
 
 static int gup_fast_devmap_pud_leaf(pud_t pud, pud_t *pudp, unsigned long addr,
                 unsigned long end, unsigned int flags, struct page **pages,
                 int *nr)
 {
         BUILD_BUG();
         return 0;
 }
 #endif
 
 static int gup_fast_pmd_leaf(pmd_t orig, pmd_t *pmdp, unsigned long addr,
                 unsigned long end, unsigned int flags, struct page **pages,
                 int *nr)
 {
         struct page *page;
         struct folio *folio;
         int refs;
 
         if (!pmd_access_permitted(orig, flags & FOLL_WRITE))
                 return 0;
 
         if (pmd_devmap(orig)) {
                 if (unlikely(flags & FOLL_LONGTERM))
                         return 0;
                 return gup_fast_devmap_pmd_leaf(orig, pmdp, addr, end, flags,
                                                 pages, nr);
         }
 
         page = pmd_page(orig);
         refs = record_subpages(page, PMD_SIZE, addr, end, pages + *nr);
 
         folio = try_grab_folio_fast(page, refs, flags);
         if (!folio)
                 return 0;
 
         if (unlikely(pmd_val(orig) != pmd_val(*pmdp))) {
                 gup_put_folio(folio, refs, flags);
                 return 0;
         }
 
         if (!gup_fast_folio_allowed(folio, flags)) {
                 gup_put_folio(folio, refs, flags);
                 return 0;
         }
         if (!pmd_write(orig) && gup_must_unshare(NULL, flags, &folio->page)) {
                 gup_put_folio(folio, refs, flags);
                 return 0;
         }
 
         *nr += refs;
         folio_set_referenced(folio);
         return 1;
 }
 
 static int gup_fast_pud_leaf(pud_t orig, pud_t *pudp, unsigned long addr,
                 unsigned long end, unsigned int flags, struct page **pages,
                 int *nr)
 {
         struct page *page;
         struct folio *folio;
         int refs;
 
         if (!pud_access_permitted(orig, flags & FOLL_WRITE))
                 return 0;
 
         if (pud_devmap(orig)) {
                 if (unlikely(flags & FOLL_LONGTERM))
                         return 0;
                 return gup_fast_devmap_pud_leaf(orig, pudp, addr, end, flags,
                                                 pages, nr);
         }
 
         page = pud_page(orig);
         refs = record_subpages(page, PUD_SIZE, addr, end, pages + *nr);
 
         folio = try_grab_folio_fast(page, refs, flags);
         if (!folio)
                 return 0;
 
         if (unlikely(pud_val(orig) != pud_val(*pudp))) {
                 gup_put_folio(folio, refs, flags);
                 return 0;
         }
 
         if (!gup_fast_folio_allowed(folio, flags)) {
                 gup_put_folio(folio, refs, flags);
                 return 0;
         }
 
         if (!pud_write(orig) && gup_must_unshare(NULL, flags, &folio->page)) {
                 gup_put_folio(folio, refs, flags);
                 return 0;
         }
 
         *nr += refs;
         folio_set_referenced(folio);
         return 1;
 }
 
 static int gup_fast_pgd_leaf(pgd_t orig, pgd_t *pgdp, unsigned long addr,
                 unsigned long end, unsigned int flags, struct page **pages,
                 int *nr)
 {
         int refs;
         struct page *page;
         struct folio *folio;
 
         if (!pgd_access_permitted(orig, flags & FOLL_WRITE))
                 return 0;
 
         BUILD_BUG_ON(pgd_devmap(orig));
 
         page = pgd_page(orig);
         refs = record_subpages(page, PGDIR_SIZE, addr, end, pages + *nr);
 
         folio = try_grab_folio_fast(page, refs, flags);
         if (!folio)
                 return 0;
 
         if (unlikely(pgd_val(orig) != pgd_val(*pgdp))) {
                 gup_put_folio(folio, refs, flags);
                 return 0;
         }
 
         if (!pgd_write(orig) && gup_must_unshare(NULL, flags, &folio->page)) {
                 gup_put_folio(folio, refs, flags);
                 return 0;
         }
 
         if (!gup_fast_folio_allowed(folio, flags)) {
                 gup_put_folio(folio, refs, flags);
                 return 0;
         }
 
         *nr += refs;
         folio_set_referenced(folio);
         return 1;
 }
 
 static int gup_fast_pmd_range(pud_t *pudp, pud_t pud, unsigned long addr,
                 unsigned long end, unsigned int flags, struct page **pages,
                 int *nr)
 {
         unsigned long next;
         pmd_t *pmdp;
 
         pmdp = pmd_offset_lockless(pudp, pud, addr);
         do {
                 pmd_t pmd = pmdp_get_lockless(pmdp);
 
                 next = pmd_addr_end(addr, end);
                 if (!pmd_present(pmd))
                         return 0;
 
                 if (unlikely(pmd_leaf(pmd))) {
                         /* See gup_fast_pte_range() */
                         if (pmd_protnone(pmd))
                                 return 0;
 
                         if (!gup_fast_pmd_leaf(pmd, pmdp, addr, next, flags,
                                 pages, nr))
                                 return 0;
 
                 } else if (!gup_fast_pte_range(pmd, pmdp, addr, next, flags,
                                                pages, nr))
                         return 0;
         } while (pmdp++, addr = next, addr != end);
 
         return 1;
 }
 
 static int gup_fast_pud_range(p4d_t *p4dp, p4d_t p4d, unsigned long addr,
                 unsigned long end, unsigned int flags, struct page **pages,
                 int *nr)
 {
         unsigned long next;
         pud_t *pudp;
 
         pudp = pud_offset_lockless(p4dp, p4d, addr);
         do {
                 pud_t pud = READ_ONCE(*pudp);
 
                 next = pud_addr_end(addr, end);
                 if (unlikely(!pud_present(pud)))
                         return 0;
                 if (unlikely(pud_leaf(pud))) {
                         if (!gup_fast_pud_leaf(pud, pudp, addr, next, flags,
                                                pages, nr))
                                 return 0;
                 } else if (!gup_fast_pmd_range(pudp, pud, addr, next, flags,
                                                pages, nr))
                         return 0;
         } while (pudp++, addr = next, addr != end);
 
         return 1;
 }
 
 static int gup_fast_p4d_range(pgd_t *pgdp, pgd_t pgd, unsigned long addr,
                 unsigned long end, unsigned int flags, struct page **pages,
                 int *nr)
 {
         unsigned long next;
         p4d_t *p4dp;
 
         p4dp = p4d_offset_lockless(pgdp, pgd, addr);
         do {
                 p4d_t p4d = READ_ONCE(*p4dp);
 
                 next = p4d_addr_end(addr, end);
                 if (!p4d_present(p4d))
                         return 0;
                 BUILD_BUG_ON(p4d_leaf(p4d));
                 if (!gup_fast_pud_range(p4dp, p4d, addr, next, flags,
                                         pages, nr))
                         return 0;
         } while (p4dp++, addr = next, addr != end);
 
         return 1;
 }
 
 static void gup_fast_pgd_range(unsigned long addr, unsigned long end,
                 unsigned int flags, struct page **pages, int *nr)
 {
         unsigned long next;
         pgd_t *pgdp;
 
         pgdp = pgd_offset(current->mm, addr);
         do {
                 pgd_t pgd = READ_ONCE(*pgdp);
 
                 next = pgd_addr_end(addr, end);
                 if (pgd_none(pgd))
                         return;
                 if (unlikely(pgd_leaf(pgd))) {
                         if (!gup_fast_pgd_leaf(pgd, pgdp, addr, next, flags,
                                                pages, nr))
                                 return;
                 } else if (!gup_fast_p4d_range(pgdp, pgd, addr, next, flags,
                                                pages, nr))
                         return;
         } while (pgdp++, addr = next, addr != end);
 }
 #else
 static inline void gup_fast_pgd_range(unsigned long addr, unsigned long end,
                 unsigned int flags, struct page **pages, int *nr)
 {
 }
 #endif /* CONFIG_HAVE_GUP_FAST */
 
 #ifndef gup_fast_permitted
 /*
  * Check if it's allowed to use get_user_pages_fast_only() for the range, or
  * we need to fall back to the slow version:
  */
 static bool gup_fast_permitted(unsigned long start, unsigned long end)
 {
         return true;
 }
 #endif
 
 static unsigned long gup_fast(unsigned long start, unsigned long end,
                 unsigned int gup_flags, struct page **pages)
 {
         unsigned long flags;
         int nr_pinned = 0;
         unsigned seq;
 
         if (!IS_ENABLED(CONFIG_HAVE_GUP_FAST) ||
             !gup_fast_permitted(start, end))
                 return 0;
 
         if (gup_flags & FOLL_PIN) {
                 seq = raw_read_seqcount(&current->mm->write_protect_seq);
                 if (seq & 1)
                         return 0;
         }
 
         /*
          * Disable interrupts. The nested form is used, in order to allow full,
          * general purpose use of this routine.
          *
          * With interrupts disabled, we block page table pages from being freed
          * from under us. See struct mmu_table_batch comments in
          * include/asm-generic/tlb.h for more details.
          *
          * We do not adopt an rcu_read_lock() here as we also want to block IPIs
          * that come from THPs splitting.
          */
         local_irq_save(flags);
         gup_fast_pgd_range(start, end, gup_flags, pages, &nr_pinned);
         local_irq_restore(flags);
 
         /*
          * When pinning pages for DMA there could be a concurrent write protect
          * from fork() via copy_page_range(), in this case always fail GUP-fast.
          */
         if (gup_flags & FOLL_PIN) {
                 if (read_seqcount_retry(&current->mm->write_protect_seq, seq)) {
                         gup_fast_unpin_user_pages(pages, nr_pinned);
                         return 0;
                 } else {
                         sanity_check_pinned_pages(pages, nr_pinned);
                 }
         }
         return nr_pinned;
 }
 
 static int gup_fast_fallback(unsigned long start, unsigned long nr_pages,
                 unsigned int gup_flags, struct page **pages)
 {
         unsigned long len, end;
         unsigned long nr_pinned;
         int locked = 0;
         int ret;
 
         if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM |
                                        FOLL_FORCE | FOLL_PIN | FOLL_GET |
                                        FOLL_FAST_ONLY | FOLL_NOFAULT |
                                        FOLL_PCI_P2PDMA | FOLL_HONOR_NUMA_FAULT)))
                 return -EINVAL;
 
         if (gup_flags & FOLL_PIN)
                 mm_set_has_pinned_flag(&current->mm->flags);
 
         if (!(gup_flags & FOLL_FAST_ONLY))
                 might_lock_read(&current->mm->mmap_lock);
 
         start = untagged_addr(start) & PAGE_MASK;
         len = nr_pages << PAGE_SHIFT;
         if (check_add_overflow(start, len, &end))
                 return -EOVERFLOW;
         if (end > TASK_SIZE_MAX)
                 return -EFAULT;
         if (unlikely(!access_ok((void __user *)start, len)))
                 return -EFAULT;
 
         nr_pinned = gup_fast(start, end, gup_flags, pages);
         if (nr_pinned == nr_pages || gup_flags & FOLL_FAST_ONLY)
                 return nr_pinned;
 
         /* Slow path: try to get the remaining pages with get_user_pages */
         start += nr_pinned << PAGE_SHIFT;
         pages += nr_pinned;
         ret = __gup_longterm_locked(current->mm, start, nr_pages - nr_pinned,
                                     pages, &locked,
                                     gup_flags | FOLL_TOUCH | FOLL_UNLOCKABLE);
         if (ret < 0) {
                 /*
                  * The caller has to unpin the pages we already pinned so
                  * returning -errno is not an option
                  */
                 if (nr_pinned)
                         return nr_pinned;
                 return ret;
         }
         return ret + nr_pinned;
 }
 
 /**
  * get_user_pages_fast_only() - pin user pages in memory
  * @start:      starting user address
  * @nr_pages:   number of pages from start to pin
  * @gup_flags:  flags modifying pin behaviour
  * @pages:      array that receives pointers to the pages pinned.
  *              Should be at least nr_pages long.
  *
  * Like get_user_pages_fast() except it's IRQ-safe in that it won't fall back to
  * the regular GUP.
  *
  * If the architecture does not support this function, simply return with no
  * pages pinned.
  *
  * Careful, careful! COW breaking can go either way, so a non-write
  * access can get ambiguous page results. If you call this function without
  * 'write' set, you'd better be sure that you're ok with that ambiguity.
  */
 int get_user_pages_fast_only(unsigned long start, int nr_pages,
                              unsigned int gup_flags, struct page **pages)
 {
         /*
          * Internally (within mm/gup.c), gup fast variants must set FOLL_GET,
          * because gup fast is always a "pin with a +1 page refcount" request.
          *
          * FOLL_FAST_ONLY is required in order to match the API description of
          * this routine: no fall back to regular ("slow") GUP.
          */
         if (!is_valid_gup_args(pages, NULL, &gup_flags,
                                FOLL_GET | FOLL_FAST_ONLY))
                 return -EINVAL;
 
         return gup_fast_fallback(start, nr_pages, gup_flags, pages);
 }
 EXPORT_SYMBOL_GPL(get_user_pages_fast_only);
 
 /**
  * get_user_pages_fast() - pin user pages in memory
  * @start:      starting user address
  * @nr_pages:   number of pages from start to pin
  * @gup_flags:  flags modifying pin behaviour
  * @pages:      array that receives pointers to the pages pinned.
  *              Should be at least nr_pages long.
  *
  * Attempt to pin user pages in memory without taking mm->mmap_lock.
  * If not successful, it will fall back to taking the lock and
  * calling get_user_pages().
  *
  * Returns number of pages pinned. This may be fewer than the number requested.
  * If nr_pages is 0 or negative, returns 0. If no pages were pinned, returns
  * -errno.
  */
 int get_user_pages_fast(unsigned long start, int nr_pages,
                         unsigned int gup_flags, struct page **pages)
 {
         /*
          * The caller may or may not have explicitly set FOLL_GET; either way is
          * OK. However, internally (within mm/gup.c), gup fast variants must set
          * FOLL_GET, because gup fast is always a "pin with a +1 page refcount"
          * request.
          */
         if (!is_valid_gup_args(pages, NULL, &gup_flags, FOLL_GET))
                 return -EINVAL;
         return gup_fast_fallback(start, nr_pages, gup_flags, pages);
 }
 EXPORT_SYMBOL_GPL(get_user_pages_fast);
 
 /**
  * pin_user_pages_fast() - pin user pages in memory without taking locks
  *
  * @start:      starting user address
  * @nr_pages:   number of pages from start to pin
  * @gup_flags:  flags modifying pin behaviour
  * @pages:      array that receives pointers to the pages pinned.
  *              Should be at least nr_pages long.
  *
  * Nearly the same as get_user_pages_fast(), except that FOLL_PIN is set. See
  * get_user_pages_fast() for documentation on the function arguments, because
  * the arguments here are identical.
  *
  * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
  * see Documentation/core-api/pin_user_pages.rst for further details.
  *
  * Note that if a zero_page is amongst the returned pages, it will not have
  * pins in it and unpin_user_page() will not remove pins from it.
  */
 int pin_user_pages_fast(unsigned long start, int nr_pages,
                         unsigned int gup_flags, struct page **pages)
 {
         if (!is_valid_gup_args(pages, NULL, &gup_flags, FOLL_PIN))
                 return -EINVAL;
         return gup_fast_fallback(start, nr_pages, gup_flags, pages);
 }
 EXPORT_SYMBOL_GPL(pin_user_pages_fast);
 
 /**
  * pin_user_pages_remote() - pin pages of a remote process
  *
  * @mm:         mm_struct of target mm
  * @start:      starting user address
  * @nr_pages:   number of pages from start to pin
  * @gup_flags:  flags modifying lookup behaviour
  * @pages:      array that receives pointers to the pages pinned.
  *              Should be at least nr_pages long.
  * @locked:     pointer to lock flag indicating whether lock is held and
  *              subsequently whether VM_FAULT_RETRY functionality can be
  *              utilised. Lock must initially be held.
  *
  * Nearly the same as get_user_pages_remote(), except that FOLL_PIN is set. See
  * get_user_pages_remote() for documentation on the function arguments, because
  * the arguments here are identical.
  *
  * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
  * see Documentation/core-api/pin_user_pages.rst for details.
  *
  * Note that if a zero_page is amongst the returned pages, it will not have
  * pins in it and unpin_user_page*() will not remove pins from it.
  */
 long pin_user_pages_remote(struct mm_struct *mm,
                            unsigned long start, unsigned long nr_pages,
                            unsigned int gup_flags, struct page **pages,
                            int *locked)
 {
         int local_locked = 1;
 
         if (!is_valid_gup_args(pages, locked, &gup_flags,
                                FOLL_PIN | FOLL_TOUCH | FOLL_REMOTE))
                 return 0;
         return __gup_longterm_locked(mm, start, nr_pages, pages,
                                      locked ? locked : &local_locked,
                                      gup_flags);
 }
 EXPORT_SYMBOL(pin_user_pages_remote);
 
 /**
  * pin_user_pages() - pin user pages in memory for use by other devices
  *
  * @start:      starting user address
  * @nr_pages:   number of pages from start to pin
  * @gup_flags:  flags modifying lookup behaviour
  * @pages:      array that receives pointers to the pages pinned.
  *              Should be at least nr_pages long.
  *
  * Nearly the same as get_user_pages(), except that FOLL_TOUCH is not set, and
  * FOLL_PIN is set.
  *
  * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
  * see Documentation/core-api/pin_user_pages.rst for details.
  *
  * Note that if a zero_page is amongst the returned pages, it will not have
  * pins in it and unpin_user_page*() will not remove pins from it.
  */
 long pin_user_pages(unsigned long start, unsigned long nr_pages,
                     unsigned int gup_flags, struct page **pages)
 {
         int locked = 1;
 
         if (!is_valid_gup_args(pages, NULL, &gup_flags, FOLL_PIN))
                 return 0;
         return __gup_longterm_locked(current->mm, start, nr_pages,
                                      pages, &locked, gup_flags);
 }
 EXPORT_SYMBOL(pin_user_pages);
 
 /*
  * pin_user_pages_unlocked() is the FOLL_PIN variant of
  * get_user_pages_unlocked(). Behavior is the same, except that this one sets
  * FOLL_PIN and rejects FOLL_GET.
  *
  * Note that if a zero_page is amongst the returned pages, it will not have
  * pins in it and unpin_user_page*() will not remove pins from it.
  */
 long pin_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
                              struct page **pages, unsigned int gup_flags)
 {
         int locked = 0;
 
         if (!is_valid_gup_args(pages, NULL, &gup_flags,
                                FOLL_PIN | FOLL_TOUCH | FOLL_UNLOCKABLE))
                 return 0;
 
         return __gup_longterm_locked(current->mm, start, nr_pages, pages,
                                      &locked, gup_flags);
 }
 EXPORT_SYMBOL(pin_user_pages_unlocked);
 
 /**
  * memfd_pin_folios() - pin folios associated with a memfd
  * @memfd:      the memfd whose folios are to be pinned
  * @start:      the first memfd offset
  * @end:        the last memfd offset (inclusive)
  * @folios:     array that receives pointers to the folios pinned
  * @max_folios: maximum number of entries in @folios
  * @offset:     the offset into the first folio
  *
  * Attempt to pin folios associated with a memfd in the contiguous range
  * [start, end]. Given that a memfd is either backed by shmem or hugetlb,
  * the folios can either be found in the page cache or need to be allocated
  * if necessary. Once the folios are located, they are all pinned via
  * FOLL_PIN and @offset is populatedwith the offset into the first folio.
  * And, eventually, these pinned folios must be released either using
  * unpin_folios() or unpin_folio().
  *
  * It must be noted that the folios may be pinned for an indefinite amount
  * of time. And, in most cases, the duration of time they may stay pinned
  * would be controlled by the userspace. This behavior is effectively the
  * same as using FOLL_LONGTERM with other GUP APIs.
  *
  * Returns number of folios pinned, which could be less than @max_folios
  * as it depends on the folio sizes that cover the range [start, end].
  * If no folios were pinned, it returns -errno.
  */
 long memfd_pin_folios(struct file *memfd, loff_t start, loff_t end,
                       struct folio **folios, unsigned int max_folios,
                       pgoff_t *offset)
 {
         unsigned int flags, nr_folios, nr_found;
         unsigned int i, pgshift = PAGE_SHIFT;
         pgoff_t start_idx, end_idx, next_idx;
         struct folio *folio = NULL;
         struct folio_batch fbatch;
         struct hstate *h;
         long ret = -EINVAL;
 
         if (start < 0 || start > end || !max_folios)
                 return -EINVAL;
 
         if (!memfd)
                 return -EINVAL;
 
         if (!shmem_file(memfd) && !is_file_hugepages(memfd))
                 return -EINVAL;
 
         if (end >= i_size_read(file_inode(memfd)))
                 return -EINVAL;
 
         if (is_file_hugepages(memfd)) {
                 h = hstate_file(memfd);
                 pgshift = huge_page_shift(h);
         }
 
         flags = memalloc_pin_save();
         do {
                 nr_folios = 0;
                 start_idx = start >> pgshift;
                 end_idx = end >> pgshift;
                 if (is_file_hugepages(memfd)) {
                         start_idx <<= huge_page_order(h);
                         end_idx <<= huge_page_order(h);
                 }
 
                 folio_batch_init(&fbatch);
                 while (start_idx <= end_idx && nr_folios < max_folios) {
                         /*
                          * In most cases, we should be able to find the folios
                          * in the page cache. If we cannot find them for some
                          * reason, we try to allocate them and add them to the
                          * page cache.
                          */
                         nr_found = filemap_get_folios_contig(memfd->f_mapping,
                                                              &start_idx,
                                                              end_idx,
                                                              &fbatch);
                         if (folio) {
                                 folio_put(folio);
                                 folio = NULL;
                         }
 
                         next_idx = 0;
                         for (i = 0; i < nr_found; i++) {
                                 /*
                                  * As there can be multiple entries for a
                                  * given folio in the batch returned by
                                  * filemap_get_folios_contig(), the below
                                  * check is to ensure that we pin and return a
                                  * unique set of folios between start and end.
                                  */
                                 if (next_idx &&
                                     next_idx != folio_index(fbatch.folios[i]))
                                         continue;
 
                                 folio = page_folio(&fbatch.folios[i]->page);
 
                                 if (try_grab_folio(folio, 1, FOLL_PIN)) {
                                         folio_batch_release(&fbatch);
                                         ret = -EINVAL;
                                         goto err;
                                 }
 
                                 if (nr_folios == 0)
                                         *offset = offset_in_folio(folio, start);
 
                                 folios[nr_folios] = folio;
                                 next_idx = folio_next_index(folio);
                                 if (++nr_folios == max_folios)
                                         break;
                         }
 
                         folio = NULL;
                         folio_batch_release(&fbatch);
                         if (!nr_found) {
                                 folio = memfd_alloc_folio(memfd, start_idx);
                                 if (IS_ERR(folio)) {
                                         ret = PTR_ERR(folio);
                                         if (ret != -EEXIST)
                                                 goto err;
                                         folio = NULL;
                                 }
                         }
                 }
 
                 ret = check_and_migrate_movable_folios(nr_folios, folios);
         } while (ret == -EAGAIN);
 
         memalloc_pin_restore(flags);
         return ret ? ret : nr_folios;
 err:
         memalloc_pin_restore(flags);
         unpin_folios(folios, nr_folios);
 
         return ret;
 }
 EXPORT_SYMBOL_GPL(memfd_pin_folios);