TOMOYO Linux Cross Reference
Linux/fs/buffer.c

   // SPDX-License-Identifier: GPL-2.0-only
   /*
    *  linux/fs/buffer.c
    *
    *  Copyright (C) 1991, 1992, 2002  Linus Torvalds
    */
   
   /*
    * Start bdflush() with kernel_thread not syscall - Paul Gortmaker, 12/95
   *
   * Removed a lot of unnecessary code and simplified things now that
   * the buffer cache isn't our primary cache - Andrew Tridgell 12/96
   *
   * Speed up hash, lru, and free list operations.  Use gfp() for allocating
   * hash table, use SLAB cache for buffer heads. SMP threading.  -DaveM
   *
   * Added 32k buffer block sizes - these are required older ARM systems. - RMK
   *
   * async buffer flushing, 1999 Andrea Arcangeli <andrea@suse.de>
   */
  
  #include <linux/kernel.h>
  #include <linux/sched/signal.h>
  #include <linux/syscalls.h>
  #include <linux/fs.h>
  #include <linux/iomap.h>
  #include <linux/mm.h>
  #include <linux/percpu.h>
  #include <linux/slab.h>
  #include <linux/capability.h>
  #include <linux/blkdev.h>
  #include <linux/file.h>
  #include <linux/quotaops.h>
  #include <linux/highmem.h>
  #include <linux/export.h>
  #include <linux/backing-dev.h>
  #include <linux/writeback.h>
  #include <linux/hash.h>
  #include <linux/suspend.h>
  #include <linux/buffer_head.h>
  #include <linux/task_io_accounting_ops.h>
  #include <linux/bio.h>
  #include <linux/cpu.h>
  #include <linux/bitops.h>
  #include <linux/mpage.h>
  #include <linux/bit_spinlock.h>
  #include <linux/pagevec.h>
  #include <linux/sched/mm.h>
  #include <trace/events/block.h>
  #include <linux/fscrypt.h>
  #include <linux/fsverity.h>
  #include <linux/sched/isolation.h>
  
  #include "internal.h"
  
  static int fsync_buffers_list(spinlock_t *lock, struct list_head *list);
  static void submit_bh_wbc(blk_opf_t opf, struct buffer_head *bh,
                            enum rw_hint hint, struct writeback_control *wbc);
  
  #define BH_ENTRY(list) list_entry((list), struct buffer_head, b_assoc_buffers)
  
  inline void touch_buffer(struct buffer_head *bh)
  {
          trace_block_touch_buffer(bh);
          folio_mark_accessed(bh->b_folio);
  }
  EXPORT_SYMBOL(touch_buffer);
  
  void __lock_buffer(struct buffer_head *bh)
  {
          wait_on_bit_lock_io(&bh->b_state, BH_Lock, TASK_UNINTERRUPTIBLE);
  }
  EXPORT_SYMBOL(__lock_buffer);
  
  void unlock_buffer(struct buffer_head *bh)
  {
          clear_bit_unlock(BH_Lock, &bh->b_state);
          smp_mb__after_atomic();
          wake_up_bit(&bh->b_state, BH_Lock);
  }
  EXPORT_SYMBOL(unlock_buffer);
  
  /*
   * Returns if the folio has dirty or writeback buffers. If all the buffers
   * are unlocked and clean then the folio_test_dirty information is stale. If
   * any of the buffers are locked, it is assumed they are locked for IO.
   */
  void buffer_check_dirty_writeback(struct folio *folio,
                                       bool *dirty, bool *writeback)
  {
          struct buffer_head *head, *bh;
          *dirty = false;
          *writeback = false;
  
          BUG_ON(!folio_test_locked(folio));
  
          head = folio_buffers(folio);
          if (!head)
                  return;
 
         if (folio_test_writeback(folio))
                 *writeback = true;
 
         bh = head;
         do {
                 if (buffer_locked(bh))
                         *writeback = true;
 
                 if (buffer_dirty(bh))
                         *dirty = true;
 
                 bh = bh->b_this_page;
         } while (bh != head);
 }
 
 /*
  * Block until a buffer comes unlocked.  This doesn't stop it
  * from becoming locked again - you have to lock it yourself
  * if you want to preserve its state.
  */
 void __wait_on_buffer(struct buffer_head * bh)
 {
         wait_on_bit_io(&bh->b_state, BH_Lock, TASK_UNINTERRUPTIBLE);
 }
 EXPORT_SYMBOL(__wait_on_buffer);
 
 static void buffer_io_error(struct buffer_head *bh, char *msg)
 {
         if (!test_bit(BH_Quiet, &bh->b_state))
                 printk_ratelimited(KERN_ERR
                         "Buffer I/O error on dev %pg, logical block %llu%s\n",
                         bh->b_bdev, (unsigned long long)bh->b_blocknr, msg);
 }
 
 /*
  * End-of-IO handler helper function which does not touch the bh after
  * unlocking it.
  * Note: unlock_buffer() sort-of does touch the bh after unlocking it, but
  * a race there is benign: unlock_buffer() only use the bh's address for
  * hashing after unlocking the buffer, so it doesn't actually touch the bh
  * itself.
  */
 static void __end_buffer_read_notouch(struct buffer_head *bh, int uptodate)
 {
         if (uptodate) {
                 set_buffer_uptodate(bh);
         } else {
                 /* This happens, due to failed read-ahead attempts. */
                 clear_buffer_uptodate(bh);
         }
         unlock_buffer(bh);
 }
 
 /*
  * Default synchronous end-of-IO handler..  Just mark it up-to-date and
  * unlock the buffer.
  */
 void end_buffer_read_sync(struct buffer_head *bh, int uptodate)
 {
         __end_buffer_read_notouch(bh, uptodate);
         put_bh(bh);
 }
 EXPORT_SYMBOL(end_buffer_read_sync);
 
 void end_buffer_write_sync(struct buffer_head *bh, int uptodate)
 {
         if (uptodate) {
                 set_buffer_uptodate(bh);
         } else {
                 buffer_io_error(bh, ", lost sync page write");
                 mark_buffer_write_io_error(bh);
                 clear_buffer_uptodate(bh);
         }
         unlock_buffer(bh);
         put_bh(bh);
 }
 EXPORT_SYMBOL(end_buffer_write_sync);
 
 /*
  * Various filesystems appear to want __find_get_block to be non-blocking.
  * But it's the page lock which protects the buffers.  To get around this,
  * we get exclusion from try_to_free_buffers with the blockdev mapping's
  * i_private_lock.
  *
  * Hack idea: for the blockdev mapping, i_private_lock contention
  * may be quite high.  This code could TryLock the page, and if that
  * succeeds, there is no need to take i_private_lock.
  */
 static struct buffer_head *
 __find_get_block_slow(struct block_device *bdev, sector_t block)
 {
         struct address_space *bd_mapping = bdev->bd_mapping;
         const int blkbits = bd_mapping->host->i_blkbits;
         struct buffer_head *ret = NULL;
         pgoff_t index;
         struct buffer_head *bh;
         struct buffer_head *head;
         struct folio *folio;
         int all_mapped = 1;
         static DEFINE_RATELIMIT_STATE(last_warned, HZ, 1);
 
         index = ((loff_t)block << blkbits) / PAGE_SIZE;
         folio = __filemap_get_folio(bd_mapping, index, FGP_ACCESSED, 0);
         if (IS_ERR(folio))
                 goto out;
 
         spin_lock(&bd_mapping->i_private_lock);
         head = folio_buffers(folio);
         if (!head)
                 goto out_unlock;
         bh = head;
         do {
                 if (!buffer_mapped(bh))
                         all_mapped = 0;
                 else if (bh->b_blocknr == block) {
                         ret = bh;
                         get_bh(bh);
                         goto out_unlock;
                 }
                 bh = bh->b_this_page;
         } while (bh != head);
 
         /* we might be here because some of the buffers on this page are
          * not mapped.  This is due to various races between
          * file io on the block device and getblk.  It gets dealt with
          * elsewhere, don't buffer_error if we had some unmapped buffers
          */
         ratelimit_set_flags(&last_warned, RATELIMIT_MSG_ON_RELEASE);
         if (all_mapped && __ratelimit(&last_warned)) {
                 printk("__find_get_block_slow() failed. block=%llu, "
                        "b_blocknr=%llu, b_state=0x%08lx, b_size=%zu, "
                        "device %pg blocksize: %d\n",
                        (unsigned long long)block,
                        (unsigned long long)bh->b_blocknr,
                        bh->b_state, bh->b_size, bdev,
                        1 << blkbits);
         }
 out_unlock:
         spin_unlock(&bd_mapping->i_private_lock);
         folio_put(folio);
 out:
         return ret;
 }
 
 static void end_buffer_async_read(struct buffer_head *bh, int uptodate)
 {
         unsigned long flags;
         struct buffer_head *first;
         struct buffer_head *tmp;
         struct folio *folio;
         int folio_uptodate = 1;
 
         BUG_ON(!buffer_async_read(bh));
 
         folio = bh->b_folio;
         if (uptodate) {
                 set_buffer_uptodate(bh);
         } else {
                 clear_buffer_uptodate(bh);
                 buffer_io_error(bh, ", async page read");
         }
 
         /*
          * Be _very_ careful from here on. Bad things can happen if
          * two buffer heads end IO at almost the same time and both
          * decide that the page is now completely done.
          */
         first = folio_buffers(folio);
         spin_lock_irqsave(&first->b_uptodate_lock, flags);
         clear_buffer_async_read(bh);
         unlock_buffer(bh);
         tmp = bh;
         do {
                 if (!buffer_uptodate(tmp))
                         folio_uptodate = 0;
                 if (buffer_async_read(tmp)) {
                         BUG_ON(!buffer_locked(tmp));
                         goto still_busy;
                 }
                 tmp = tmp->b_this_page;
         } while (tmp != bh);
         spin_unlock_irqrestore(&first->b_uptodate_lock, flags);
 
         folio_end_read(folio, folio_uptodate);
         return;
 
 still_busy:
         spin_unlock_irqrestore(&first->b_uptodate_lock, flags);
         return;
 }
 
 struct postprocess_bh_ctx {
         struct work_struct work;
         struct buffer_head *bh;
 };
 
 static void verify_bh(struct work_struct *work)
 {
         struct postprocess_bh_ctx *ctx =
                 container_of(work, struct postprocess_bh_ctx, work);
         struct buffer_head *bh = ctx->bh;
         bool valid;
 
         valid = fsverity_verify_blocks(bh->b_folio, bh->b_size, bh_offset(bh));
         end_buffer_async_read(bh, valid);
         kfree(ctx);
 }
 
 static bool need_fsverity(struct buffer_head *bh)
 {
         struct folio *folio = bh->b_folio;
         struct inode *inode = folio->mapping->host;
 
         return fsverity_active(inode) &&
                 /* needed by ext4 */
                 folio->index < DIV_ROUND_UP(inode->i_size, PAGE_SIZE);
 }
 
 static void decrypt_bh(struct work_struct *work)
 {
         struct postprocess_bh_ctx *ctx =
                 container_of(work, struct postprocess_bh_ctx, work);
         struct buffer_head *bh = ctx->bh;
         int err;
 
         err = fscrypt_decrypt_pagecache_blocks(bh->b_folio, bh->b_size,
                                                bh_offset(bh));
         if (err == 0 && need_fsverity(bh)) {
                 /*
                  * We use different work queues for decryption and for verity
                  * because verity may require reading metadata pages that need
                  * decryption, and we shouldn't recurse to the same workqueue.
                  */
                 INIT_WORK(&ctx->work, verify_bh);
                 fsverity_enqueue_verify_work(&ctx->work);
                 return;
         }
         end_buffer_async_read(bh, err == 0);
         kfree(ctx);
 }
 
 /*
  * I/O completion handler for block_read_full_folio() - pages
  * which come unlocked at the end of I/O.
  */
 static void end_buffer_async_read_io(struct buffer_head *bh, int uptodate)
 {
         struct inode *inode = bh->b_folio->mapping->host;
         bool decrypt = fscrypt_inode_uses_fs_layer_crypto(inode);
         bool verify = need_fsverity(bh);
 
         /* Decrypt (with fscrypt) and/or verify (with fsverity) if needed. */
         if (uptodate && (decrypt || verify)) {
                 struct postprocess_bh_ctx *ctx =
                         kmalloc(sizeof(*ctx), GFP_ATOMIC);
 
                 if (ctx) {
                         ctx->bh = bh;
                         if (decrypt) {
                                 INIT_WORK(&ctx->work, decrypt_bh);
                                 fscrypt_enqueue_decrypt_work(&ctx->work);
                         } else {
                                 INIT_WORK(&ctx->work, verify_bh);
                                 fsverity_enqueue_verify_work(&ctx->work);
                         }
                         return;
                 }
                 uptodate = 0;
         }
         end_buffer_async_read(bh, uptodate);
 }
 
 /*
  * Completion handler for block_write_full_folio() - folios which are unlocked
  * during I/O, and which have the writeback flag cleared upon I/O completion.
  */
 static void end_buffer_async_write(struct buffer_head *bh, int uptodate)
 {
         unsigned long flags;
         struct buffer_head *first;
         struct buffer_head *tmp;
         struct folio *folio;
 
         BUG_ON(!buffer_async_write(bh));
 
         folio = bh->b_folio;
         if (uptodate) {
                 set_buffer_uptodate(bh);
         } else {
                 buffer_io_error(bh, ", lost async page write");
                 mark_buffer_write_io_error(bh);
                 clear_buffer_uptodate(bh);
         }
 
         first = folio_buffers(folio);
         spin_lock_irqsave(&first->b_uptodate_lock, flags);
 
         clear_buffer_async_write(bh);
         unlock_buffer(bh);
         tmp = bh->b_this_page;
         while (tmp != bh) {
                 if (buffer_async_write(tmp)) {
                         BUG_ON(!buffer_locked(tmp));
                         goto still_busy;
                 }
                 tmp = tmp->b_this_page;
         }
         spin_unlock_irqrestore(&first->b_uptodate_lock, flags);
         folio_end_writeback(folio);
         return;
 
 still_busy:
         spin_unlock_irqrestore(&first->b_uptodate_lock, flags);
         return;
 }
 
 /*
  * If a page's buffers are under async readin (end_buffer_async_read
  * completion) then there is a possibility that another thread of
  * control could lock one of the buffers after it has completed
  * but while some of the other buffers have not completed.  This
  * locked buffer would confuse end_buffer_async_read() into not unlocking
  * the page.  So the absence of BH_Async_Read tells end_buffer_async_read()
  * that this buffer is not under async I/O.
  *
  * The page comes unlocked when it has no locked buffer_async buffers
  * left.
  *
  * PageLocked prevents anyone starting new async I/O reads any of
  * the buffers.
  *
  * PageWriteback is used to prevent simultaneous writeout of the same
  * page.
  *
  * PageLocked prevents anyone from starting writeback of a page which is
  * under read I/O (PageWriteback is only ever set against a locked page).
  */
 static void mark_buffer_async_read(struct buffer_head *bh)
 {
         bh->b_end_io = end_buffer_async_read_io;
         set_buffer_async_read(bh);
 }
 
 static void mark_buffer_async_write_endio(struct buffer_head *bh,
                                           bh_end_io_t *handler)
 {
         bh->b_end_io = handler;
         set_buffer_async_write(bh);
 }
 
 void mark_buffer_async_write(struct buffer_head *bh)
 {
         mark_buffer_async_write_endio(bh, end_buffer_async_write);
 }
 EXPORT_SYMBOL(mark_buffer_async_write);
 
 
 /*
  * fs/buffer.c contains helper functions for buffer-backed address space's
  * fsync functions.  A common requirement for buffer-based filesystems is
  * that certain data from the backing blockdev needs to be written out for
  * a successful fsync().  For example, ext2 indirect blocks need to be
  * written back and waited upon before fsync() returns.
  *
  * The functions mark_buffer_dirty_inode(), fsync_inode_buffers(),
  * inode_has_buffers() and invalidate_inode_buffers() are provided for the
  * management of a list of dependent buffers at ->i_mapping->i_private_list.
  *
  * Locking is a little subtle: try_to_free_buffers() will remove buffers
  * from their controlling inode's queue when they are being freed.  But
  * try_to_free_buffers() will be operating against the *blockdev* mapping
  * at the time, not against the S_ISREG file which depends on those buffers.
  * So the locking for i_private_list is via the i_private_lock in the address_space
  * which backs the buffers.  Which is different from the address_space 
  * against which the buffers are listed.  So for a particular address_space,
  * mapping->i_private_lock does *not* protect mapping->i_private_list!  In fact,
  * mapping->i_private_list will always be protected by the backing blockdev's
  * ->i_private_lock.
  *
  * Which introduces a requirement: all buffers on an address_space's
  * ->i_private_list must be from the same address_space: the blockdev's.
  *
  * address_spaces which do not place buffers at ->i_private_list via these
  * utility functions are free to use i_private_lock and i_private_list for
  * whatever they want.  The only requirement is that list_empty(i_private_list)
  * be true at clear_inode() time.
  *
  * FIXME: clear_inode should not call invalidate_inode_buffers().  The
  * filesystems should do that.  invalidate_inode_buffers() should just go
  * BUG_ON(!list_empty).
  *
  * FIXME: mark_buffer_dirty_inode() is a data-plane operation.  It should
  * take an address_space, not an inode.  And it should be called
  * mark_buffer_dirty_fsync() to clearly define why those buffers are being
  * queued up.
  *
  * FIXME: mark_buffer_dirty_inode() doesn't need to add the buffer to the
  * list if it is already on a list.  Because if the buffer is on a list,
  * it *must* already be on the right one.  If not, the filesystem is being
  * silly.  This will save a ton of locking.  But first we have to ensure
  * that buffers are taken *off* the old inode's list when they are freed
  * (presumably in truncate).  That requires careful auditing of all
  * filesystems (do it inside bforget()).  It could also be done by bringing
  * b_inode back.
  */
 
 /*
  * The buffer's backing address_space's i_private_lock must be held
  */
 static void __remove_assoc_queue(struct buffer_head *bh)
 {
         list_del_init(&bh->b_assoc_buffers);
         WARN_ON(!bh->b_assoc_map);
         bh->b_assoc_map = NULL;
 }
 
 int inode_has_buffers(struct inode *inode)
 {
         return !list_empty(&inode->i_data.i_private_list);
 }
 
 /*
  * osync is designed to support O_SYNC io.  It waits synchronously for
  * all already-submitted IO to complete, but does not queue any new
  * writes to the disk.
  *
  * To do O_SYNC writes, just queue the buffer writes with write_dirty_buffer
  * as you dirty the buffers, and then use osync_inode_buffers to wait for
  * completion.  Any other dirty buffers which are not yet queued for
  * write will not be flushed to disk by the osync.
  */
 static int osync_buffers_list(spinlock_t *lock, struct list_head *list)
 {
         struct buffer_head *bh;
         struct list_head *p;
         int err = 0;
 
         spin_lock(lock);
 repeat:
         list_for_each_prev(p, list) {
                 bh = BH_ENTRY(p);
                 if (buffer_locked(bh)) {
                         get_bh(bh);
                         spin_unlock(lock);
                         wait_on_buffer(bh);
                         if (!buffer_uptodate(bh))
                                 err = -EIO;
                         brelse(bh);
                         spin_lock(lock);
                         goto repeat;
                 }
         }
         spin_unlock(lock);
         return err;
 }
 
 /**
  * sync_mapping_buffers - write out & wait upon a mapping's "associated" buffers
  * @mapping: the mapping which wants those buffers written
  *
  * Starts I/O against the buffers at mapping->i_private_list, and waits upon
  * that I/O.
  *
  * Basically, this is a convenience function for fsync().
  * @mapping is a file or directory which needs those buffers to be written for
  * a successful fsync().
  */
 int sync_mapping_buffers(struct address_space *mapping)
 {
         struct address_space *buffer_mapping = mapping->i_private_data;
 
         if (buffer_mapping == NULL || list_empty(&mapping->i_private_list))
                 return 0;
 
         return fsync_buffers_list(&buffer_mapping->i_private_lock,
                                         &mapping->i_private_list);
 }
 EXPORT_SYMBOL(sync_mapping_buffers);
 
 /**
  * generic_buffers_fsync_noflush - generic buffer fsync implementation
  * for simple filesystems with no inode lock
  *
  * @file:       file to synchronize
  * @start:      start offset in bytes
  * @end:        end offset in bytes (inclusive)
  * @datasync:   only synchronize essential metadata if true
  *
  * This is a generic implementation of the fsync method for simple
  * filesystems which track all non-inode metadata in the buffers list
  * hanging off the address_space structure.
  */
 int generic_buffers_fsync_noflush(struct file *file, loff_t start, loff_t end,
                                   bool datasync)
 {
         struct inode *inode = file->f_mapping->host;
         int err;
         int ret;
 
         err = file_write_and_wait_range(file, start, end);
         if (err)
                 return err;
 
         ret = sync_mapping_buffers(inode->i_mapping);
         if (!(inode->i_state & I_DIRTY_ALL))
                 goto out;
         if (datasync && !(inode->i_state & I_DIRTY_DATASYNC))
                 goto out;
 
         err = sync_inode_metadata(inode, 1);
         if (ret == 0)
                 ret = err;
 
 out:
         /* check and advance again to catch errors after syncing out buffers */
         err = file_check_and_advance_wb_err(file);
         if (ret == 0)
                 ret = err;
         return ret;
 }
 EXPORT_SYMBOL(generic_buffers_fsync_noflush);
 
 /**
  * generic_buffers_fsync - generic buffer fsync implementation
  * for simple filesystems with no inode lock
  *
  * @file:       file to synchronize
  * @start:      start offset in bytes
  * @end:        end offset in bytes (inclusive)
  * @datasync:   only synchronize essential metadata if true
  *
  * This is a generic implementation of the fsync method for simple
  * filesystems which track all non-inode metadata in the buffers list
  * hanging off the address_space structure. This also makes sure that
  * a device cache flush operation is called at the end.
  */
 int generic_buffers_fsync(struct file *file, loff_t start, loff_t end,
                           bool datasync)
 {
         struct inode *inode = file->f_mapping->host;
         int ret;
 
         ret = generic_buffers_fsync_noflush(file, start, end, datasync);
         if (!ret)
                 ret = blkdev_issue_flush(inode->i_sb->s_bdev);
         return ret;
 }
 EXPORT_SYMBOL(generic_buffers_fsync);
 
 /*
  * Called when we've recently written block `bblock', and it is known that
  * `bblock' was for a buffer_boundary() buffer.  This means that the block at
  * `bblock + 1' is probably a dirty indirect block.  Hunt it down and, if it's
  * dirty, schedule it for IO.  So that indirects merge nicely with their data.
  */
 void write_boundary_block(struct block_device *bdev,
                         sector_t bblock, unsigned blocksize)
 {
         struct buffer_head *bh = __find_get_block(bdev, bblock + 1, blocksize);
         if (bh) {
                 if (buffer_dirty(bh))
                         write_dirty_buffer(bh, 0);
                 put_bh(bh);
         }
 }
 
 void mark_buffer_dirty_inode(struct buffer_head *bh, struct inode *inode)
 {
         struct address_space *mapping = inode->i_mapping;
         struct address_space *buffer_mapping = bh->b_folio->mapping;
 
         mark_buffer_dirty(bh);
         if (!mapping->i_private_data) {
                 mapping->i_private_data = buffer_mapping;
         } else {
                 BUG_ON(mapping->i_private_data != buffer_mapping);
         }
         if (!bh->b_assoc_map) {
                 spin_lock(&buffer_mapping->i_private_lock);
                 list_move_tail(&bh->b_assoc_buffers,
                                 &mapping->i_private_list);
                 bh->b_assoc_map = mapping;
                 spin_unlock(&buffer_mapping->i_private_lock);
         }
 }
 EXPORT_SYMBOL(mark_buffer_dirty_inode);
 
 /**
  * block_dirty_folio - Mark a folio as dirty.
  * @mapping: The address space containing this folio.
  * @folio: The folio to mark dirty.
  *
  * Filesystems which use buffer_heads can use this function as their
  * ->dirty_folio implementation.  Some filesystems need to do a little
  * work before calling this function.  Filesystems which do not use
  * buffer_heads should call filemap_dirty_folio() instead.
  *
  * If the folio has buffers, the uptodate buffers are set dirty, to
  * preserve dirty-state coherency between the folio and the buffers.
  * Buffers added to a dirty folio are created dirty.
  *
  * The buffers are dirtied before the folio is dirtied.  There's a small
  * race window in which writeback may see the folio cleanness but not the
  * buffer dirtiness.  That's fine.  If this code were to set the folio
  * dirty before the buffers, writeback could clear the folio dirty flag,
  * see a bunch of clean buffers and we'd end up with dirty buffers/clean
  * folio on the dirty folio list.
  *
  * We use i_private_lock to lock against try_to_free_buffers() while
  * using the folio's buffer list.  This also prevents clean buffers
  * being added to the folio after it was set dirty.
  *
  * Context: May only be called from process context.  Does not sleep.
  * Caller must ensure that @folio cannot be truncated during this call,
  * typically by holding the folio lock or having a page in the folio
  * mapped and holding the page table lock.
  *
  * Return: True if the folio was dirtied; false if it was already dirtied.
  */
 bool block_dirty_folio(struct address_space *mapping, struct folio *folio)
 {
         struct buffer_head *head;
         bool newly_dirty;
 
         spin_lock(&mapping->i_private_lock);
         head = folio_buffers(folio);
         if (head) {
                 struct buffer_head *bh = head;
 
                 do {
                         set_buffer_dirty(bh);
                         bh = bh->b_this_page;
                 } while (bh != head);
         }
         /*
          * Lock out page's memcg migration to keep PageDirty
          * synchronized with per-memcg dirty page counters.
          */
         folio_memcg_lock(folio);
         newly_dirty = !folio_test_set_dirty(folio);
         spin_unlock(&mapping->i_private_lock);
 
         if (newly_dirty)
                 __folio_mark_dirty(folio, mapping, 1);
 
         folio_memcg_unlock(folio);
 
         if (newly_dirty)
                 __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
 
         return newly_dirty;
 }
 EXPORT_SYMBOL(block_dirty_folio);
 
 /*
  * Write out and wait upon a list of buffers.
  *
  * We have conflicting pressures: we want to make sure that all
  * initially dirty buffers get waited on, but that any subsequently
  * dirtied buffers don't.  After all, we don't want fsync to last
  * forever if somebody is actively writing to the file.
  *
  * Do this in two main stages: first we copy dirty buffers to a
  * temporary inode list, queueing the writes as we go.  Then we clean
  * up, waiting for those writes to complete.
  * 
  * During this second stage, any subsequent updates to the file may end
  * up refiling the buffer on the original inode's dirty list again, so
  * there is a chance we will end up with a buffer queued for write but
  * not yet completed on that list.  So, as a final cleanup we go through
  * the osync code to catch these locked, dirty buffers without requeuing
  * any newly dirty buffers for write.
  */
 static int fsync_buffers_list(spinlock_t *lock, struct list_head *list)
 {
         struct buffer_head *bh;
         struct list_head tmp;
         struct address_space *mapping;
         int err = 0, err2;
         struct blk_plug plug;
 
         INIT_LIST_HEAD(&tmp);
         blk_start_plug(&plug);
 
         spin_lock(lock);
         while (!list_empty(list)) {
                 bh = BH_ENTRY(list->next);
                 mapping = bh->b_assoc_map;
                 __remove_assoc_queue(bh);
                 /* Avoid race with mark_buffer_dirty_inode() which does
                  * a lockless check and we rely on seeing the dirty bit */
                 smp_mb();
                 if (buffer_dirty(bh) || buffer_locked(bh)) {
                         list_add(&bh->b_assoc_buffers, &tmp);
                         bh->b_assoc_map = mapping;
                         if (buffer_dirty(bh)) {
                                 get_bh(bh);
                                 spin_unlock(lock);
                                 /*
                                  * Ensure any pending I/O completes so that
                                  * write_dirty_buffer() actually writes the
                                  * current contents - it is a noop if I/O is
                                  * still in flight on potentially older
                                  * contents.
                                  */
                                 write_dirty_buffer(bh, REQ_SYNC);
 
                                 /*
                                  * Kick off IO for the previous mapping. Note
                                  * that we will not run the very last mapping,
                                  * wait_on_buffer() will do that for us
                                  * through sync_buffer().
                                  */
                                 brelse(bh);
                                 spin_lock(lock);
                         }
                 }
         }
 
         spin_unlock(lock);
         blk_finish_plug(&plug);
         spin_lock(lock);
 
         while (!list_empty(&tmp)) {
                 bh = BH_ENTRY(tmp.prev);
                 get_bh(bh);
                 mapping = bh->b_assoc_map;
                 __remove_assoc_queue(bh);
                 /* Avoid race with mark_buffer_dirty_inode() which does
                  * a lockless check and we rely on seeing the dirty bit */
                 smp_mb();
                 if (buffer_dirty(bh)) {
                         list_add(&bh->b_assoc_buffers,
                                  &mapping->i_private_list);
                         bh->b_assoc_map = mapping;
                 }
                 spin_unlock(lock);
                 wait_on_buffer(bh);
                 if (!buffer_uptodate(bh))
                         err = -EIO;
                 brelse(bh);
                 spin_lock(lock);
         }
         
         spin_unlock(lock);
         err2 = osync_buffers_list(lock, list);
         if (err)
                 return err;
         else
                 return err2;
 }
 
 /*
  * Invalidate any and all dirty buffers on a given inode.  We are
  * probably unmounting the fs, but that doesn't mean we have already
  * done a sync().  Just drop the buffers from the inode list.
  *
  * NOTE: we take the inode's blockdev's mapping's i_private_lock.  Which
  * assumes that all the buffers are against the blockdev.  Not true
  * for reiserfs.
  */
 void invalidate_inode_buffers(struct inode *inode)
 {
         if (inode_has_buffers(inode)) {
                 struct address_space *mapping = &inode->i_data;
                 struct list_head *list = &mapping->i_private_list;
                 struct address_space *buffer_mapping = mapping->i_private_data;
 
                 spin_lock(&buffer_mapping->i_private_lock);
                 while (!list_empty(list))
                         __remove_assoc_queue(BH_ENTRY(list->next));
                 spin_unlock(&buffer_mapping->i_private_lock);
         }
 }
 EXPORT_SYMBOL(invalidate_inode_buffers);
 
 /*
  * Remove any clean buffers from the inode's buffer list.  This is called
  * when we're trying to free the inode itself.  Those buffers can pin it.
  *
  * Returns true if all buffers were removed.
  */
 int remove_inode_buffers(struct inode *inode)
 {
         int ret = 1;
 
         if (inode_has_buffers(inode)) {
                 struct address_space *mapping = &inode->i_data;
                 struct list_head *list = &mapping->i_private_list;
                 struct address_space *buffer_mapping = mapping->i_private_data;
 
                 spin_lock(&buffer_mapping->i_private_lock);
                 while (!list_empty(list)) {
                         struct buffer_head *bh = BH_ENTRY(list->next);
                         if (buffer_dirty(bh)) {
                                 ret = 0;
                                 break;
                         }
                         __remove_assoc_queue(bh);
                 }
                 spin_unlock(&buffer_mapping->i_private_lock);
         }
         return ret;
 }
 
 /*
  * Create the appropriate buffers when given a folio for data area and
  * the size of each buffer.. Use the bh->b_this_page linked list to
  * follow the buffers created.  Return NULL if unable to create more
  * buffers.
  *
  * The retry flag is used to differentiate async IO (paging, swapping)
  * which may not fail from ordinary buffer allocations.
  */
 struct buffer_head *folio_alloc_buffers(struct folio *folio, unsigned long size,
                                         gfp_t gfp)
 {
         struct buffer_head *bh, *head;
         long offset;
         struct mem_cgroup *memcg, *old_memcg;
 
         /* The folio lock pins the memcg */
         memcg = folio_memcg(folio);
         old_memcg = set_active_memcg(memcg);
 
         head = NULL;
         offset = folio_size(folio);
         while ((offset -= size) >= 0) {
                 bh = alloc_buffer_head(gfp);
                 if (!bh)
                         goto no_grow;
 
                 bh->b_this_page = head;
                 bh->b_blocknr = -1;
                 head = bh;
 
                 bh->b_size = size;
 
                 /* Link the buffer to its folio */
                 folio_set_bh(bh, folio, offset);
         }
 out:
         set_active_memcg(old_memcg);
         return head;
 /*
  * In case anything failed, we just free everything we got.
  */
 no_grow:
         if (head) {
                 do {
                         bh = head;
                         head = head->b_this_page;
                         free_buffer_head(bh);
                 } while (head);
         }
 
         goto out;
 }
 EXPORT_SYMBOL_GPL(folio_alloc_buffers);
 
 struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
                                        bool retry)
 {
         gfp_t gfp = GFP_NOFS | __GFP_ACCOUNT;
         if (retry)
                 gfp |= __GFP_NOFAIL;
 
         return folio_alloc_buffers(page_folio(page), size, gfp);
 }
 EXPORT_SYMBOL_GPL(alloc_page_buffers);
 
 static inline void link_dev_buffers(struct folio *folio,
                 struct buffer_head *head)
 {
         struct buffer_head *bh, *tail;
 
         bh = head;
         do {
                 tail = bh;
                 bh = bh->b_this_page;
         } while (bh);
         tail->b_this_page = head;
         folio_attach_private(folio, head);
 }
 
 static sector_t blkdev_max_block(struct block_device *bdev, unsigned int size)
 {
         sector_t retval = ~((sector_t)0);
         loff_t sz = bdev_nr_bytes(bdev);
 
         if (sz) {
                 unsigned int sizebits = blksize_bits(size);
                 retval = (sz >> sizebits);
         }
         return retval;
 }
 
 /*
  * Initialise the state of a blockdev folio's buffers.
  */ 
 static sector_t folio_init_buffers(struct folio *folio,
                 struct block_device *bdev, unsigned size)
 {
         struct buffer_head *head = folio_buffers(folio);
         struct buffer_head *bh = head;
         bool uptodate = folio_test_uptodate(folio);
         sector_t block = div_u64(folio_pos(folio), size);
         sector_t end_block = blkdev_max_block(bdev, size);
 
         do {
                 if (!buffer_mapped(bh)) {
                         bh->b_end_io = NULL;
                         bh->b_private = NULL;
                         bh->b_bdev = bdev;
                         bh->b_blocknr = block;
                         if (uptodate)
                                 set_buffer_uptodate(bh);
                         if (block < end_block)
                                 set_buffer_mapped(bh);
                 }
                 block++;
                 bh = bh->b_this_page;
         } while (bh != head);
 
         /*
          * Caller needs to validate requested block against end of device.
          */
         return end_block;
 }
 
 /*
  * Create the page-cache folio that contains the requested block.
  *
  * This is used purely for blockdev mappings.
  *
  * Returns false if we have a failure which cannot be cured by retrying
  * without sleeping.  Returns true if we succeeded, or the caller should retry.
  */
 static bool grow_dev_folio(struct block_device *bdev, sector_t block,
                 pgoff_t index, unsigned size, gfp_t gfp)
 {
         struct address_space *mapping = bdev->bd_mapping;
         struct folio *folio;
         struct buffer_head *bh;
         sector_t end_block = 0;
 
         folio = __filemap_get_folio(mapping, index,
                         FGP_LOCK | FGP_ACCESSED | FGP_CREAT, gfp);
         if (IS_ERR(folio))
                 return false;
 
         bh = folio_buffers(folio);
         if (bh) {
                 if (bh->b_size == size) {
                         end_block = folio_init_buffers(folio, bdev, size);
                         goto unlock;
                 }
 
                 /*
                  * Retrying may succeed; for example the folio may finish
                  * writeback, or buffers may be cleaned.  This should not
                  * happen very often; maybe we have old buffers attached to
                  * this blockdev's page cache and we're trying to change
                  * the block size?
                  */
                 if (!try_to_free_buffers(folio)) {
                         end_block = ~0ULL;
                         goto unlock;
                 }
         }
 
         bh = folio_alloc_buffers(folio, size, gfp | __GFP_ACCOUNT);
         if (!bh)
                 goto unlock;
 
         /*
          * Link the folio to the buffers and initialise them.  Take the
          * lock to be atomic wrt __find_get_block(), which does not
          * run under the folio lock.
          */
         spin_lock(&mapping->i_private_lock);
         link_dev_buffers(folio, bh);
         end_block = folio_init_buffers(folio, bdev, size);
         spin_unlock(&mapping->i_private_lock);
 unlock:
         folio_unlock(folio);
         folio_put(folio);
         return block < end_block;
 }
 
 /*
  * Create buffers for the specified block device block's folio.  If
  * that folio was dirty, the buffers are set dirty also.  Returns false
  * if we've hit a permanent error.
  */
 static bool grow_buffers(struct block_device *bdev, sector_t block,
                 unsigned size, gfp_t gfp)
 {
         loff_t pos;
 
         /*
          * Check for a block which lies outside our maximum possible
          * pagecache index.
          */
         if (check_mul_overflow(block, (sector_t)size, &pos) || pos > MAX_LFS_FILESIZE) {
                 printk(KERN_ERR "%s: requested out-of-range block %llu for device %pg\n",
                         __func__, (unsigned long long)block,
                         bdev);
                 return false;
         }
 
         /* Create a folio with the proper size buffers */
         return grow_dev_folio(bdev, block, pos / PAGE_SIZE, size, gfp);
 }
 
 static struct buffer_head *
 __getblk_slow(struct block_device *bdev, sector_t block,
              unsigned size, gfp_t gfp)
 {
         /* Size must be multiple of hard sectorsize */
         if (unlikely(size & (bdev_logical_block_size(bdev)-1) ||
                         (size < 512 || size > PAGE_SIZE))) {
                 printk(KERN_ERR "getblk(): invalid block size %d requested\n",
                                         size);
                 printk(KERN_ERR "logical block size: %d\n",
                                         bdev_logical_block_size(bdev));
 
                 dump_stack();
                 return NULL;
         }
 
         for (;;) {
                 struct buffer_head *bh;
 
                 bh = __find_get_block(bdev, block, size);
                 if (bh)
                         return bh;
 
                 if (!grow_buffers(bdev, block, size, gfp))
                         return NULL;
         }
 }
 
 /*
  * The relationship between dirty buffers and dirty pages:
  *
  * Whenever a page has any dirty buffers, the page's dirty bit is set, and
  * the page is tagged dirty in the page cache.
  *
  * At all times, the dirtiness of the buffers represents the dirtiness of
  * subsections of the page.  If the page has buffers, the page dirty bit is
  * merely a hint about the true dirty state.
  *
  * When a page is set dirty in its entirety, all its buffers are marked dirty
  * (if the page has buffers).
  *
  * When a buffer is marked dirty, its page is dirtied, but the page's other
  * buffers are not.
  *
  * Also.  When blockdev buffers are explicitly read with bread(), they
  * individually become uptodate.  But their backing page remains not
  * uptodate - even if all of its buffers are uptodate.  A subsequent
  * block_read_full_folio() against that folio will discover all the uptodate
  * buffers, will set the folio uptodate and will perform no I/O.
  */
 
 /**
  * mark_buffer_dirty - mark a buffer_head as needing writeout
  * @bh: the buffer_head to mark dirty
  *
  * mark_buffer_dirty() will set the dirty bit against the buffer, then set
  * its backing page dirty, then tag the page as dirty in the page cache
  * and then attach the address_space's inode to its superblock's dirty
  * inode list.
  *
  * mark_buffer_dirty() is atomic.  It takes bh->b_folio->mapping->i_private_lock,
  * i_pages lock and mapping->host->i_lock.
  */
 void mark_buffer_dirty(struct buffer_head *bh)
 {
         WARN_ON_ONCE(!buffer_uptodate(bh));
 
         trace_block_dirty_buffer(bh);
 
         /*
          * Very *carefully* optimize the it-is-already-dirty case.
          *
          * Don't let the final "is it dirty" escape to before we
          * perhaps modified the buffer.
          */
         if (buffer_dirty(bh)) {
                 smp_mb();
                 if (buffer_dirty(bh))
                         return;
         }
 
         if (!test_set_buffer_dirty(bh)) {
                 struct folio *folio = bh->b_folio;
                 struct address_space *mapping = NULL;
 
                 folio_memcg_lock(folio);
                 if (!folio_test_set_dirty(folio)) {
                         mapping = folio->mapping;
                         if (mapping)
                                 __folio_mark_dirty(folio, mapping, 0);
                 }
                 folio_memcg_unlock(folio);
                 if (mapping)
                         __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
         }
 }
 EXPORT_SYMBOL(mark_buffer_dirty);
 
 void mark_buffer_write_io_error(struct buffer_head *bh)
 {
         set_buffer_write_io_error(bh);
         /* FIXME: do we need to set this in both places? */
         if (bh->b_folio && bh->b_folio->mapping)
                 mapping_set_error(bh->b_folio->mapping, -EIO);
         if (bh->b_assoc_map) {
                 mapping_set_error(bh->b_assoc_map, -EIO);
                 errseq_set(&bh->b_assoc_map->host->i_sb->s_wb_err, -EIO);
         }
 }
 EXPORT_SYMBOL(mark_buffer_write_io_error);
 
 /**
  * __brelse - Release a buffer.
  * @bh: The buffer to release.
  *
  * This variant of brelse() can be called if @bh is guaranteed to not be NULL.
  */
 void __brelse(struct buffer_head *bh)
 {
         if (atomic_read(&bh->b_count)) {
                 put_bh(bh);
                 return;
         }
         WARN(1, KERN_ERR "VFS: brelse: Trying to free free buffer\n");
 }
 EXPORT_SYMBOL(__brelse);
 
 /**
  * __bforget - Discard any dirty data in a buffer.
  * @bh: The buffer to forget.
  *
  * This variant of bforget() can be called if @bh is guaranteed to not
  * be NULL.
  */
 void __bforget(struct buffer_head *bh)
 {
         clear_buffer_dirty(bh);
         if (bh->b_assoc_map) {
                 struct address_space *buffer_mapping = bh->b_folio->mapping;
 
                 spin_lock(&buffer_mapping->i_private_lock);
                 list_del_init(&bh->b_assoc_buffers);
                 bh->b_assoc_map = NULL;
                 spin_unlock(&buffer_mapping->i_private_lock);
         }
         __brelse(bh);
 }
 EXPORT_SYMBOL(__bforget);
 
 static struct buffer_head *__bread_slow(struct buffer_head *bh)
 {
         lock_buffer(bh);
         if (buffer_uptodate(bh)) {
                 unlock_buffer(bh);
                 return bh;
         } else {
                 get_bh(bh);
                 bh->b_end_io = end_buffer_read_sync;
                 submit_bh(REQ_OP_READ, bh);
                 wait_on_buffer(bh);
                 if (buffer_uptodate(bh))
                         return bh;
         }
         brelse(bh);
         return NULL;
 }
 
 /*
  * Per-cpu buffer LRU implementation.  To reduce the cost of __find_get_block().
  * The bhs[] array is sorted - newest buffer is at bhs[0].  Buffers have their
  * refcount elevated by one when they're in an LRU.  A buffer can only appear
  * once in a particular CPU's LRU.  A single buffer can be present in multiple
  * CPU's LRUs at the same time.
  *
  * This is a transparent caching front-end to sb_bread(), sb_getblk() and
  * sb_find_get_block().
  *
  * The LRUs themselves only need locking against invalidate_bh_lrus.  We use
  * a local interrupt disable for that.
  */
 
 #define BH_LRU_SIZE     16
 
 struct bh_lru {
         struct buffer_head *bhs[BH_LRU_SIZE];
 };
 
 static DEFINE_PER_CPU(struct bh_lru, bh_lrus) = {{ NULL }};
 
 #ifdef CONFIG_SMP
 #define bh_lru_lock()   local_irq_disable()
 #define bh_lru_unlock() local_irq_enable()
 #else
 #define bh_lru_lock()   preempt_disable()
 #define bh_lru_unlock() preempt_enable()
 #endif
 
 static inline void check_irqs_on(void)
 {
 #ifdef irqs_disabled
         BUG_ON(irqs_disabled());
 #endif
 }
 
 /*
  * Install a buffer_head into this cpu's LRU.  If not already in the LRU, it is
  * inserted at the front, and the buffer_head at the back if any is evicted.
  * Or, if already in the LRU it is moved to the front.
  */
 static void bh_lru_install(struct buffer_head *bh)
 {
         struct buffer_head *evictee = bh;
         struct bh_lru *b;
         int i;
 
         check_irqs_on();
         bh_lru_lock();
 
         /*
          * the refcount of buffer_head in bh_lru prevents dropping the
          * attached page(i.e., try_to_free_buffers) so it could cause
          * failing page migration.
          * Skip putting upcoming bh into bh_lru until migration is done.
          */
         if (lru_cache_disabled() || cpu_is_isolated(smp_processor_id())) {
                 bh_lru_unlock();
                 return;
         }
 
         b = this_cpu_ptr(&bh_lrus);
         for (i = 0; i < BH_LRU_SIZE; i++) {
                 swap(evictee, b->bhs[i]);
                 if (evictee == bh) {
                         bh_lru_unlock();
                         return;
                 }
         }
 
         get_bh(bh);
         bh_lru_unlock();
         brelse(evictee);
 }
 
 /*
  * Look up the bh in this cpu's LRU.  If it's there, move it to the head.
  */
 static struct buffer_head *
 lookup_bh_lru(struct block_device *bdev, sector_t block, unsigned size)
 {
         struct buffer_head *ret = NULL;
         unsigned int i;
 
         check_irqs_on();
         bh_lru_lock();
         if (cpu_is_isolated(smp_processor_id())) {
                 bh_lru_unlock();
                 return NULL;
         }
         for (i = 0; i < BH_LRU_SIZE; i++) {
                 struct buffer_head *bh = __this_cpu_read(bh_lrus.bhs[i]);
 
                 if (bh && bh->b_blocknr == block && bh->b_bdev == bdev &&
                     bh->b_size == size) {
                         if (i) {
                                 while (i) {
                                         __this_cpu_write(bh_lrus.bhs[i],
                                                 __this_cpu_read(bh_lrus.bhs[i - 1]));
                                         i--;
                                 }
                                 __this_cpu_write(bh_lrus.bhs[0], bh);
                         }
                         get_bh(bh);
                         ret = bh;
                         break;
                 }
         }
         bh_lru_unlock();
         return ret;
 }
 
 /*
  * Perform a pagecache lookup for the matching buffer.  If it's there, refresh
  * it in the LRU and mark it as accessed.  If it is not present then return
  * NULL
  */
 struct buffer_head *
 __find_get_block(struct block_device *bdev, sector_t block, unsigned size)
 {
         struct buffer_head *bh = lookup_bh_lru(bdev, block, size);
 
         if (bh == NULL) {
                 /* __find_get_block_slow will mark the page accessed */
                 bh = __find_get_block_slow(bdev, block);
                 if (bh)
                         bh_lru_install(bh);
         } else
                 touch_buffer(bh);
 
         return bh;
 }
 EXPORT_SYMBOL(__find_get_block);
 
 /**
  * bdev_getblk - Get a buffer_head in a block device's buffer cache.
  * @bdev: The block device.
  * @block: The block number.
  * @size: The size of buffer_heads for this @bdev.
  * @gfp: The memory allocation flags to use.
  *
  * The returned buffer head has its reference count incremented, but is
  * not locked.  The caller should call brelse() when it has finished
  * with the buffer.  The buffer may not be uptodate.  If needed, the
  * caller can bring it uptodate either by reading it or overwriting it.
  *
  * Return: The buffer head, or NULL if memory could not be allocated.
  */
 struct buffer_head *bdev_getblk(struct block_device *bdev, sector_t block,
                 unsigned size, gfp_t gfp)
 {
         struct buffer_head *bh = __find_get_block(bdev, block, size);
 
         might_alloc(gfp);
         if (bh)
                 return bh;
 
         return __getblk_slow(bdev, block, size, gfp);
 }
 EXPORT_SYMBOL(bdev_getblk);
 
 /*
  * Do async read-ahead on a buffer..
  */
 void __breadahead(struct block_device *bdev, sector_t block, unsigned size)
 {
         struct buffer_head *bh = bdev_getblk(bdev, block, size,
                         GFP_NOWAIT | __GFP_MOVABLE);
 
         if (likely(bh)) {
                 bh_readahead(bh, REQ_RAHEAD);
                 brelse(bh);
         }
 }
 EXPORT_SYMBOL(__breadahead);
 
 /**
  * __bread_gfp() - Read a block.
  * @bdev: The block device to read from.
  * @block: Block number in units of block size.
  * @size: The block size of this device in bytes.
  * @gfp: Not page allocation flags; see below.
  *
  * You are not expected to call this function.  You should use one of
  * sb_bread(), sb_bread_unmovable() or __bread().
  *
  * Read a specified block, and return the buffer head that refers to it.
  * If @gfp is 0, the memory will be allocated using the block device's
  * default GFP flags.  If @gfp is __GFP_MOVABLE, the memory may be
  * allocated from a movable area.  Do not pass in a complete set of
  * GFP flags.
  *
  * The returned buffer head has its refcount increased.  The caller should
  * call brelse() when it has finished with the buffer.
  *
  * Context: May sleep waiting for I/O.
  * Return: NULL if the block was unreadable.
  */
 struct buffer_head *__bread_gfp(struct block_device *bdev, sector_t block,
                 unsigned size, gfp_t gfp)
 {
         struct buffer_head *bh;
 
         gfp |= mapping_gfp_constraint(bdev->bd_mapping, ~__GFP_FS);
 
         /*
          * Prefer looping in the allocator rather than here, at least that
          * code knows what it's doing.
          */
         gfp |= __GFP_NOFAIL;
 
         bh = bdev_getblk(bdev, block, size, gfp);
 
         if (likely(bh) && !buffer_uptodate(bh))
                 bh = __bread_slow(bh);
         return bh;
 }
 EXPORT_SYMBOL(__bread_gfp);
 
 static void __invalidate_bh_lrus(struct bh_lru *b)
 {
         int i;
 
         for (i = 0; i < BH_LRU_SIZE; i++) {
                 brelse(b->bhs[i]);
                 b->bhs[i] = NULL;
         }
 }
 /*
  * invalidate_bh_lrus() is called rarely - but not only at unmount.
  * This doesn't race because it runs in each cpu either in irq
  * or with preempt disabled.
  */
 static void invalidate_bh_lru(void *arg)
 {
         struct bh_lru *b = &get_cpu_var(bh_lrus);
 
         __invalidate_bh_lrus(b);
         put_cpu_var(bh_lrus);
 }
 
 bool has_bh_in_lru(int cpu, void *dummy)
 {
         struct bh_lru *b = per_cpu_ptr(&bh_lrus, cpu);
         int i;
         
         for (i = 0; i < BH_LRU_SIZE; i++) {
                 if (b->bhs[i])
                         return true;
         }
 
         return false;
 }
 
 void invalidate_bh_lrus(void)
 {
         on_each_cpu_cond(has_bh_in_lru, invalidate_bh_lru, NULL, 1);
 }
 EXPORT_SYMBOL_GPL(invalidate_bh_lrus);
 
 /*
  * It's called from workqueue context so we need a bh_lru_lock to close
  * the race with preemption/irq.
  */
 void invalidate_bh_lrus_cpu(void)
 {
         struct bh_lru *b;
 
         bh_lru_lock();
         b = this_cpu_ptr(&bh_lrus);
         __invalidate_bh_lrus(b);
         bh_lru_unlock();
 }
 
 void folio_set_bh(struct buffer_head *bh, struct folio *folio,
                   unsigned long offset)
 {
         bh->b_folio = folio;
         BUG_ON(offset >= folio_size(folio));
         if (folio_test_highmem(folio))
                 /*
                  * This catches illegal uses and preserves the offset:
                  */
                 bh->b_data = (char *)(0 + offset);
         else
                 bh->b_data = folio_address(folio) + offset;
 }
 EXPORT_SYMBOL(folio_set_bh);
 
 /*
  * Called when truncating a buffer on a page completely.
  */
 
 /* Bits that are cleared during an invalidate */
 #define BUFFER_FLAGS_DISCARD \
         (1 << BH_Mapped | 1 << BH_New | 1 << BH_Req | \
          1 << BH_Delay | 1 << BH_Unwritten)
 
 static void discard_buffer(struct buffer_head * bh)
 {
         unsigned long b_state;
 
         lock_buffer(bh);
         clear_buffer_dirty(bh);
         bh->b_bdev = NULL;
         b_state = READ_ONCE(bh->b_state);
         do {
         } while (!try_cmpxchg(&bh->b_state, &b_state,
                               b_state & ~BUFFER_FLAGS_DISCARD));
         unlock_buffer(bh);
 }
 
 /**
  * block_invalidate_folio - Invalidate part or all of a buffer-backed folio.
  * @folio: The folio which is affected.
  * @offset: start of the range to invalidate
  * @length: length of the range to invalidate
  *
  * block_invalidate_folio() is called when all or part of the folio has been
  * invalidated by a truncate operation.
  *
  * block_invalidate_folio() does not have to release all buffers, but it must
  * ensure that no dirty buffer is left outside @offset and that no I/O
  * is underway against any of the blocks which are outside the truncation
  * point.  Because the caller is about to free (and possibly reuse) those
  * blocks on-disk.
  */
 void block_invalidate_folio(struct folio *folio, size_t offset, size_t length)
 {
         struct buffer_head *head, *bh, *next;
         size_t curr_off = 0;
         size_t stop = length + offset;
 
         BUG_ON(!folio_test_locked(folio));
 
         /*
          * Check for overflow
          */
         BUG_ON(stop > folio_size(folio) || stop < length);
 
         head = folio_buffers(folio);
         if (!head)
                 return;
 
         bh = head;
         do {
                 size_t next_off = curr_off + bh->b_size;
                 next = bh->b_this_page;
 
                 /*
                  * Are we still fully in range ?
                  */
                 if (next_off > stop)
                         goto out;
 
                 /*
                  * is this block fully invalidated?
                  */
                 if (offset <= curr_off)
                         discard_buffer(bh);
                 curr_off = next_off;
                 bh = next;
         } while (bh != head);
 
         /*
          * We release buffers only if the entire folio is being invalidated.
          * The get_block cached value has been unconditionally invalidated,
          * so real IO is not possible anymore.
          */
         if (length == folio_size(folio))
                 filemap_release_folio(folio, 0);
 out:
         return;
 }
 EXPORT_SYMBOL(block_invalidate_folio);
 
 /*
  * We attach and possibly dirty the buffers atomically wrt
  * block_dirty_folio() via i_private_lock.  try_to_free_buffers
  * is already excluded via the folio lock.
  */
 struct buffer_head *create_empty_buffers(struct folio *folio,
                 unsigned long blocksize, unsigned long b_state)
 {
         struct buffer_head *bh, *head, *tail;
         gfp_t gfp = GFP_NOFS | __GFP_ACCOUNT | __GFP_NOFAIL;
 
         head = folio_alloc_buffers(folio, blocksize, gfp);
         bh = head;
         do {
                 bh->b_state |= b_state;
                 tail = bh;
                 bh = bh->b_this_page;
         } while (bh);
         tail->b_this_page = head;
 
         spin_lock(&folio->mapping->i_private_lock);
         if (folio_test_uptodate(folio) || folio_test_dirty(folio)) {
                 bh = head;
                 do {
                         if (folio_test_dirty(folio))
                                 set_buffer_dirty(bh);
                         if (folio_test_uptodate(folio))
                                 set_buffer_uptodate(bh);
                         bh = bh->b_this_page;
                 } while (bh != head);
         }
         folio_attach_private(folio, head);
         spin_unlock(&folio->mapping->i_private_lock);
 
         return head;
 }
 EXPORT_SYMBOL(create_empty_buffers);
 
 /**
  * clean_bdev_aliases: clean a range of buffers in block device
  * @bdev: Block device to clean buffers in
  * @block: Start of a range of blocks to clean
  * @len: Number of blocks to clean
  *
  * We are taking a range of blocks for data and we don't want writeback of any
  * buffer-cache aliases starting from return from this function and until the
  * moment when something will explicitly mark the buffer dirty (hopefully that
  * will not happen until we will free that block ;-) We don't even need to mark
  * it not-uptodate - nobody can expect anything from a newly allocated buffer
  * anyway. We used to use unmap_buffer() for such invalidation, but that was
  * wrong. We definitely don't want to mark the alias unmapped, for example - it
  * would confuse anyone who might pick it with bread() afterwards...
  *
  * Also..  Note that bforget() doesn't lock the buffer.  So there can be
  * writeout I/O going on against recently-freed buffers.  We don't wait on that
  * I/O in bforget() - it's more efficient to wait on the I/O only if we really
  * need to.  That happens here.
  */
 void clean_bdev_aliases(struct block_device *bdev, sector_t block, sector_t len)
 {
         struct address_space *bd_mapping = bdev->bd_mapping;
         const int blkbits = bd_mapping->host->i_blkbits;
         struct folio_batch fbatch;
         pgoff_t index = ((loff_t)block << blkbits) / PAGE_SIZE;
         pgoff_t end;
         int i, count;
         struct buffer_head *bh;
         struct buffer_head *head;
 
         end = ((loff_t)(block + len - 1) << blkbits) / PAGE_SIZE;
         folio_batch_init(&fbatch);
         while (filemap_get_folios(bd_mapping, &index, end, &fbatch)) {
                 count = folio_batch_count(&fbatch);
                 for (i = 0; i < count; i++) {
                         struct folio *folio = fbatch.folios[i];
 
                         if (!folio_buffers(folio))
                                 continue;
                         /*
                          * We use folio lock instead of bd_mapping->i_private_lock
                          * to pin buffers here since we can afford to sleep and
                          * it scales better than a global spinlock lock.
                          */
                         folio_lock(folio);
                         /* Recheck when the folio is locked which pins bhs */
                         head = folio_buffers(folio);
                         if (!head)
                                 goto unlock_page;
                         bh = head;
                         do {
                                 if (!buffer_mapped(bh) || (bh->b_blocknr < block))
                                         goto next;
                                 if (bh->b_blocknr >= block + len)
                                         break;
                                 clear_buffer_dirty(bh);
                                 wait_on_buffer(bh);
                                 clear_buffer_req(bh);
 next:
                                 bh = bh->b_this_page;
                         } while (bh != head);
 unlock_page:
                         folio_unlock(folio);
                 }
                 folio_batch_release(&fbatch);
                 cond_resched();
                 /* End of range already reached? */
                 if (index > end || !index)
                         break;
         }
 }
 EXPORT_SYMBOL(clean_bdev_aliases);
 
 static struct buffer_head *folio_create_buffers(struct folio *folio,
                                                 struct inode *inode,
                                                 unsigned int b_state)
 {
         struct buffer_head *bh;
 
         BUG_ON(!folio_test_locked(folio));
 
         bh = folio_buffers(folio);
         if (!bh)
                 bh = create_empty_buffers(folio,
                                 1 << READ_ONCE(inode->i_blkbits), b_state);
         return bh;
 }
 
 /*
  * NOTE! All mapped/uptodate combinations are valid:
  *
  *      Mapped  Uptodate        Meaning
  *
  *      No      No              "unknown" - must do get_block()
  *      No      Yes             "hole" - zero-filled
  *      Yes     No              "allocated" - allocated on disk, not read in
  *      Yes     Yes             "valid" - allocated and up-to-date in memory.
  *
  * "Dirty" is valid only with the last case (mapped+uptodate).
  */
 
 /*
  * While block_write_full_folio is writing back the dirty buffers under
  * the page lock, whoever dirtied the buffers may decide to clean them
  * again at any time.  We handle that by only looking at the buffer
  * state inside lock_buffer().
  *
  * If block_write_full_folio() is called for regular writeback
  * (wbc->sync_mode == WB_SYNC_NONE) then it will redirty a page which has a
  * locked buffer.   This only can happen if someone has written the buffer
  * directly, with submit_bh().  At the address_space level PageWriteback
  * prevents this contention from occurring.
  *
  * If block_write_full_folio() is called with wbc->sync_mode ==
  * WB_SYNC_ALL, the writes are posted using REQ_SYNC; this
  * causes the writes to be flagged as synchronous writes.
  */
 int __block_write_full_folio(struct inode *inode, struct folio *folio,
                         get_block_t *get_block, struct writeback_control *wbc)
 {
         int err;
         sector_t block;
         sector_t last_block;
         struct buffer_head *bh, *head;
         size_t blocksize;
         int nr_underway = 0;
         blk_opf_t write_flags = wbc_to_write_flags(wbc);
 
         head = folio_create_buffers(folio, inode,
                                     (1 << BH_Dirty) | (1 << BH_Uptodate));
 
         /*
          * Be very careful.  We have no exclusion from block_dirty_folio
          * here, and the (potentially unmapped) buffers may become dirty at
          * any time.  If a buffer becomes dirty here after we've inspected it
          * then we just miss that fact, and the folio stays dirty.
          *
          * Buffers outside i_size may be dirtied by block_dirty_folio;
          * handle that here by just cleaning them.
          */
 
         bh = head;
         blocksize = bh->b_size;
 
         block = div_u64(folio_pos(folio), blocksize);
         last_block = div_u64(i_size_read(inode) - 1, blocksize);
 
         /*
          * Get all the dirty buffers mapped to disk addresses and
          * handle any aliases from the underlying blockdev's mapping.
          */
         do {
                 if (block > last_block) {
                         /*
                          * mapped buffers outside i_size will occur, because
                          * this folio can be outside i_size when there is a
                          * truncate in progress.
                          */
                         /*
                          * The buffer was zeroed by block_write_full_folio()
                          */
                         clear_buffer_dirty(bh);
                         set_buffer_uptodate(bh);
                 } else if ((!buffer_mapped(bh) || buffer_delay(bh)) &&
                            buffer_dirty(bh)) {
                         WARN_ON(bh->b_size != blocksize);
                         err = get_block(inode, block, bh, 1);
                         if (err)
                                 goto recover;
                         clear_buffer_delay(bh);
                         if (buffer_new(bh)) {
                                 /* blockdev mappings never come here */
                                 clear_buffer_new(bh);
                                 clean_bdev_bh_alias(bh);
                         }
                 }
                 bh = bh->b_this_page;
                 block++;
         } while (bh != head);
 
         do {
                 if (!buffer_mapped(bh))
                         continue;
                 /*
                  * If it's a fully non-blocking write attempt and we cannot
                  * lock the buffer then redirty the folio.  Note that this can
                  * potentially cause a busy-wait loop from writeback threads
                  * and kswapd activity, but those code paths have their own
                  * higher-level throttling.
                  */
                 if (wbc->sync_mode != WB_SYNC_NONE) {
                         lock_buffer(bh);
                 } else if (!trylock_buffer(bh)) {
                         folio_redirty_for_writepage(wbc, folio);
                         continue;
                 }
                 if (test_clear_buffer_dirty(bh)) {
                         mark_buffer_async_write_endio(bh,
                                 end_buffer_async_write);
                 } else {
                         unlock_buffer(bh);
                 }
         } while ((bh = bh->b_this_page) != head);
 
         /*
          * The folio and its buffers are protected by the writeback flag,
          * so we can drop the bh refcounts early.
          */
         BUG_ON(folio_test_writeback(folio));
         folio_start_writeback(folio);
 
         do {
                 struct buffer_head *next = bh->b_this_page;
                 if (buffer_async_write(bh)) {
                         submit_bh_wbc(REQ_OP_WRITE | write_flags, bh,
                                       inode->i_write_hint, wbc);
                         nr_underway++;
                 }
                 bh = next;
         } while (bh != head);
         folio_unlock(folio);
 
         err = 0;
 done:
         if (nr_underway == 0) {
                 /*
                  * The folio was marked dirty, but the buffers were
                  * clean.  Someone wrote them back by hand with
                  * write_dirty_buffer/submit_bh.  A rare case.
                  */
                 folio_end_writeback(folio);
 
                 /*
                  * The folio and buffer_heads can be released at any time from
                  * here on.
                  */
         }
         return err;
 
 recover:
         /*
          * ENOSPC, or some other error.  We may already have added some
          * blocks to the file, so we need to write these out to avoid
          * exposing stale data.
          * The folio is currently locked and not marked for writeback
          */
         bh = head;
         /* Recovery: lock and submit the mapped buffers */
         do {
                 if (buffer_mapped(bh) && buffer_dirty(bh) &&
                     !buffer_delay(bh)) {
                         lock_buffer(bh);
                         mark_buffer_async_write_endio(bh,
                                 end_buffer_async_write);
                 } else {
                         /*
                          * The buffer may have been set dirty during
                          * attachment to a dirty folio.
                          */
                         clear_buffer_dirty(bh);
                 }
         } while ((bh = bh->b_this_page) != head);
         BUG_ON(folio_test_writeback(folio));
         mapping_set_error(folio->mapping, err);
         folio_start_writeback(folio);
         do {
                 struct buffer_head *next = bh->b_this_page;
                 if (buffer_async_write(bh)) {
                         clear_buffer_dirty(bh);
                         submit_bh_wbc(REQ_OP_WRITE | write_flags, bh,
                                       inode->i_write_hint, wbc);
                         nr_underway++;
                 }
                 bh = next;
         } while (bh != head);
         folio_unlock(folio);
         goto done;
 }
 EXPORT_SYMBOL(__block_write_full_folio);
 
 /*
  * If a folio has any new buffers, zero them out here, and mark them uptodate
  * and dirty so they'll be written out (in order to prevent uninitialised
  * block data from leaking). And clear the new bit.
  */
 void folio_zero_new_buffers(struct folio *folio, size_t from, size_t to)
 {
         size_t block_start, block_end;
         struct buffer_head *head, *bh;
 
         BUG_ON(!folio_test_locked(folio));
         head = folio_buffers(folio);
         if (!head)
                 return;
 
         bh = head;
         block_start = 0;
         do {
                 block_end = block_start + bh->b_size;
 
                 if (buffer_new(bh)) {
                         if (block_end > from && block_start < to) {
                                 if (!folio_test_uptodate(folio)) {
                                         size_t start, xend;
 
                                         start = max(from, block_start);
                                         xend = min(to, block_end);
 
                                         folio_zero_segment(folio, start, xend);
                                         set_buffer_uptodate(bh);
                                 }
 
                                 clear_buffer_new(bh);
                                 mark_buffer_dirty(bh);
                         }
                 }
 
                 block_start = block_end;
                 bh = bh->b_this_page;
         } while (bh != head);
 }
 EXPORT_SYMBOL(folio_zero_new_buffers);
 
 static int
 iomap_to_bh(struct inode *inode, sector_t block, struct buffer_head *bh,
                 const struct iomap *iomap)
 {
         loff_t offset = (loff_t)block << inode->i_blkbits;
 
         bh->b_bdev = iomap->bdev;
 
         /*
          * Block points to offset in file we need to map, iomap contains
          * the offset at which the map starts. If the map ends before the
          * current block, then do not map the buffer and let the caller
          * handle it.
          */
         if (offset >= iomap->offset + iomap->length)
                 return -EIO;
 
         switch (iomap->type) {
         case IOMAP_HOLE:
                 /*
                  * If the buffer is not up to date or beyond the current EOF,
                  * we need to mark it as new to ensure sub-block zeroing is
                  * executed if necessary.
                  */
                 if (!buffer_uptodate(bh) ||
                     (offset >= i_size_read(inode)))
                         set_buffer_new(bh);
                 return 0;
         case IOMAP_DELALLOC:
                 if (!buffer_uptodate(bh) ||
                     (offset >= i_size_read(inode)))
                         set_buffer_new(bh);
                 set_buffer_uptodate(bh);
                 set_buffer_mapped(bh);
                 set_buffer_delay(bh);
                 return 0;
         case IOMAP_UNWRITTEN:
                 /*
                  * For unwritten regions, we always need to ensure that regions
                  * in the block we are not writing to are zeroed. Mark the
                  * buffer as new to ensure this.
                  */
                 set_buffer_new(bh);
                 set_buffer_unwritten(bh);
                 fallthrough;
         case IOMAP_MAPPED:
                 if ((iomap->flags & IOMAP_F_NEW) ||
                     offset >= i_size_read(inode)) {
                         /*
                          * This can happen if truncating the block device races
                          * with the check in the caller as i_size updates on
                          * block devices aren't synchronized by i_rwsem for
                          * block devices.
                          */
                         if (S_ISBLK(inode->i_mode))
                                 return -EIO;
                         set_buffer_new(bh);
                 }
                 bh->b_blocknr = (iomap->addr + offset - iomap->offset) >>
                                 inode->i_blkbits;
                 set_buffer_mapped(bh);
                 return 0;
         default:
                 WARN_ON_ONCE(1);
                 return -EIO;
         }
 }
 
 int __block_write_begin_int(struct folio *folio, loff_t pos, unsigned len,
                 get_block_t *get_block, const struct iomap *iomap)
 {
         size_t from = offset_in_folio(folio, pos);
         size_t to = from + len;
         struct inode *inode = folio->mapping->host;
         size_t block_start, block_end;
         sector_t block;
         int err = 0;
         size_t blocksize;
         struct buffer_head *bh, *head, *wait[2], **wait_bh=wait;
 
         BUG_ON(!folio_test_locked(folio));
         BUG_ON(to > folio_size(folio));
         BUG_ON(from > to);
 
         head = folio_create_buffers(folio, inode, 0);
         blocksize = head->b_size;
         block = div_u64(folio_pos(folio), blocksize);
 
         for (bh = head, block_start = 0; bh != head || !block_start;
             block++, block_start=block_end, bh = bh->b_this_page) {
                 block_end = block_start + blocksize;
                 if (block_end <= from || block_start >= to) {
                         if (folio_test_uptodate(folio)) {
                                 if (!buffer_uptodate(bh))
                                         set_buffer_uptodate(bh);
                         }
                         continue;
                 }
                 if (buffer_new(bh))
                         clear_buffer_new(bh);
                 if (!buffer_mapped(bh)) {
                         WARN_ON(bh->b_size != blocksize);
                         if (get_block)
                                 err = get_block(inode, block, bh, 1);
                         else
                                 err = iomap_to_bh(inode, block, bh, iomap);
                         if (err)
                                 break;
 
                         if (buffer_new(bh)) {
                                 clean_bdev_bh_alias(bh);
                                 if (folio_test_uptodate(folio)) {
                                         clear_buffer_new(bh);
                                         set_buffer_uptodate(bh);
                                         mark_buffer_dirty(bh);
                                         continue;
                                 }
                                 if (block_end > to || block_start < from)
                                         folio_zero_segments(folio,
                                                 to, block_end,
                                                 block_start, from);
                                 continue;
                         }
                 }
                 if (folio_test_uptodate(folio)) {
                         if (!buffer_uptodate(bh))
                                 set_buffer_uptodate(bh);
                         continue; 
                 }
                 if (!buffer_uptodate(bh) && !buffer_delay(bh) &&
                     !buffer_unwritten(bh) &&
                      (block_start < from || block_end > to)) {
                         bh_read_nowait(bh, 0);
                         *wait_bh++=bh;
                 }
         }
         /*
          * If we issued read requests - let them complete.
          */
         while(wait_bh > wait) {
                 wait_on_buffer(*--wait_bh);
                 if (!buffer_uptodate(*wait_bh))
                         err = -EIO;
         }
         if (unlikely(err))
                 folio_zero_new_buffers(folio, from, to);
         return err;
 }
 
 int __block_write_begin(struct page *page, loff_t pos, unsigned len,
                 get_block_t *get_block)
 {
         return __block_write_begin_int(page_folio(page), pos, len, get_block,
                                        NULL);
 }
 EXPORT_SYMBOL(__block_write_begin);
 
 static void __block_commit_write(struct folio *folio, size_t from, size_t to)
 {
         size_t block_start, block_end;
         bool partial = false;
         unsigned blocksize;
         struct buffer_head *bh, *head;
 
         bh = head = folio_buffers(folio);
         if (!bh)
                 return;
         blocksize = bh->b_size;
 
         block_start = 0;
         do {
                 block_end = block_start + blocksize;
                 if (block_end <= from || block_start >= to) {
                         if (!buffer_uptodate(bh))
                                 partial = true;
                 } else {
                         set_buffer_uptodate(bh);
                         mark_buffer_dirty(bh);
                 }
                 if (buffer_new(bh))
                         clear_buffer_new(bh);
 
                 block_start = block_end;
                 bh = bh->b_this_page;
         } while (bh != head);
 
         /*
          * If this is a partial write which happened to make all buffers
          * uptodate then we can optimize away a bogus read_folio() for
          * the next read(). Here we 'discover' whether the folio went
          * uptodate as a result of this (potentially partial) write.
          */
         if (!partial)
                 folio_mark_uptodate(folio);
 }
 
 /*
  * block_write_begin takes care of the basic task of block allocation and
  * bringing partial write blocks uptodate first.
  *
  * The filesystem needs to handle block truncation upon failure.
  */
 int block_write_begin(struct address_space *mapping, loff_t pos, unsigned len,
                 struct page **pagep, get_block_t *get_block)
 {
         pgoff_t index = pos >> PAGE_SHIFT;
         struct page *page;
         int status;
 
         page = grab_cache_page_write_begin(mapping, index);
         if (!page)
                 return -ENOMEM;
 
         status = __block_write_begin(page, pos, len, get_block);
         if (unlikely(status)) {
                 unlock_page(page);
                 put_page(page);
                 page = NULL;
         }
 
         *pagep = page;
         return status;
 }
 EXPORT_SYMBOL(block_write_begin);
 
 int block_write_end(struct file *file, struct address_space *mapping,
                         loff_t pos, unsigned len, unsigned copied,
                         struct page *page, void *fsdata)
 {
         struct folio *folio = page_folio(page);
         size_t start = pos - folio_pos(folio);
 
         if (unlikely(copied < len)) {
                 /*
                  * The buffers that were written will now be uptodate, so
                  * we don't have to worry about a read_folio reading them
                  * and overwriting a partial write. However if we have
                  * encountered a short write and only partially written
                  * into a buffer, it will not be marked uptodate, so a
                  * read_folio might come in and destroy our partial write.
                  *
                  * Do the simplest thing, and just treat any short write to a
                  * non uptodate folio as a zero-length write, and force the
                  * caller to redo the whole thing.
                  */
                 if (!folio_test_uptodate(folio))
                         copied = 0;
 
                 folio_zero_new_buffers(folio, start+copied, start+len);
         }
         flush_dcache_folio(folio);
 
         /* This could be a short (even 0-length) commit */
         __block_commit_write(folio, start, start + copied);
 
         return copied;
 }
 EXPORT_SYMBOL(block_write_end);
 
 int generic_write_end(struct file *file, struct address_space *mapping,
                         loff_t pos, unsigned len, unsigned copied,
                         struct page *page, void *fsdata)
 {
         struct inode *inode = mapping->host;
         loff_t old_size = inode->i_size;
         bool i_size_changed = false;
 
         copied = block_write_end(file, mapping, pos, len, copied, page, fsdata);
 
         /*
          * No need to use i_size_read() here, the i_size cannot change under us
          * because we hold i_rwsem.
          *
          * But it's important to update i_size while still holding page lock:
          * page writeout could otherwise come in and zero beyond i_size.
          */
         if (pos + copied > inode->i_size) {
                 i_size_write(inode, pos + copied);
                 i_size_changed = true;
         }
 
         unlock_page(page);
         put_page(page);
 
         if (old_size < pos)
                 pagecache_isize_extended(inode, old_size, pos);
         /*
          * Don't mark the inode dirty under page lock. First, it unnecessarily
          * makes the holding time of page lock longer. Second, it forces lock
          * ordering of page lock and transaction start for journaling
          * filesystems.
          */
         if (i_size_changed)
                 mark_inode_dirty(inode);
         return copied;
 }
 EXPORT_SYMBOL(generic_write_end);
 
 /*
  * block_is_partially_uptodate checks whether buffers within a folio are
  * uptodate or not.
  *
  * Returns true if all buffers which correspond to the specified part
  * of the folio are uptodate.
  */
 bool block_is_partially_uptodate(struct folio *folio, size_t from, size_t count)
 {
         unsigned block_start, block_end, blocksize;
         unsigned to;
         struct buffer_head *bh, *head;
         bool ret = true;
 
         head = folio_buffers(folio);
         if (!head)
                 return false;
         blocksize = head->b_size;
         to = min_t(unsigned, folio_size(folio) - from, count);
         to = from + to;
         if (from < blocksize && to > folio_size(folio) - blocksize)
                 return false;
 
         bh = head;
         block_start = 0;
         do {
                 block_end = block_start + blocksize;
                 if (block_end > from && block_start < to) {
                         if (!buffer_uptodate(bh)) {
                                 ret = false;
                                 break;
                         }
                         if (block_end >= to)
                                 break;
                 }
                 block_start = block_end;
                 bh = bh->b_this_page;
         } while (bh != head);
 
         return ret;
 }
 EXPORT_SYMBOL(block_is_partially_uptodate);
 
 /*
  * Generic "read_folio" function for block devices that have the normal
  * get_block functionality. This is most of the block device filesystems.
  * Reads the folio asynchronously --- the unlock_buffer() and
  * set/clear_buffer_uptodate() functions propagate buffer state into the
  * folio once IO has completed.
  */
 int block_read_full_folio(struct folio *folio, get_block_t *get_block)
 {
         struct inode *inode = folio->mapping->host;
         sector_t iblock, lblock;
         struct buffer_head *bh, *head, *arr[MAX_BUF_PER_PAGE];
         size_t blocksize;
         int nr, i;
         int fully_mapped = 1;
         bool page_error = false;
         loff_t limit = i_size_read(inode);
 
         /* This is needed for ext4. */
         if (IS_ENABLED(CONFIG_FS_VERITY) && IS_VERITY(inode))
                 limit = inode->i_sb->s_maxbytes;
 
         VM_BUG_ON_FOLIO(folio_test_large(folio), folio);
 
         head = folio_create_buffers(folio, inode, 0);
         blocksize = head->b_size;
 
         iblock = div_u64(folio_pos(folio), blocksize);
         lblock = div_u64(limit + blocksize - 1, blocksize);
         bh = head;
         nr = 0;
         i = 0;
 
         do {
                 if (buffer_uptodate(bh))
                         continue;
 
                 if (!buffer_mapped(bh)) {
                         int err = 0;
 
                         fully_mapped = 0;
                         if (iblock < lblock) {
                                 WARN_ON(bh->b_size != blocksize);
                                 err = get_block(inode, iblock, bh, 0);
                                 if (err)
                                         page_error = true;
                         }
                         if (!buffer_mapped(bh)) {
                                 folio_zero_range(folio, i * blocksize,
                                                 blocksize);
                                 if (!err)
                                         set_buffer_uptodate(bh);
                                 continue;
                         }
                         /*
                          * get_block() might have updated the buffer
                          * synchronously
                          */
                         if (buffer_uptodate(bh))
                                 continue;
                 }
                 arr[nr++] = bh;
         } while (i++, iblock++, (bh = bh->b_this_page) != head);
 
         if (fully_mapped)
                 folio_set_mappedtodisk(folio);
 
         if (!nr) {
                 /*
                  * All buffers are uptodate or get_block() returned an
                  * error when trying to map them - we can finish the read.
                  */
                 folio_end_read(folio, !page_error);
                 return 0;
         }
 
         /* Stage two: lock the buffers */
         for (i = 0; i < nr; i++) {
                 bh = arr[i];
                 lock_buffer(bh);
                 mark_buffer_async_read(bh);
         }
 
         /*
          * Stage 3: start the IO.  Check for uptodateness
          * inside the buffer lock in case another process reading
          * the underlying blockdev brought it uptodate (the sct fix).
          */
         for (i = 0; i < nr; i++) {
                 bh = arr[i];
                 if (buffer_uptodate(bh))
                         end_buffer_async_read(bh, 1);
                 else
                         submit_bh(REQ_OP_READ, bh);
         }
         return 0;
 }
 EXPORT_SYMBOL(block_read_full_folio);
 
 /* utility function for filesystems that need to do work on expanding
  * truncates.  Uses filesystem pagecache writes to allow the filesystem to
  * deal with the hole.  
  */
 int generic_cont_expand_simple(struct inode *inode, loff_t size)
 {
         struct address_space *mapping = inode->i_mapping;
         const struct address_space_operations *aops = mapping->a_ops;
         struct page *page;
         void *fsdata = NULL;
         int err;
 
         err = inode_newsize_ok(inode, size);
         if (err)
                 goto out;
 
         err = aops->write_begin(NULL, mapping, size, 0, &page, &fsdata);
         if (err)
                 goto out;
 
         err = aops->write_end(NULL, mapping, size, 0, 0, page, fsdata);
         BUG_ON(err > 0);
 
 out:
         return err;
 }
 EXPORT_SYMBOL(generic_cont_expand_simple);
 
 static int cont_expand_zero(struct file *file, struct address_space *mapping,
                             loff_t pos, loff_t *bytes)
 {
         struct inode *inode = mapping->host;
         const struct address_space_operations *aops = mapping->a_ops;
         unsigned int blocksize = i_blocksize(inode);
         struct page *page;
         void *fsdata = NULL;
         pgoff_t index, curidx;
         loff_t curpos;
         unsigned zerofrom, offset, len;
         int err = 0;
 
         index = pos >> PAGE_SHIFT;
         offset = pos & ~PAGE_MASK;
 
         while (index > (curidx = (curpos = *bytes)>>PAGE_SHIFT)) {
                 zerofrom = curpos & ~PAGE_MASK;
                 if (zerofrom & (blocksize-1)) {
                         *bytes |= (blocksize-1);
                         (*bytes)++;
                 }
                 len = PAGE_SIZE - zerofrom;
 
                 err = aops->write_begin(file, mapping, curpos, len,
                                             &page, &fsdata);
                 if (err)
                         goto out;
                 zero_user(page, zerofrom, len);
                 err = aops->write_end(file, mapping, curpos, len, len,
                                                 page, fsdata);
                 if (err < 0)
                         goto out;
                 BUG_ON(err != len);
                 err = 0;
 
                 balance_dirty_pages_ratelimited(mapping);
 
                 if (fatal_signal_pending(current)) {
                         err = -EINTR;
                         goto out;
                 }
         }
 
         /* page covers the boundary, find the boundary offset */
         if (index == curidx) {
                 zerofrom = curpos & ~PAGE_MASK;
                 /* if we will expand the thing last block will be filled */
                 if (offset <= zerofrom) {
                         goto out;
                 }
                 if (zerofrom & (blocksize-1)) {
                         *bytes |= (blocksize-1);
                         (*bytes)++;
                 }
                 len = offset - zerofrom;
 
                 err = aops->write_begin(file, mapping, curpos, len,
                                             &page, &fsdata);
                 if (err)
                         goto out;
                 zero_user(page, zerofrom, len);
                 err = aops->write_end(file, mapping, curpos, len, len,
                                                 page, fsdata);
                 if (err < 0)
                         goto out;
                 BUG_ON(err != len);
                 err = 0;
         }
 out:
         return err;
 }
 
 /*
  * For moronic filesystems that do not allow holes in file.
  * We may have to extend the file.
  */
 int cont_write_begin(struct file *file, struct address_space *mapping,
                         loff_t pos, unsigned len,
                         struct page **pagep, void **fsdata,
                         get_block_t *get_block, loff_t *bytes)
 {
         struct inode *inode = mapping->host;
         unsigned int blocksize = i_blocksize(inode);
         unsigned int zerofrom;
         int err;
 
         err = cont_expand_zero(file, mapping, pos, bytes);
         if (err)
                 return err;
 
         zerofrom = *bytes & ~PAGE_MASK;
         if (pos+len > *bytes && zerofrom & (blocksize-1)) {
                 *bytes |= (blocksize-1);
                 (*bytes)++;
         }
 
         return block_write_begin(mapping, pos, len, pagep, get_block);
 }
 EXPORT_SYMBOL(cont_write_begin);
 
 void block_commit_write(struct page *page, unsigned from, unsigned to)
 {
         struct folio *folio = page_folio(page);
         __block_commit_write(folio, from, to);
 }
 EXPORT_SYMBOL(block_commit_write);
 
 /*
  * block_page_mkwrite() is not allowed to change the file size as it gets
  * called from a page fault handler when a page is first dirtied. Hence we must
  * be careful to check for EOF conditions here. We set the page up correctly
  * for a written page which means we get ENOSPC checking when writing into
  * holes and correct delalloc and unwritten extent mapping on filesystems that
  * support these features.
  *
  * We are not allowed to take the i_mutex here so we have to play games to
  * protect against truncate races as the page could now be beyond EOF.  Because
  * truncate writes the inode size before removing pages, once we have the
  * page lock we can determine safely if the page is beyond EOF. If it is not
  * beyond EOF, then the page is guaranteed safe against truncation until we
  * unlock the page.
  *
  * Direct callers of this function should protect against filesystem freezing
  * using sb_start_pagefault() - sb_end_pagefault() functions.
  */
 int block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
                          get_block_t get_block)
 {
         struct folio *folio = page_folio(vmf->page);
         struct inode *inode = file_inode(vma->vm_file);
         unsigned long end;
         loff_t size;
         int ret;
 
         folio_lock(folio);
         size = i_size_read(inode);
         if ((folio->mapping != inode->i_mapping) ||
             (folio_pos(folio) >= size)) {
                 /* We overload EFAULT to mean page got truncated */
                 ret = -EFAULT;
                 goto out_unlock;
         }
 
         end = folio_size(folio);
         /* folio is wholly or partially inside EOF */
         if (folio_pos(folio) + end > size)
                 end = size - folio_pos(folio);
 
         ret = __block_write_begin_int(folio, 0, end, get_block, NULL);
         if (unlikely(ret))
                 goto out_unlock;
 
         __block_commit_write(folio, 0, end);
 
         folio_mark_dirty(folio);
         folio_wait_stable(folio);
         return 0;
 out_unlock:
         folio_unlock(folio);
         return ret;
 }
 EXPORT_SYMBOL(block_page_mkwrite);
 
 int block_truncate_page(struct address_space *mapping,
                         loff_t from, get_block_t *get_block)
 {
         pgoff_t index = from >> PAGE_SHIFT;
         unsigned blocksize;
         sector_t iblock;
         size_t offset, length, pos;
         struct inode *inode = mapping->host;
         struct folio *folio;
         struct buffer_head *bh;
         int err = 0;
 
         blocksize = i_blocksize(inode);
         length = from & (blocksize - 1);
 
         /* Block boundary? Nothing to do */
         if (!length)
                 return 0;
 
         length = blocksize - length;
         iblock = ((loff_t)index * PAGE_SIZE) >> inode->i_blkbits;
 
         folio = filemap_grab_folio(mapping, index);
         if (IS_ERR(folio))
                 return PTR_ERR(folio);
 
         bh = folio_buffers(folio);
         if (!bh)
                 bh = create_empty_buffers(folio, blocksize, 0);
 
         /* Find the buffer that contains "offset" */
         offset = offset_in_folio(folio, from);
         pos = blocksize;
         while (offset >= pos) {
                 bh = bh->b_this_page;
                 iblock++;
                 pos += blocksize;
         }
 
         if (!buffer_mapped(bh)) {
                 WARN_ON(bh->b_size != blocksize);
                 err = get_block(inode, iblock, bh, 0);
                 if (err)
                         goto unlock;
                 /* unmapped? It's a hole - nothing to do */
                 if (!buffer_mapped(bh))
                         goto unlock;
         }
 
         /* Ok, it's mapped. Make sure it's up-to-date */
         if (folio_test_uptodate(folio))
                 set_buffer_uptodate(bh);
 
         if (!buffer_uptodate(bh) && !buffer_delay(bh) && !buffer_unwritten(bh)) {
                 err = bh_read(bh, 0);
                 /* Uhhuh. Read error. Complain and punt. */
                 if (err < 0)
                         goto unlock;
         }
 
         folio_zero_range(folio, offset, length);
         mark_buffer_dirty(bh);
 
 unlock:
         folio_unlock(folio);
         folio_put(folio);
 
         return err;
 }
 EXPORT_SYMBOL(block_truncate_page);
 
 /*
  * The generic ->writepage function for buffer-backed address_spaces
  */
 int block_write_full_folio(struct folio *folio, struct writeback_control *wbc,
                 void *get_block)
 {
         struct inode * const inode = folio->mapping->host;
         loff_t i_size = i_size_read(inode);
 
         /* Is the folio fully inside i_size? */
         if (folio_pos(folio) + folio_size(folio) <= i_size)
                 return __block_write_full_folio(inode, folio, get_block, wbc);
 
         /* Is the folio fully outside i_size? (truncate in progress) */
         if (folio_pos(folio) >= i_size) {
                 folio_unlock(folio);
                 return 0; /* don't care */
         }
 
         /*
          * The folio straddles i_size.  It must be zeroed out on each and every
          * writepage invocation because it may be mmapped.  "A file is mapped
          * in multiples of the page size.  For a file that is not a multiple of
          * the page size, the remaining memory is zeroed when mapped, and
          * writes to that region are not written out to the file."
          */
         folio_zero_segment(folio, offset_in_folio(folio, i_size),
                         folio_size(folio));
         return __block_write_full_folio(inode, folio, get_block, wbc);
 }
 
 sector_t generic_block_bmap(struct address_space *mapping, sector_t block,
                             get_block_t *get_block)
 {
         struct inode *inode = mapping->host;
         struct buffer_head tmp = {
                 .b_size = i_blocksize(inode),
         };
 
         get_block(inode, block, &tmp, 0);
         return tmp.b_blocknr;
 }
 EXPORT_SYMBOL(generic_block_bmap);
 
 static void end_bio_bh_io_sync(struct bio *bio)
 {
         struct buffer_head *bh = bio->bi_private;
 
         if (unlikely(bio_flagged(bio, BIO_QUIET)))
                 set_bit(BH_Quiet, &bh->b_state);
 
         bh->b_end_io(bh, !bio->bi_status);
         bio_put(bio);
 }
 
 static void submit_bh_wbc(blk_opf_t opf, struct buffer_head *bh,
                           enum rw_hint write_hint,
                           struct writeback_control *wbc)
 {
         const enum req_op op = opf & REQ_OP_MASK;
         struct bio *bio;
 
         BUG_ON(!buffer_locked(bh));
         BUG_ON(!buffer_mapped(bh));
         BUG_ON(!bh->b_end_io);
         BUG_ON(buffer_delay(bh));
         BUG_ON(buffer_unwritten(bh));
 
         /*
          * Only clear out a write error when rewriting
          */
         if (test_set_buffer_req(bh) && (op == REQ_OP_WRITE))
                 clear_buffer_write_io_error(bh);
 
         if (buffer_meta(bh))
                 opf |= REQ_META;
         if (buffer_prio(bh))
                 opf |= REQ_PRIO;
 
         bio = bio_alloc(bh->b_bdev, 1, opf, GFP_NOIO);
 
         fscrypt_set_bio_crypt_ctx_bh(bio, bh, GFP_NOIO);
 
         bio->bi_iter.bi_sector = bh->b_blocknr * (bh->b_size >> 9);
         bio->bi_write_hint = write_hint;
 
         __bio_add_page(bio, bh->b_page, bh->b_size, bh_offset(bh));
 
         bio->bi_end_io = end_bio_bh_io_sync;
         bio->bi_private = bh;
 
         /* Take care of bh's that straddle the end of the device */
         guard_bio_eod(bio);
 
         if (wbc) {
                 wbc_init_bio(wbc, bio);
                 wbc_account_cgroup_owner(wbc, bh->b_page, bh->b_size);
         }
 
         submit_bio(bio);
 }
 
 void submit_bh(blk_opf_t opf, struct buffer_head *bh)
 {
         submit_bh_wbc(opf, bh, WRITE_LIFE_NOT_SET, NULL);
 }
 EXPORT_SYMBOL(submit_bh);
 
 void write_dirty_buffer(struct buffer_head *bh, blk_opf_t op_flags)
 {
         lock_buffer(bh);
         if (!test_clear_buffer_dirty(bh)) {
                 unlock_buffer(bh);
                 return;
         }
         bh->b_end_io = end_buffer_write_sync;
         get_bh(bh);
         submit_bh(REQ_OP_WRITE | op_flags, bh);
 }
 EXPORT_SYMBOL(write_dirty_buffer);
 
 /*
  * For a data-integrity writeout, we need to wait upon any in-progress I/O
  * and then start new I/O and then wait upon it.  The caller must have a ref on
  * the buffer_head.
  */
 int __sync_dirty_buffer(struct buffer_head *bh, blk_opf_t op_flags)
 {
         WARN_ON(atomic_read(&bh->b_count) < 1);
         lock_buffer(bh);
         if (test_clear_buffer_dirty(bh)) {
                 /*
                  * The bh should be mapped, but it might not be if the
                  * device was hot-removed. Not much we can do but fail the I/O.
                  */
                 if (!buffer_mapped(bh)) {
                         unlock_buffer(bh);
                         return -EIO;
                 }
 
                 get_bh(bh);
                 bh->b_end_io = end_buffer_write_sync;
                 submit_bh(REQ_OP_WRITE | op_flags, bh);
                 wait_on_buffer(bh);
                 if (!buffer_uptodate(bh))
                         return -EIO;
         } else {
                 unlock_buffer(bh);
         }
         return 0;
 }
 EXPORT_SYMBOL(__sync_dirty_buffer);
 
 int sync_dirty_buffer(struct buffer_head *bh)
 {
         return __sync_dirty_buffer(bh, REQ_SYNC);
 }
 EXPORT_SYMBOL(sync_dirty_buffer);
 
 static inline int buffer_busy(struct buffer_head *bh)
 {
         return atomic_read(&bh->b_count) |
                 (bh->b_state & ((1 << BH_Dirty) | (1 << BH_Lock)));
 }
 
 static bool
 drop_buffers(struct folio *folio, struct buffer_head **buffers_to_free)
 {
         struct buffer_head *head = folio_buffers(folio);
         struct buffer_head *bh;
 
         bh = head;
         do {
                 if (buffer_busy(bh))
                         goto failed;
                 bh = bh->b_this_page;
         } while (bh != head);
 
         do {
                 struct buffer_head *next = bh->b_this_page;
 
                 if (bh->b_assoc_map)
                         __remove_assoc_queue(bh);
                 bh = next;
         } while (bh != head);
         *buffers_to_free = head;
         folio_detach_private(folio);
         return true;
 failed:
         return false;
 }
 
 /**
  * try_to_free_buffers - Release buffers attached to this folio.
  * @folio: The folio.
  *
  * If any buffers are in use (dirty, under writeback, elevated refcount),
  * no buffers will be freed.
  *
  * If the folio is dirty but all the buffers are clean then we need to
  * be sure to mark the folio clean as well.  This is because the folio
  * may be against a block device, and a later reattachment of buffers
  * to a dirty folio will set *all* buffers dirty.  Which would corrupt
  * filesystem data on the same device.
  *
  * The same applies to regular filesystem folios: if all the buffers are
  * clean then we set the folio clean and proceed.  To do that, we require
  * total exclusion from block_dirty_folio().  That is obtained with
  * i_private_lock.
  *
  * Exclusion against try_to_free_buffers may be obtained by either
  * locking the folio or by holding its mapping's i_private_lock.
  *
  * Context: Process context.  @folio must be locked.  Will not sleep.
  * Return: true if all buffers attached to this folio were freed.
  */
 bool try_to_free_buffers(struct folio *folio)
 {
         struct address_space * const mapping = folio->mapping;
         struct buffer_head *buffers_to_free = NULL;
         bool ret = 0;
 
         BUG_ON(!folio_test_locked(folio));
         if (folio_test_writeback(folio))
                 return false;
 
         if (mapping == NULL) {          /* can this still happen? */
                 ret = drop_buffers(folio, &buffers_to_free);
                 goto out;
         }
 
         spin_lock(&mapping->i_private_lock);
         ret = drop_buffers(folio, &buffers_to_free);
 
         /*
          * If the filesystem writes its buffers by hand (eg ext3)
          * then we can have clean buffers against a dirty folio.  We
          * clean the folio here; otherwise the VM will never notice
          * that the filesystem did any IO at all.
          *
          * Also, during truncate, discard_buffer will have marked all
          * the folio's buffers clean.  We discover that here and clean
          * the folio also.
          *
          * i_private_lock must be held over this entire operation in order
          * to synchronise against block_dirty_folio and prevent the
          * dirty bit from being lost.
          */
         if (ret)
                 folio_cancel_dirty(folio);
         spin_unlock(&mapping->i_private_lock);
 out:
         if (buffers_to_free) {
                 struct buffer_head *bh = buffers_to_free;
 
                 do {
                         struct buffer_head *next = bh->b_this_page;
                         free_buffer_head(bh);
                         bh = next;
                 } while (bh != buffers_to_free);
         }
         return ret;
 }
 EXPORT_SYMBOL(try_to_free_buffers);
 
 /*
  * Buffer-head allocation
  */
 static struct kmem_cache *bh_cachep __ro_after_init;
 
 /*
  * Once the number of bh's in the machine exceeds this level, we start
  * stripping them in writeback.
  */
 static unsigned long max_buffer_heads __ro_after_init;
 
 int buffer_heads_over_limit;
 
 struct bh_accounting {
         int nr;                 /* Number of live bh's */
         int ratelimit;          /* Limit cacheline bouncing */
 };
 
 static DEFINE_PER_CPU(struct bh_accounting, bh_accounting) = {0, 0};
 
 static void recalc_bh_state(void)
 {
         int i;
         int tot = 0;
 
         if (__this_cpu_inc_return(bh_accounting.ratelimit) - 1 < 4096)
                 return;
         __this_cpu_write(bh_accounting.ratelimit, 0);
         for_each_online_cpu(i)
                 tot += per_cpu(bh_accounting, i).nr;
         buffer_heads_over_limit = (tot > max_buffer_heads);
 }
 
 struct buffer_head *alloc_buffer_head(gfp_t gfp_flags)
 {
         struct buffer_head *ret = kmem_cache_zalloc(bh_cachep, gfp_flags);
         if (ret) {
                 INIT_LIST_HEAD(&ret->b_assoc_buffers);
                 spin_lock_init(&ret->b_uptodate_lock);
                 preempt_disable();
                 __this_cpu_inc(bh_accounting.nr);
                 recalc_bh_state();
                 preempt_enable();
         }
         return ret;
 }
 EXPORT_SYMBOL(alloc_buffer_head);
 
 void free_buffer_head(struct buffer_head *bh)
 {
         BUG_ON(!list_empty(&bh->b_assoc_buffers));
         kmem_cache_free(bh_cachep, bh);
         preempt_disable();
         __this_cpu_dec(bh_accounting.nr);
         recalc_bh_state();
         preempt_enable();
 }
 EXPORT_SYMBOL(free_buffer_head);
 
 static int buffer_exit_cpu_dead(unsigned int cpu)
 {
         int i;
         struct bh_lru *b = &per_cpu(bh_lrus, cpu);
 
         for (i = 0; i < BH_LRU_SIZE; i++) {
                 brelse(b->bhs[i]);
                 b->bhs[i] = NULL;
         }
         this_cpu_add(bh_accounting.nr, per_cpu(bh_accounting, cpu).nr);
         per_cpu(bh_accounting, cpu).nr = 0;
         return 0;
 }
 
 /**
  * bh_uptodate_or_lock - Test whether the buffer is uptodate
  * @bh: struct buffer_head
  *
  * Return true if the buffer is up-to-date and false,
  * with the buffer locked, if not.
  */
 int bh_uptodate_or_lock(struct buffer_head *bh)
 {
         if (!buffer_uptodate(bh)) {
                 lock_buffer(bh);
                 if (!buffer_uptodate(bh))
                         return 0;
                 unlock_buffer(bh);
         }
         return 1;
 }
 EXPORT_SYMBOL(bh_uptodate_or_lock);
 
 /**
  * __bh_read - Submit read for a locked buffer
  * @bh: struct buffer_head
  * @op_flags: appending REQ_OP_* flags besides REQ_OP_READ
  * @wait: wait until reading finish
  *
  * Returns zero on success or don't wait, and -EIO on error.
  */
 int __bh_read(struct buffer_head *bh, blk_opf_t op_flags, bool wait)
 {
         int ret = 0;
 
         BUG_ON(!buffer_locked(bh));
 
         get_bh(bh);
         bh->b_end_io = end_buffer_read_sync;
         submit_bh(REQ_OP_READ | op_flags, bh);
         if (wait) {
                 wait_on_buffer(bh);
                 if (!buffer_uptodate(bh))
                         ret = -EIO;
         }
         return ret;
 }
 EXPORT_SYMBOL(__bh_read);
 
 /**
  * __bh_read_batch - Submit read for a batch of unlocked buffers
  * @nr: entry number of the buffer batch
  * @bhs: a batch of struct buffer_head
  * @op_flags: appending REQ_OP_* flags besides REQ_OP_READ
  * @force_lock: force to get a lock on the buffer if set, otherwise drops any
  *              buffer that cannot lock.
  *
  * Returns zero on success or don't wait, and -EIO on error.
  */
 void __bh_read_batch(int nr, struct buffer_head *bhs[],
                      blk_opf_t op_flags, bool force_lock)
 {
         int i;
 
         for (i = 0; i < nr; i++) {
                 struct buffer_head *bh = bhs[i];
 
                 if (buffer_uptodate(bh))
                         continue;
 
                 if (force_lock)
                         lock_buffer(bh);
                 else
                         if (!trylock_buffer(bh))
                                 continue;
 
                 if (buffer_uptodate(bh)) {
                         unlock_buffer(bh);
                         continue;
                 }
 
                 bh->b_end_io = end_buffer_read_sync;
                 get_bh(bh);
                 submit_bh(REQ_OP_READ | op_flags, bh);
         }
 }
 EXPORT_SYMBOL(__bh_read_batch);
 
 void __init buffer_init(void)
 {
         unsigned long nrpages;
         int ret;
 
         bh_cachep = KMEM_CACHE(buffer_head,
                                 SLAB_RECLAIM_ACCOUNT|SLAB_PANIC);
         /*
          * Limit the bh occupancy to 10% of ZONE_NORMAL
          */
         nrpages = (nr_free_buffer_pages() * 10) / 100;
         max_buffer_heads = nrpages * (PAGE_SIZE / sizeof(struct buffer_head));
         ret = cpuhp_setup_state_nocalls(CPUHP_FS_BUFF_DEAD, "fs/buffer:dead",
                                         NULL, buffer_exit_cpu_dead);
         WARN_ON(ret < 0);
 }
 

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