TOMOYO Linux Cross Reference
Linux/fs/super.c

   // SPDX-License-Identifier: GPL-2.0
   /*
    *  linux/fs/super.c
    *
    *  Copyright (C) 1991, 1992  Linus Torvalds
    *
    *  super.c contains code to handle: - mount structures
    *                                   - super-block tables
    *                                   - filesystem drivers list
   *                                   - mount system call
   *                                   - umount system call
   *                                   - ustat system call
   *
   * GK 2/5/95  -  Changed to support mounting the root fs via NFS
   *
   *  Added kerneld support: Jacques Gelinas and Bjorn Ekwall
   *  Added change_root: Werner Almesberger & Hans Lermen, Feb '96
   *  Added options to /proc/mounts:
   *    Torbjörn Lindh (torbjorn.lindh@gopta.se), April 14, 1996.
   *  Added devfs support: Richard Gooch <rgooch@atnf.csiro.au>, 13-JAN-1998
   *  Heavily rewritten for 'one fs - one tree' dcache architecture. AV, Mar 2000
   */
  
  #include <linux/export.h>
  #include <linux/slab.h>
  #include <linux/blkdev.h>
  #include <linux/mount.h>
  #include <linux/security.h>
  #include <linux/writeback.h>            /* for the emergency remount stuff */
  #include <linux/idr.h>
  #include <linux/mutex.h>
  #include <linux/backing-dev.h>
  #include <linux/rculist_bl.h>
  #include <linux/fscrypt.h>
  #include <linux/fsnotify.h>
  #include <linux/lockdep.h>
  #include <linux/user_namespace.h>
  #include <linux/fs_context.h>
  #include <uapi/linux/mount.h>
  #include "internal.h"
  
  static int thaw_super_locked(struct super_block *sb, enum freeze_holder who);
  
  static LIST_HEAD(super_blocks);
  static DEFINE_SPINLOCK(sb_lock);
  
  static char *sb_writers_name[SB_FREEZE_LEVELS] = {
          "sb_writers",
          "sb_pagefaults",
          "sb_internal",
  };
  
  static inline void __super_lock(struct super_block *sb, bool excl)
  {
          if (excl)
                  down_write(&sb->s_umount);
          else
                  down_read(&sb->s_umount);
  }
  
  static inline void super_unlock(struct super_block *sb, bool excl)
  {
          if (excl)
                  up_write(&sb->s_umount);
          else
                  up_read(&sb->s_umount);
  }
  
  static inline void __super_lock_excl(struct super_block *sb)
  {
          __super_lock(sb, true);
  }
  
  static inline void super_unlock_excl(struct super_block *sb)
  {
          super_unlock(sb, true);
  }
  
  static inline void super_unlock_shared(struct super_block *sb)
  {
          super_unlock(sb, false);
  }
  
  static bool super_flags(const struct super_block *sb, unsigned int flags)
  {
          /*
           * Pairs with smp_store_release() in super_wake() and ensures
           * that we see @flags after we're woken.
           */
          return smp_load_acquire(&sb->s_flags) & flags;
  }
  
  /**
   * super_lock - wait for superblock to become ready and lock it
   * @sb: superblock to wait for
   * @excl: whether exclusive access is required
   *
   * If the superblock has neither passed through vfs_get_tree() or
   * generic_shutdown_super() yet wait for it to happen. Either superblock
  * creation will succeed and SB_BORN is set by vfs_get_tree() or we're
  * woken and we'll see SB_DYING.
  *
  * The caller must have acquired a temporary reference on @sb->s_count.
  *
  * Return: The function returns true if SB_BORN was set and with
  *         s_umount held. The function returns false if SB_DYING was
  *         set and without s_umount held.
  */
 static __must_check bool super_lock(struct super_block *sb, bool excl)
 {
         lockdep_assert_not_held(&sb->s_umount);
 
         /* wait until the superblock is ready or dying */
         wait_var_event(&sb->s_flags, super_flags(sb, SB_BORN | SB_DYING));
 
         /* Don't pointlessly acquire s_umount. */
         if (super_flags(sb, SB_DYING))
                 return false;
 
         __super_lock(sb, excl);
 
         /*
          * Has gone through generic_shutdown_super() in the meantime.
          * @sb->s_root is NULL and @sb->s_active is 0. No one needs to
          * grab a reference to this. Tell them so.
          */
         if (sb->s_flags & SB_DYING) {
                 super_unlock(sb, excl);
                 return false;
         }
 
         WARN_ON_ONCE(!(sb->s_flags & SB_BORN));
         return true;
 }
 
 /* wait and try to acquire read-side of @sb->s_umount */
 static inline bool super_lock_shared(struct super_block *sb)
 {
         return super_lock(sb, false);
 }
 
 /* wait and try to acquire write-side of @sb->s_umount */
 static inline bool super_lock_excl(struct super_block *sb)
 {
         return super_lock(sb, true);
 }
 
 /* wake waiters */
 #define SUPER_WAKE_FLAGS (SB_BORN | SB_DYING | SB_DEAD)
 static void super_wake(struct super_block *sb, unsigned int flag)
 {
         WARN_ON_ONCE((flag & ~SUPER_WAKE_FLAGS));
         WARN_ON_ONCE(hweight32(flag & SUPER_WAKE_FLAGS) > 1);
 
         /*
          * Pairs with smp_load_acquire() in super_lock() to make sure
          * all initializations in the superblock are seen by the user
          * seeing SB_BORN sent.
          */
         smp_store_release(&sb->s_flags, sb->s_flags | flag);
         /*
          * Pairs with the barrier in prepare_to_wait_event() to make sure
          * ___wait_var_event() either sees SB_BORN set or
          * waitqueue_active() check in wake_up_var() sees the waiter.
          */
         smp_mb();
         wake_up_var(&sb->s_flags);
 }
 
 /*
  * One thing we have to be careful of with a per-sb shrinker is that we don't
  * drop the last active reference to the superblock from within the shrinker.
  * If that happens we could trigger unregistering the shrinker from within the
  * shrinker path and that leads to deadlock on the shrinker_mutex. Hence we
  * take a passive reference to the superblock to avoid this from occurring.
  */
 static unsigned long super_cache_scan(struct shrinker *shrink,
                                       struct shrink_control *sc)
 {
         struct super_block *sb;
         long    fs_objects = 0;
         long    total_objects;
         long    freed = 0;
         long    dentries;
         long    inodes;
 
         sb = shrink->private_data;
 
         /*
          * Deadlock avoidance.  We may hold various FS locks, and we don't want
          * to recurse into the FS that called us in clear_inode() and friends..
          */
         if (!(sc->gfp_mask & __GFP_FS))
                 return SHRINK_STOP;
 
         if (!super_trylock_shared(sb))
                 return SHRINK_STOP;
 
         if (sb->s_op->nr_cached_objects)
                 fs_objects = sb->s_op->nr_cached_objects(sb, sc);
 
         inodes = list_lru_shrink_count(&sb->s_inode_lru, sc);
         dentries = list_lru_shrink_count(&sb->s_dentry_lru, sc);
         total_objects = dentries + inodes + fs_objects + 1;
         if (!total_objects)
                 total_objects = 1;
 
         /* proportion the scan between the caches */
         dentries = mult_frac(sc->nr_to_scan, dentries, total_objects);
         inodes = mult_frac(sc->nr_to_scan, inodes, total_objects);
         fs_objects = mult_frac(sc->nr_to_scan, fs_objects, total_objects);
 
         /*
          * prune the dcache first as the icache is pinned by it, then
          * prune the icache, followed by the filesystem specific caches
          *
          * Ensure that we always scan at least one object - memcg kmem
          * accounting uses this to fully empty the caches.
          */
         sc->nr_to_scan = dentries + 1;
         freed = prune_dcache_sb(sb, sc);
         sc->nr_to_scan = inodes + 1;
         freed += prune_icache_sb(sb, sc);
 
         if (fs_objects) {
                 sc->nr_to_scan = fs_objects + 1;
                 freed += sb->s_op->free_cached_objects(sb, sc);
         }
 
         super_unlock_shared(sb);
         return freed;
 }
 
 static unsigned long super_cache_count(struct shrinker *shrink,
                                        struct shrink_control *sc)
 {
         struct super_block *sb;
         long    total_objects = 0;
 
         sb = shrink->private_data;
 
         /*
          * We don't call super_trylock_shared() here as it is a scalability
          * bottleneck, so we're exposed to partial setup state. The shrinker
          * rwsem does not protect filesystem operations backing
          * list_lru_shrink_count() or s_op->nr_cached_objects(). Counts can
          * change between super_cache_count and super_cache_scan, so we really
          * don't need locks here.
          *
          * However, if we are currently mounting the superblock, the underlying
          * filesystem might be in a state of partial construction and hence it
          * is dangerous to access it.  super_trylock_shared() uses a SB_BORN check
          * to avoid this situation, so do the same here. The memory barrier is
          * matched with the one in mount_fs() as we don't hold locks here.
          */
         if (!(sb->s_flags & SB_BORN))
                 return 0;
         smp_rmb();
 
         if (sb->s_op && sb->s_op->nr_cached_objects)
                 total_objects = sb->s_op->nr_cached_objects(sb, sc);
 
         total_objects += list_lru_shrink_count(&sb->s_dentry_lru, sc);
         total_objects += list_lru_shrink_count(&sb->s_inode_lru, sc);
 
         if (!total_objects)
                 return SHRINK_EMPTY;
 
         total_objects = vfs_pressure_ratio(total_objects);
         return total_objects;
 }
 
 static void destroy_super_work(struct work_struct *work)
 {
         struct super_block *s = container_of(work, struct super_block,
                                                         destroy_work);
         fsnotify_sb_free(s);
         security_sb_free(s);
         put_user_ns(s->s_user_ns);
         kfree(s->s_subtype);
         for (int i = 0; i < SB_FREEZE_LEVELS; i++)
                 percpu_free_rwsem(&s->s_writers.rw_sem[i]);
         kfree(s);
 }
 
 static void destroy_super_rcu(struct rcu_head *head)
 {
         struct super_block *s = container_of(head, struct super_block, rcu);
         INIT_WORK(&s->destroy_work, destroy_super_work);
         schedule_work(&s->destroy_work);
 }
 
 /* Free a superblock that has never been seen by anyone */
 static void destroy_unused_super(struct super_block *s)
 {
         if (!s)
                 return;
         super_unlock_excl(s);
         list_lru_destroy(&s->s_dentry_lru);
         list_lru_destroy(&s->s_inode_lru);
         shrinker_free(s->s_shrink);
         /* no delays needed */
         destroy_super_work(&s->destroy_work);
 }
 
 /**
  *      alloc_super     -       create new superblock
  *      @type:  filesystem type superblock should belong to
  *      @flags: the mount flags
  *      @user_ns: User namespace for the super_block
  *
  *      Allocates and initializes a new &struct super_block.  alloc_super()
  *      returns a pointer new superblock or %NULL if allocation had failed.
  */
 static struct super_block *alloc_super(struct file_system_type *type, int flags,
                                        struct user_namespace *user_ns)
 {
         struct super_block *s = kzalloc(sizeof(struct super_block), GFP_KERNEL);
         static const struct super_operations default_op;
         int i;
 
         if (!s)
                 return NULL;
 
         INIT_LIST_HEAD(&s->s_mounts);
         s->s_user_ns = get_user_ns(user_ns);
         init_rwsem(&s->s_umount);
         lockdep_set_class(&s->s_umount, &type->s_umount_key);
         /*
          * sget() can have s_umount recursion.
          *
          * When it cannot find a suitable sb, it allocates a new
          * one (this one), and tries again to find a suitable old
          * one.
          *
          * In case that succeeds, it will acquire the s_umount
          * lock of the old one. Since these are clearly distrinct
          * locks, and this object isn't exposed yet, there's no
          * risk of deadlocks.
          *
          * Annotate this by putting this lock in a different
          * subclass.
          */
         down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING);
 
         if (security_sb_alloc(s))
                 goto fail;
 
         for (i = 0; i < SB_FREEZE_LEVELS; i++) {
                 if (__percpu_init_rwsem(&s->s_writers.rw_sem[i],
                                         sb_writers_name[i],
                                         &type->s_writers_key[i]))
                         goto fail;
         }
         s->s_bdi = &noop_backing_dev_info;
         s->s_flags = flags;
         if (s->s_user_ns != &init_user_ns)
                 s->s_iflags |= SB_I_NODEV;
         INIT_HLIST_NODE(&s->s_instances);
         INIT_HLIST_BL_HEAD(&s->s_roots);
         mutex_init(&s->s_sync_lock);
         INIT_LIST_HEAD(&s->s_inodes);
         spin_lock_init(&s->s_inode_list_lock);
         INIT_LIST_HEAD(&s->s_inodes_wb);
         spin_lock_init(&s->s_inode_wblist_lock);
 
         s->s_count = 1;
         atomic_set(&s->s_active, 1);
         mutex_init(&s->s_vfs_rename_mutex);
         lockdep_set_class(&s->s_vfs_rename_mutex, &type->s_vfs_rename_key);
         init_rwsem(&s->s_dquot.dqio_sem);
         s->s_maxbytes = MAX_NON_LFS;
         s->s_op = &default_op;
         s->s_time_gran = 1000000000;
         s->s_time_min = TIME64_MIN;
         s->s_time_max = TIME64_MAX;
 
         s->s_shrink = shrinker_alloc(SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE,
                                      "sb-%s", type->name);
         if (!s->s_shrink)
                 goto fail;
 
         s->s_shrink->scan_objects = super_cache_scan;
         s->s_shrink->count_objects = super_cache_count;
         s->s_shrink->batch = 1024;
         s->s_shrink->private_data = s;
 
         if (list_lru_init_memcg(&s->s_dentry_lru, s->s_shrink))
                 goto fail;
         if (list_lru_init_memcg(&s->s_inode_lru, s->s_shrink))
                 goto fail;
         return s;
 
 fail:
         destroy_unused_super(s);
         return NULL;
 }
 
 /* Superblock refcounting  */
 
 /*
  * Drop a superblock's refcount.  The caller must hold sb_lock.
  */
 static void __put_super(struct super_block *s)
 {
         if (!--s->s_count) {
                 list_del_init(&s->s_list);
                 WARN_ON(s->s_dentry_lru.node);
                 WARN_ON(s->s_inode_lru.node);
                 WARN_ON(!list_empty(&s->s_mounts));
                 call_rcu(&s->rcu, destroy_super_rcu);
         }
 }
 
 /**
  *      put_super       -       drop a temporary reference to superblock
  *      @sb: superblock in question
  *
  *      Drops a temporary reference, frees superblock if there's no
  *      references left.
  */
 void put_super(struct super_block *sb)
 {
         spin_lock(&sb_lock);
         __put_super(sb);
         spin_unlock(&sb_lock);
 }
 
 static void kill_super_notify(struct super_block *sb)
 {
         lockdep_assert_not_held(&sb->s_umount);
 
         /* already notified earlier */
         if (sb->s_flags & SB_DEAD)
                 return;
 
         /*
          * Remove it from @fs_supers so it isn't found by new
          * sget{_fc}() walkers anymore. Any concurrent mounter still
          * managing to grab a temporary reference is guaranteed to
          * already see SB_DYING and will wait until we notify them about
          * SB_DEAD.
          */
         spin_lock(&sb_lock);
         hlist_del_init(&sb->s_instances);
         spin_unlock(&sb_lock);
 
         /*
          * Let concurrent mounts know that this thing is really dead.
          * We don't need @sb->s_umount here as every concurrent caller
          * will see SB_DYING and either discard the superblock or wait
          * for SB_DEAD.
          */
         super_wake(sb, SB_DEAD);
 }
 
 /**
  *      deactivate_locked_super -       drop an active reference to superblock
  *      @s: superblock to deactivate
  *
  *      Drops an active reference to superblock, converting it into a temporary
  *      one if there is no other active references left.  In that case we
  *      tell fs driver to shut it down and drop the temporary reference we
  *      had just acquired.
  *
  *      Caller holds exclusive lock on superblock; that lock is released.
  */
 void deactivate_locked_super(struct super_block *s)
 {
         struct file_system_type *fs = s->s_type;
         if (atomic_dec_and_test(&s->s_active)) {
                 shrinker_free(s->s_shrink);
                 fs->kill_sb(s);
 
                 kill_super_notify(s);
 
                 /*
                  * Since list_lru_destroy() may sleep, we cannot call it from
                  * put_super(), where we hold the sb_lock. Therefore we destroy
                  * the lru lists right now.
                  */
                 list_lru_destroy(&s->s_dentry_lru);
                 list_lru_destroy(&s->s_inode_lru);
 
                 put_filesystem(fs);
                 put_super(s);
         } else {
                 super_unlock_excl(s);
         }
 }
 
 EXPORT_SYMBOL(deactivate_locked_super);
 
 /**
  *      deactivate_super        -       drop an active reference to superblock
  *      @s: superblock to deactivate
  *
  *      Variant of deactivate_locked_super(), except that superblock is *not*
  *      locked by caller.  If we are going to drop the final active reference,
  *      lock will be acquired prior to that.
  */
 void deactivate_super(struct super_block *s)
 {
         if (!atomic_add_unless(&s->s_active, -1, 1)) {
                 __super_lock_excl(s);
                 deactivate_locked_super(s);
         }
 }
 
 EXPORT_SYMBOL(deactivate_super);
 
 /**
  * grab_super - acquire an active reference to a superblock
  * @sb: superblock to acquire
  *
  * Acquire a temporary reference on a superblock and try to trade it for
  * an active reference. This is used in sget{_fc}() to wait for a
  * superblock to either become SB_BORN or for it to pass through
  * sb->kill() and be marked as SB_DEAD.
  *
  * Return: This returns true if an active reference could be acquired,
  *         false if not.
  */
 static bool grab_super(struct super_block *sb)
 {
         bool locked;
 
         sb->s_count++;
         spin_unlock(&sb_lock);
         locked = super_lock_excl(sb);
         if (locked) {
                 if (atomic_inc_not_zero(&sb->s_active)) {
                         put_super(sb);
                         return true;
                 }
                 super_unlock_excl(sb);
         }
         wait_var_event(&sb->s_flags, super_flags(sb, SB_DEAD));
         put_super(sb);
         return false;
 }
 
 /*
  *      super_trylock_shared - try to grab ->s_umount shared
  *      @sb: reference we are trying to grab
  *
  *      Try to prevent fs shutdown.  This is used in places where we
  *      cannot take an active reference but we need to ensure that the
  *      filesystem is not shut down while we are working on it. It returns
  *      false if we cannot acquire s_umount or if we lose the race and
  *      filesystem already got into shutdown, and returns true with the s_umount
  *      lock held in read mode in case of success. On successful return,
  *      the caller must drop the s_umount lock when done.
  *
  *      Note that unlike get_super() et.al. this one does *not* bump ->s_count.
  *      The reason why it's safe is that we are OK with doing trylock instead
  *      of down_read().  There's a couple of places that are OK with that, but
  *      it's very much not a general-purpose interface.
  */
 bool super_trylock_shared(struct super_block *sb)
 {
         if (down_read_trylock(&sb->s_umount)) {
                 if (!(sb->s_flags & SB_DYING) && sb->s_root &&
                     (sb->s_flags & SB_BORN))
                         return true;
                 super_unlock_shared(sb);
         }
 
         return false;
 }
 
 /**
  *      retire_super    -       prevents superblock from being reused
  *      @sb: superblock to retire
  *
  *      The function marks superblock to be ignored in superblock test, which
  *      prevents it from being reused for any new mounts.  If the superblock has
  *      a private bdi, it also unregisters it, but doesn't reduce the refcount
  *      of the superblock to prevent potential races.  The refcount is reduced
  *      by generic_shutdown_super().  The function can not be called
  *      concurrently with generic_shutdown_super().  It is safe to call the
  *      function multiple times, subsequent calls have no effect.
  *
  *      The marker will affect the re-use only for block-device-based
  *      superblocks.  Other superblocks will still get marked if this function
  *      is used, but that will not affect their reusability.
  */
 void retire_super(struct super_block *sb)
 {
         WARN_ON(!sb->s_bdev);
         __super_lock_excl(sb);
         if (sb->s_iflags & SB_I_PERSB_BDI) {
                 bdi_unregister(sb->s_bdi);
                 sb->s_iflags &= ~SB_I_PERSB_BDI;
         }
         sb->s_iflags |= SB_I_RETIRED;
         super_unlock_excl(sb);
 }
 EXPORT_SYMBOL(retire_super);
 
 /**
  *      generic_shutdown_super  -       common helper for ->kill_sb()
  *      @sb: superblock to kill
  *
  *      generic_shutdown_super() does all fs-independent work on superblock
  *      shutdown.  Typical ->kill_sb() should pick all fs-specific objects
  *      that need destruction out of superblock, call generic_shutdown_super()
  *      and release aforementioned objects.  Note: dentries and inodes _are_
  *      taken care of and do not need specific handling.
  *
  *      Upon calling this function, the filesystem may no longer alter or
  *      rearrange the set of dentries belonging to this super_block, nor may it
  *      change the attachments of dentries to inodes.
  */
 void generic_shutdown_super(struct super_block *sb)
 {
         const struct super_operations *sop = sb->s_op;
 
         if (sb->s_root) {
                 shrink_dcache_for_umount(sb);
                 sync_filesystem(sb);
                 sb->s_flags &= ~SB_ACTIVE;
 
                 cgroup_writeback_umount();
 
                 /* Evict all inodes with zero refcount. */
                 evict_inodes(sb);
 
                 /*
                  * Clean up and evict any inodes that still have references due
                  * to fsnotify or the security policy.
                  */
                 fsnotify_sb_delete(sb);
                 security_sb_delete(sb);
 
                 if (sb->s_dio_done_wq) {
                         destroy_workqueue(sb->s_dio_done_wq);
                         sb->s_dio_done_wq = NULL;
                 }
 
                 if (sop->put_super)
                         sop->put_super(sb);
 
                 /*
                  * Now that all potentially-encrypted inodes have been evicted,
                  * the fscrypt keyring can be destroyed.
                  */
                 fscrypt_destroy_keyring(sb);
 
                 if (CHECK_DATA_CORRUPTION(!list_empty(&sb->s_inodes),
                                 "VFS: Busy inodes after unmount of %s (%s)",
                                 sb->s_id, sb->s_type->name)) {
                         /*
                          * Adding a proper bailout path here would be hard, but
                          * we can at least make it more likely that a later
                          * iput_final() or such crashes cleanly.
                          */
                         struct inode *inode;
 
                         spin_lock(&sb->s_inode_list_lock);
                         list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
                                 inode->i_op = VFS_PTR_POISON;
                                 inode->i_sb = VFS_PTR_POISON;
                                 inode->i_mapping = VFS_PTR_POISON;
                         }
                         spin_unlock(&sb->s_inode_list_lock);
                 }
         }
         /*
          * Broadcast to everyone that grabbed a temporary reference to this
          * superblock before we removed it from @fs_supers that the superblock
          * is dying. Every walker of @fs_supers outside of sget{_fc}() will now
          * discard this superblock and treat it as dead.
          *
          * We leave the superblock on @fs_supers so it can be found by
          * sget{_fc}() until we passed sb->kill_sb().
          */
         super_wake(sb, SB_DYING);
         super_unlock_excl(sb);
         if (sb->s_bdi != &noop_backing_dev_info) {
                 if (sb->s_iflags & SB_I_PERSB_BDI)
                         bdi_unregister(sb->s_bdi);
                 bdi_put(sb->s_bdi);
                 sb->s_bdi = &noop_backing_dev_info;
         }
 }
 
 EXPORT_SYMBOL(generic_shutdown_super);
 
 bool mount_capable(struct fs_context *fc)
 {
         if (!(fc->fs_type->fs_flags & FS_USERNS_MOUNT))
                 return capable(CAP_SYS_ADMIN);
         else
                 return ns_capable(fc->user_ns, CAP_SYS_ADMIN);
 }
 
 /**
  * sget_fc - Find or create a superblock
  * @fc: Filesystem context.
  * @test: Comparison callback
  * @set: Setup callback
  *
  * Create a new superblock or find an existing one.
  *
  * The @test callback is used to find a matching existing superblock.
  * Whether or not the requested parameters in @fc are taken into account
  * is specific to the @test callback that is used. They may even be
  * completely ignored.
  *
  * If an extant superblock is matched, it will be returned unless:
  *
  * (1) the namespace the filesystem context @fc and the extant
  *     superblock's namespace differ
  *
  * (2) the filesystem context @fc has requested that reusing an extant
  *     superblock is not allowed
  *
  * In both cases EBUSY will be returned.
  *
  * If no match is made, a new superblock will be allocated and basic
  * initialisation will be performed (s_type, s_fs_info and s_id will be
  * set and the @set callback will be invoked), the superblock will be
  * published and it will be returned in a partially constructed state
  * with SB_BORN and SB_ACTIVE as yet unset.
  *
  * Return: On success, an extant or newly created superblock is
  *         returned. On failure an error pointer is returned.
  */
 struct super_block *sget_fc(struct fs_context *fc,
                             int (*test)(struct super_block *, struct fs_context *),
                             int (*set)(struct super_block *, struct fs_context *))
 {
         struct super_block *s = NULL;
         struct super_block *old;
         struct user_namespace *user_ns = fc->global ? &init_user_ns : fc->user_ns;
         int err;
 
         /*
          * Never allow s_user_ns != &init_user_ns when FS_USERNS_MOUNT is
          * not set, as the filesystem is likely unprepared to handle it.
          * This can happen when fsconfig() is called from init_user_ns with
          * an fs_fd opened in another user namespace.
          */
         if (user_ns != &init_user_ns && !(fc->fs_type->fs_flags & FS_USERNS_MOUNT)) {
                 errorfc(fc, "VFS: Mounting from non-initial user namespace is not allowed");
                 return ERR_PTR(-EPERM);
         }
 
 retry:
         spin_lock(&sb_lock);
         if (test) {
                 hlist_for_each_entry(old, &fc->fs_type->fs_supers, s_instances) {
                         if (test(old, fc))
                                 goto share_extant_sb;
                 }
         }
         if (!s) {
                 spin_unlock(&sb_lock);
                 s = alloc_super(fc->fs_type, fc->sb_flags, user_ns);
                 if (!s)
                         return ERR_PTR(-ENOMEM);
                 goto retry;
         }
 
         s->s_fs_info = fc->s_fs_info;
         err = set(s, fc);
         if (err) {
                 s->s_fs_info = NULL;
                 spin_unlock(&sb_lock);
                 destroy_unused_super(s);
                 return ERR_PTR(err);
         }
         fc->s_fs_info = NULL;
         s->s_type = fc->fs_type;
         s->s_iflags |= fc->s_iflags;
         strscpy(s->s_id, s->s_type->name, sizeof(s->s_id));
         /*
          * Make the superblock visible on @super_blocks and @fs_supers.
          * It's in a nascent state and users should wait on SB_BORN or
          * SB_DYING to be set.
          */
         list_add_tail(&s->s_list, &super_blocks);
         hlist_add_head(&s->s_instances, &s->s_type->fs_supers);
         spin_unlock(&sb_lock);
         get_filesystem(s->s_type);
         shrinker_register(s->s_shrink);
         return s;
 
 share_extant_sb:
         if (user_ns != old->s_user_ns || fc->exclusive) {
                 spin_unlock(&sb_lock);
                 destroy_unused_super(s);
                 if (fc->exclusive)
                         warnfc(fc, "reusing existing filesystem not allowed");
                 else
                         warnfc(fc, "reusing existing filesystem in another namespace not allowed");
                 return ERR_PTR(-EBUSY);
         }
         if (!grab_super(old))
                 goto retry;
         destroy_unused_super(s);
         return old;
 }
 EXPORT_SYMBOL(sget_fc);
 
 /**
  *      sget    -       find or create a superblock
  *      @type:    filesystem type superblock should belong to
  *      @test:    comparison callback
  *      @set:     setup callback
  *      @flags:   mount flags
  *      @data:    argument to each of them
  */
 struct super_block *sget(struct file_system_type *type,
                         int (*test)(struct super_block *,void *),
                         int (*set)(struct super_block *,void *),
                         int flags,
                         void *data)
 {
         struct user_namespace *user_ns = current_user_ns();
         struct super_block *s = NULL;
         struct super_block *old;
         int err;
 
         /* We don't yet pass the user namespace of the parent
          * mount through to here so always use &init_user_ns
          * until that changes.
          */
         if (flags & SB_SUBMOUNT)
                 user_ns = &init_user_ns;
 
 retry:
         spin_lock(&sb_lock);
         if (test) {
                 hlist_for_each_entry(old, &type->fs_supers, s_instances) {
                         if (!test(old, data))
                                 continue;
                         if (user_ns != old->s_user_ns) {
                                 spin_unlock(&sb_lock);
                                 destroy_unused_super(s);
                                 return ERR_PTR(-EBUSY);
                         }
                         if (!grab_super(old))
                                 goto retry;
                         destroy_unused_super(s);
                         return old;
                 }
         }
         if (!s) {
                 spin_unlock(&sb_lock);
                 s = alloc_super(type, (flags & ~SB_SUBMOUNT), user_ns);
                 if (!s)
                         return ERR_PTR(-ENOMEM);
                 goto retry;
         }
 
         err = set(s, data);
         if (err) {
                 spin_unlock(&sb_lock);
                 destroy_unused_super(s);
                 return ERR_PTR(err);
         }
         s->s_type = type;
         strscpy(s->s_id, type->name, sizeof(s->s_id));
         list_add_tail(&s->s_list, &super_blocks);
         hlist_add_head(&s->s_instances, &type->fs_supers);
         spin_unlock(&sb_lock);
         get_filesystem(type);
         shrinker_register(s->s_shrink);
         return s;
 }
 EXPORT_SYMBOL(sget);
 
 void drop_super(struct super_block *sb)
 {
         super_unlock_shared(sb);
         put_super(sb);
 }
 
 EXPORT_SYMBOL(drop_super);
 
 void drop_super_exclusive(struct super_block *sb)
 {
         super_unlock_excl(sb);
         put_super(sb);
 }
 EXPORT_SYMBOL(drop_super_exclusive);
 
 static void __iterate_supers(void (*f)(struct super_block *))
 {
         struct super_block *sb, *p = NULL;
 
         spin_lock(&sb_lock);
         list_for_each_entry(sb, &super_blocks, s_list) {
                 if (super_flags(sb, SB_DYING))
                         continue;
                 sb->s_count++;
                 spin_unlock(&sb_lock);
 
                 f(sb);
 
                 spin_lock(&sb_lock);
                 if (p)
                         __put_super(p);
                 p = sb;
         }
         if (p)
                 __put_super(p);
         spin_unlock(&sb_lock);
 }
 /**
  *      iterate_supers - call function for all active superblocks
  *      @f: function to call
  *      @arg: argument to pass to it
  *
  *      Scans the superblock list and calls given function, passing it
  *      locked superblock and given argument.
  */
 void iterate_supers(void (*f)(struct super_block *, void *), void *arg)
 {
         struct super_block *sb, *p = NULL;
 
         spin_lock(&sb_lock);
         list_for_each_entry(sb, &super_blocks, s_list) {
                 bool locked;
 
                 sb->s_count++;
                 spin_unlock(&sb_lock);
 
                 locked = super_lock_shared(sb);
                 if (locked) {
                         if (sb->s_root)
                                 f(sb, arg);
                         super_unlock_shared(sb);
                 }
 
                 spin_lock(&sb_lock);
                 if (p)
                         __put_super(p);
                 p = sb;
         }
         if (p)
                 __put_super(p);
         spin_unlock(&sb_lock);
 }
 
 /**
  *      iterate_supers_type - call function for superblocks of given type
  *      @type: fs type
  *      @f: function to call
  *      @arg: argument to pass to it
  *
  *      Scans the superblock list and calls given function, passing it
  *      locked superblock and given argument.
  */
 void iterate_supers_type(struct file_system_type *type,
         void (*f)(struct super_block *, void *), void *arg)
 {
         struct super_block *sb, *p = NULL;
 
         spin_lock(&sb_lock);
         hlist_for_each_entry(sb, &type->fs_supers, s_instances) {
                 bool locked;
 
                 sb->s_count++;
                 spin_unlock(&sb_lock);
 
                 locked = super_lock_shared(sb);
                 if (locked) {
                         if (sb->s_root)
                                 f(sb, arg);
                         super_unlock_shared(sb);
                 }
 
                 spin_lock(&sb_lock);
                 if (p)
                         __put_super(p);
                 p = sb;
         }
         if (p)
                 __put_super(p);
         spin_unlock(&sb_lock);
 }
 
 EXPORT_SYMBOL(iterate_supers_type);
 
 struct super_block *user_get_super(dev_t dev, bool excl)
 {
         struct super_block *sb;
 
         spin_lock(&sb_lock);
         list_for_each_entry(sb, &super_blocks, s_list) {
                 if (sb->s_dev ==  dev) {
                         bool locked;
 
                         sb->s_count++;
                         spin_unlock(&sb_lock);
                         /* still alive? */
                         locked = super_lock(sb, excl);
                         if (locked) {
                                 if (sb->s_root)
                                         return sb;
                                 super_unlock(sb, excl);
                         }
                         /* nope, got unmounted */
                         spin_lock(&sb_lock);
                         __put_super(sb);
                         break;
                 }
         }
         spin_unlock(&sb_lock);
         return NULL;
 }
 
 /**
  * reconfigure_super - asks filesystem to change superblock parameters
  * @fc: The superblock and configuration
  *
  * Alters the configuration parameters of a live superblock.
  */
 int reconfigure_super(struct fs_context *fc)
 {
         struct super_block *sb = fc->root->d_sb;
         int retval;
         bool remount_ro = false;
         bool remount_rw = false;
         bool force = fc->sb_flags & SB_FORCE;
 
         if (fc->sb_flags_mask & ~MS_RMT_MASK)
                 return -EINVAL;
         if (sb->s_writers.frozen != SB_UNFROZEN)
                 return -EBUSY;
 
         retval = security_sb_remount(sb, fc->security);
         if (retval)
                 return retval;
 
         if (fc->sb_flags_mask & SB_RDONLY) {
 #ifdef CONFIG_BLOCK
                 if (!(fc->sb_flags & SB_RDONLY) && sb->s_bdev &&
                     bdev_read_only(sb->s_bdev))
                         return -EACCES;
 #endif
                 remount_rw = !(fc->sb_flags & SB_RDONLY) && sb_rdonly(sb);
                 remount_ro = (fc->sb_flags & SB_RDONLY) && !sb_rdonly(sb);
         }
 
         if (remount_ro) {
                 if (!hlist_empty(&sb->s_pins)) {
                         super_unlock_excl(sb);
                         group_pin_kill(&sb->s_pins);
                         __super_lock_excl(sb);
                         if (!sb->s_root)
                                 return 0;
                         if (sb->s_writers.frozen != SB_UNFROZEN)
                                 return -EBUSY;
                         remount_ro = !sb_rdonly(sb);
                 }
         }
         shrink_dcache_sb(sb);
 
         /* If we are reconfiguring to RDONLY and current sb is read/write,
          * make sure there are no files open for writing.
          */
         if (remount_ro) {
                 if (force) {
                         sb_start_ro_state_change(sb);
                 } else {
                         retval = sb_prepare_remount_readonly(sb);
                         if (retval)
                                 return retval;
                 }
         } else if (remount_rw) {
                 /*
                  * Protect filesystem's reconfigure code from writes from
                  * userspace until reconfigure finishes.
                  */
                 sb_start_ro_state_change(sb);
         }
 
         if (fc->ops->reconfigure) {
                 retval = fc->ops->reconfigure(fc);
                 if (retval) {
                         if (!force)
                                 goto cancel_readonly;
                         /* If forced remount, go ahead despite any errors */
                         WARN(1, "forced remount of a %s fs returned %i\n",
                              sb->s_type->name, retval);
                 }
         }
 
         WRITE_ONCE(sb->s_flags, ((sb->s_flags & ~fc->sb_flags_mask) |
                                  (fc->sb_flags & fc->sb_flags_mask)));
         sb_end_ro_state_change(sb);
 
         /*
          * Some filesystems modify their metadata via some other path than the
          * bdev buffer cache (eg. use a private mapping, or directories in
          * pagecache, etc). Also file data modifications go via their own
          * mappings. So If we try to mount readonly then copy the filesystem
          * from bdev, we could get stale data, so invalidate it to give a best
          * effort at coherency.
          */
         if (remount_ro && sb->s_bdev)
                 invalidate_bdev(sb->s_bdev);
         return 0;
 
 cancel_readonly:
         sb_end_ro_state_change(sb);
         return retval;
 }
 
 static void do_emergency_remount_callback(struct super_block *sb)
 {
         bool locked = super_lock_excl(sb);
 
         if (locked && sb->s_root && sb->s_bdev && !sb_rdonly(sb)) {
                 struct fs_context *fc;
 
                 fc = fs_context_for_reconfigure(sb->s_root,
                                         SB_RDONLY | SB_FORCE, SB_RDONLY);
                 if (!IS_ERR(fc)) {
                         if (parse_monolithic_mount_data(fc, NULL) == 0)
                                 (void)reconfigure_super(fc);
                         put_fs_context(fc);
                 }
         }
         if (locked)
                 super_unlock_excl(sb);
 }
 
 static void do_emergency_remount(struct work_struct *work)
 {
         __iterate_supers(do_emergency_remount_callback);
         kfree(work);
         printk("Emergency Remount complete\n");
 }
 
 void emergency_remount(void)
 {
         struct work_struct *work;
 
         work = kmalloc(sizeof(*work), GFP_ATOMIC);
         if (work) {
                 INIT_WORK(work, do_emergency_remount);
                 schedule_work(work);
         }
 }
 
 static void do_thaw_all_callback(struct super_block *sb)
 {
         bool locked = super_lock_excl(sb);
 
         if (locked && sb->s_root) {
                 if (IS_ENABLED(CONFIG_BLOCK))
                         while (sb->s_bdev && !bdev_thaw(sb->s_bdev))
                                 pr_warn("Emergency Thaw on %pg\n", sb->s_bdev);
                 thaw_super_locked(sb, FREEZE_HOLDER_USERSPACE);
                 return;
         }
         if (locked)
                 super_unlock_excl(sb);
 }
 
 static void do_thaw_all(struct work_struct *work)
 {
         __iterate_supers(do_thaw_all_callback);
         kfree(work);
         printk(KERN_WARNING "Emergency Thaw complete\n");
 }
 
 /**
  * emergency_thaw_all -- forcibly thaw every frozen filesystem
  *
  * Used for emergency unfreeze of all filesystems via SysRq
  */
 void emergency_thaw_all(void)
 {
         struct work_struct *work;
 
         work = kmalloc(sizeof(*work), GFP_ATOMIC);
         if (work) {
                 INIT_WORK(work, do_thaw_all);
                 schedule_work(work);
         }
 }
 
 static DEFINE_IDA(unnamed_dev_ida);
 
 /**
  * get_anon_bdev - Allocate a block device for filesystems which don't have one.
  * @p: Pointer to a dev_t.
  *
  * Filesystems which don't use real block devices can call this function
  * to allocate a virtual block device.
  *
  * Context: Any context.  Frequently called while holding sb_lock.
  * Return: 0 on success, -EMFILE if there are no anonymous bdevs left
  * or -ENOMEM if memory allocation failed.
  */
 int get_anon_bdev(dev_t *p)
 {
         int dev;
 
         /*
          * Many userspace utilities consider an FSID of 0 invalid.
          * Always return at least 1 from get_anon_bdev.
          */
         dev = ida_alloc_range(&unnamed_dev_ida, 1, (1 << MINORBITS) - 1,
                         GFP_ATOMIC);
         if (dev == -ENOSPC)
                 dev = -EMFILE;
         if (dev < 0)
                 return dev;
 
         *p = MKDEV(0, dev);
         return 0;
 }
 EXPORT_SYMBOL(get_anon_bdev);
 
 void free_anon_bdev(dev_t dev)
 {
         ida_free(&unnamed_dev_ida, MINOR(dev));
 }
 EXPORT_SYMBOL(free_anon_bdev);
 
 int set_anon_super(struct super_block *s, void *data)
 {
         return get_anon_bdev(&s->s_dev);
 }
 EXPORT_SYMBOL(set_anon_super);
 
 void kill_anon_super(struct super_block *sb)
 {
         dev_t dev = sb->s_dev;
         generic_shutdown_super(sb);
         kill_super_notify(sb);
         free_anon_bdev(dev);
 }
 EXPORT_SYMBOL(kill_anon_super);
 
 void kill_litter_super(struct super_block *sb)
 {
         if (sb->s_root)
                 d_genocide(sb->s_root);
         kill_anon_super(sb);
 }
 EXPORT_SYMBOL(kill_litter_super);
 
 int set_anon_super_fc(struct super_block *sb, struct fs_context *fc)
 {
         return set_anon_super(sb, NULL);
 }
 EXPORT_SYMBOL(set_anon_super_fc);
 
 static int test_keyed_super(struct super_block *sb, struct fs_context *fc)
 {
         return sb->s_fs_info == fc->s_fs_info;
 }
 
 static int test_single_super(struct super_block *s, struct fs_context *fc)
 {
         return 1;
 }
 
 static int vfs_get_super(struct fs_context *fc,
                 int (*test)(struct super_block *, struct fs_context *),
                 int (*fill_super)(struct super_block *sb,
                                   struct fs_context *fc))
 {
         struct super_block *sb;
         int err;
 
         sb = sget_fc(fc, test, set_anon_super_fc);
         if (IS_ERR(sb))
                 return PTR_ERR(sb);
 
         if (!sb->s_root) {
                 err = fill_super(sb, fc);
                 if (err)
                         goto error;
 
                 sb->s_flags |= SB_ACTIVE;
         }
 
         fc->root = dget(sb->s_root);
         return 0;
 
 error:
         deactivate_locked_super(sb);
         return err;
 }
 
 int get_tree_nodev(struct fs_context *fc,
                   int (*fill_super)(struct super_block *sb,
                                     struct fs_context *fc))
 {
         return vfs_get_super(fc, NULL, fill_super);
 }
 EXPORT_SYMBOL(get_tree_nodev);
 
 int get_tree_single(struct fs_context *fc,
                   int (*fill_super)(struct super_block *sb,
                                     struct fs_context *fc))
 {
         return vfs_get_super(fc, test_single_super, fill_super);
 }
 EXPORT_SYMBOL(get_tree_single);
 
 int get_tree_keyed(struct fs_context *fc,
                   int (*fill_super)(struct super_block *sb,
                                     struct fs_context *fc),
                 void *key)
 {
         fc->s_fs_info = key;
         return vfs_get_super(fc, test_keyed_super, fill_super);
 }
 EXPORT_SYMBOL(get_tree_keyed);
 
 static int set_bdev_super(struct super_block *s, void *data)
 {
         s->s_dev = *(dev_t *)data;
         return 0;
 }
 
 static int super_s_dev_set(struct super_block *s, struct fs_context *fc)
 {
         return set_bdev_super(s, fc->sget_key);
 }
 
 static int super_s_dev_test(struct super_block *s, struct fs_context *fc)
 {
         return !(s->s_iflags & SB_I_RETIRED) &&
                 s->s_dev == *(dev_t *)fc->sget_key;
 }
 
 /**
  * sget_dev - Find or create a superblock by device number
  * @fc: Filesystem context.
  * @dev: device number
  *
  * Find or create a superblock using the provided device number that
  * will be stored in fc->sget_key.
  *
  * If an extant superblock is matched, then that will be returned with
  * an elevated reference count that the caller must transfer or discard.
  *
  * If no match is made, a new superblock will be allocated and basic
  * initialisation will be performed (s_type, s_fs_info, s_id, s_dev will
  * be set). The superblock will be published and it will be returned in
  * a partially constructed state with SB_BORN and SB_ACTIVE as yet
  * unset.
  *
  * Return: an existing or newly created superblock on success, an error
  *         pointer on failure.
  */
 struct super_block *sget_dev(struct fs_context *fc, dev_t dev)
 {
         fc->sget_key = &dev;
         return sget_fc(fc, super_s_dev_test, super_s_dev_set);
 }
 EXPORT_SYMBOL(sget_dev);
 
 #ifdef CONFIG_BLOCK
 /*
  * Lock the superblock that is holder of the bdev. Returns the superblock
  * pointer if we successfully locked the superblock and it is alive. Otherwise
  * we return NULL and just unlock bdev->bd_holder_lock.
  *
  * The function must be called with bdev->bd_holder_lock and releases it.
  */
 static struct super_block *bdev_super_lock(struct block_device *bdev, bool excl)
         __releases(&bdev->bd_holder_lock)
 {
         struct super_block *sb = bdev->bd_holder;
         bool locked;
 
         lockdep_assert_held(&bdev->bd_holder_lock);
         lockdep_assert_not_held(&sb->s_umount);
         lockdep_assert_not_held(&bdev->bd_disk->open_mutex);
 
         /* Make sure sb doesn't go away from under us */
         spin_lock(&sb_lock);
         sb->s_count++;
         spin_unlock(&sb_lock);
 
         mutex_unlock(&bdev->bd_holder_lock);
 
         locked = super_lock(sb, excl);
 
         /*
          * If the superblock wasn't already SB_DYING then we hold
          * s_umount and can safely drop our temporary reference.
          */
         put_super(sb);
 
         if (!locked)
                 return NULL;
 
         if (!sb->s_root || !(sb->s_flags & SB_ACTIVE)) {
                 super_unlock(sb, excl);
                 return NULL;
         }
 
         return sb;
 }
 
 static void fs_bdev_mark_dead(struct block_device *bdev, bool surprise)
 {
         struct super_block *sb;
 
         sb = bdev_super_lock(bdev, false);
         if (!sb)
                 return;
 
         if (!surprise)
                 sync_filesystem(sb);
         shrink_dcache_sb(sb);
         invalidate_inodes(sb);
         if (sb->s_op->shutdown)
                 sb->s_op->shutdown(sb);
 
         super_unlock_shared(sb);
 }
 
 static void fs_bdev_sync(struct block_device *bdev)
 {
         struct super_block *sb;
 
         sb = bdev_super_lock(bdev, false);
         if (!sb)
                 return;
 
         sync_filesystem(sb);
         super_unlock_shared(sb);
 }
 
 static struct super_block *get_bdev_super(struct block_device *bdev)
 {
         bool active = false;
         struct super_block *sb;
 
         sb = bdev_super_lock(bdev, true);
         if (sb) {
                 active = atomic_inc_not_zero(&sb->s_active);
                 super_unlock_excl(sb);
         }
         if (!active)
                 return NULL;
         return sb;
 }
 
 /**
  * fs_bdev_freeze - freeze owning filesystem of block device
  * @bdev: block device
  *
  * Freeze the filesystem that owns this block device if it is still
  * active.
  *
  * A filesystem that owns multiple block devices may be frozen from each
  * block device and won't be unfrozen until all block devices are
  * unfrozen. Each block device can only freeze the filesystem once as we
  * nest freezes for block devices in the block layer.
  *
  * Return: If the freeze was successful zero is returned. If the freeze
  *         failed a negative error code is returned.
  */
 static int fs_bdev_freeze(struct block_device *bdev)
 {
         struct super_block *sb;
         int error = 0;
 
         lockdep_assert_held(&bdev->bd_fsfreeze_mutex);
 
         sb = get_bdev_super(bdev);
         if (!sb)
                 return -EINVAL;
 
         if (sb->s_op->freeze_super)
                 error = sb->s_op->freeze_super(sb,
                                 FREEZE_MAY_NEST | FREEZE_HOLDER_USERSPACE);
         else
                 error = freeze_super(sb,
                                 FREEZE_MAY_NEST | FREEZE_HOLDER_USERSPACE);
         if (!error)
                 error = sync_blockdev(bdev);
         deactivate_super(sb);
         return error;
 }
 
 /**
  * fs_bdev_thaw - thaw owning filesystem of block device
  * @bdev: block device
  *
  * Thaw the filesystem that owns this block device.
  *
  * A filesystem that owns multiple block devices may be frozen from each
  * block device and won't be unfrozen until all block devices are
  * unfrozen. Each block device can only freeze the filesystem once as we
  * nest freezes for block devices in the block layer.
  *
  * Return: If the thaw was successful zero is returned. If the thaw
  *         failed a negative error code is returned. If this function
  *         returns zero it doesn't mean that the filesystem is unfrozen
  *         as it may have been frozen multiple times (kernel may hold a
  *         freeze or might be frozen from other block devices).
  */
 static int fs_bdev_thaw(struct block_device *bdev)
 {
         struct super_block *sb;
         int error;
 
         lockdep_assert_held(&bdev->bd_fsfreeze_mutex);
 
         /*
          * The block device may have been frozen before it was claimed by a
          * filesystem. Concurrently another process might try to mount that
          * frozen block device and has temporarily claimed the block device for
          * that purpose causing a concurrent fs_bdev_thaw() to end up here. The
          * mounter is already about to abort mounting because they still saw an
          * elevanted bdev->bd_fsfreeze_count so get_bdev_super() will return
          * NULL in that case.
          */
         sb = get_bdev_super(bdev);
         if (!sb)
                 return -EINVAL;
 
         if (sb->s_op->thaw_super)
                 error = sb->s_op->thaw_super(sb,
                                 FREEZE_MAY_NEST | FREEZE_HOLDER_USERSPACE);
         else
                 error = thaw_super(sb,
                                 FREEZE_MAY_NEST | FREEZE_HOLDER_USERSPACE);
         deactivate_super(sb);
         return error;
 }
 
 const struct blk_holder_ops fs_holder_ops = {
         .mark_dead              = fs_bdev_mark_dead,
         .sync                   = fs_bdev_sync,
         .freeze                 = fs_bdev_freeze,
         .thaw                   = fs_bdev_thaw,
 };
 EXPORT_SYMBOL_GPL(fs_holder_ops);
 
 int setup_bdev_super(struct super_block *sb, int sb_flags,
                 struct fs_context *fc)
 {
         blk_mode_t mode = sb_open_mode(sb_flags);
         struct file *bdev_file;
         struct block_device *bdev;
 
         bdev_file = bdev_file_open_by_dev(sb->s_dev, mode, sb, &fs_holder_ops);
         if (IS_ERR(bdev_file)) {
                 if (fc)
                         errorf(fc, "%s: Can't open blockdev", fc->source);
                 return PTR_ERR(bdev_file);
         }
         bdev = file_bdev(bdev_file);
 
         /*
          * This really should be in blkdev_get_by_dev, but right now can't due
          * to legacy issues that require us to allow opening a block device node
          * writable from userspace even for a read-only block device.
          */
         if ((mode & BLK_OPEN_WRITE) && bdev_read_only(bdev)) {
                 bdev_fput(bdev_file);
                 return -EACCES;
         }
 
         /*
          * It is enough to check bdev was not frozen before we set
          * s_bdev as freezing will wait until SB_BORN is set.
          */
         if (atomic_read(&bdev->bd_fsfreeze_count) > 0) {
                 if (fc)
                         warnf(fc, "%pg: Can't mount, blockdev is frozen", bdev);
                 bdev_fput(bdev_file);
                 return -EBUSY;
         }
         spin_lock(&sb_lock);
         sb->s_bdev_file = bdev_file;
         sb->s_bdev = bdev;
         sb->s_bdi = bdi_get(bdev->bd_disk->bdi);
         if (bdev_stable_writes(bdev))
                 sb->s_iflags |= SB_I_STABLE_WRITES;
         spin_unlock(&sb_lock);
 
         snprintf(sb->s_id, sizeof(sb->s_id), "%pg", bdev);
         shrinker_debugfs_rename(sb->s_shrink, "sb-%s:%s", sb->s_type->name,
                                 sb->s_id);
         sb_set_blocksize(sb, block_size(bdev));
         return 0;
 }
 EXPORT_SYMBOL_GPL(setup_bdev_super);
 
 /**
  * get_tree_bdev - Get a superblock based on a single block device
  * @fc: The filesystem context holding the parameters
  * @fill_super: Helper to initialise a new superblock
  */
 int get_tree_bdev(struct fs_context *fc,
                 int (*fill_super)(struct super_block *,
                                   struct fs_context *))
 {
         struct super_block *s;
         int error = 0;
         dev_t dev;
 
         if (!fc->source)
                 return invalf(fc, "No source specified");
 
         error = lookup_bdev(fc->source, &dev);
         if (error) {
                 errorf(fc, "%s: Can't lookup blockdev", fc->source);
                 return error;
         }
 
         fc->sb_flags |= SB_NOSEC;
         s = sget_dev(fc, dev);
         if (IS_ERR(s))
                 return PTR_ERR(s);
 
         if (s->s_root) {
                 /* Don't summarily change the RO/RW state. */
                 if ((fc->sb_flags ^ s->s_flags) & SB_RDONLY) {
                         warnf(fc, "%pg: Can't mount, would change RO state", s->s_bdev);
                         deactivate_locked_super(s);
                         return -EBUSY;
                 }
         } else {
                 error = setup_bdev_super(s, fc->sb_flags, fc);
                 if (!error)
                         error = fill_super(s, fc);
                 if (error) {
                         deactivate_locked_super(s);
                         return error;
                 }
                 s->s_flags |= SB_ACTIVE;
         }
 
         BUG_ON(fc->root);
         fc->root = dget(s->s_root);
         return 0;
 }
 EXPORT_SYMBOL(get_tree_bdev);
 
 static int test_bdev_super(struct super_block *s, void *data)
 {
         return !(s->s_iflags & SB_I_RETIRED) && s->s_dev == *(dev_t *)data;
 }
 
 struct dentry *mount_bdev(struct file_system_type *fs_type,
         int flags, const char *dev_name, void *data,
         int (*fill_super)(struct super_block *, void *, int))
 {
         struct super_block *s;
         int error;
         dev_t dev;
 
         error = lookup_bdev(dev_name, &dev);
         if (error)
                 return ERR_PTR(error);
 
         flags |= SB_NOSEC;
         s = sget(fs_type, test_bdev_super, set_bdev_super, flags, &dev);
         if (IS_ERR(s))
                 return ERR_CAST(s);
 
         if (s->s_root) {
                 if ((flags ^ s->s_flags) & SB_RDONLY) {
                         deactivate_locked_super(s);
                         return ERR_PTR(-EBUSY);
                 }
         } else {
                 error = setup_bdev_super(s, flags, NULL);
                 if (!error)
                         error = fill_super(s, data, flags & SB_SILENT ? 1 : 0);
                 if (error) {
                         deactivate_locked_super(s);
                         return ERR_PTR(error);
                 }
 
                 s->s_flags |= SB_ACTIVE;
         }
 
         return dget(s->s_root);
 }
 EXPORT_SYMBOL(mount_bdev);
 
 void kill_block_super(struct super_block *sb)
 {
         struct block_device *bdev = sb->s_bdev;
 
         generic_shutdown_super(sb);
         if (bdev) {
                 sync_blockdev(bdev);
                 bdev_fput(sb->s_bdev_file);
         }
 }
 
 EXPORT_SYMBOL(kill_block_super);
 #endif
 
 struct dentry *mount_nodev(struct file_system_type *fs_type,
         int flags, void *data,
         int (*fill_super)(struct super_block *, void *, int))
 {
         int error;
         struct super_block *s = sget(fs_type, NULL, set_anon_super, flags, NULL);
 
         if (IS_ERR(s))
                 return ERR_CAST(s);
 
         error = fill_super(s, data, flags & SB_SILENT ? 1 : 0);
         if (error) {
                 deactivate_locked_super(s);
                 return ERR_PTR(error);
         }
         s->s_flags |= SB_ACTIVE;
         return dget(s->s_root);
 }
 EXPORT_SYMBOL(mount_nodev);
 
 int reconfigure_single(struct super_block *s,
                        int flags, void *data)
 {
         struct fs_context *fc;
         int ret;
 
         /* The caller really need to be passing fc down into mount_single(),
          * then a chunk of this can be removed.  [Bollocks -- AV]
          * Better yet, reconfiguration shouldn't happen, but rather the second
          * mount should be rejected if the parameters are not compatible.
          */
         fc = fs_context_for_reconfigure(s->s_root, flags, MS_RMT_MASK);
         if (IS_ERR(fc))
                 return PTR_ERR(fc);
 
         ret = parse_monolithic_mount_data(fc, data);
         if (ret < 0)
                 goto out;
 
         ret = reconfigure_super(fc);
 out:
         put_fs_context(fc);
         return ret;
 }
 
 static int compare_single(struct super_block *s, void *p)
 {
         return 1;
 }
 
 struct dentry *mount_single(struct file_system_type *fs_type,
         int flags, void *data,
         int (*fill_super)(struct super_block *, void *, int))
 {
         struct super_block *s;
         int error;
 
         s = sget(fs_type, compare_single, set_anon_super, flags, NULL);
         if (IS_ERR(s))
                 return ERR_CAST(s);
         if (!s->s_root) {
                 error = fill_super(s, data, flags & SB_SILENT ? 1 : 0);
                 if (!error)
                         s->s_flags |= SB_ACTIVE;
         } else {
                 error = reconfigure_single(s, flags, data);
         }
         if (unlikely(error)) {
                 deactivate_locked_super(s);
                 return ERR_PTR(error);
         }
         return dget(s->s_root);
 }
 EXPORT_SYMBOL(mount_single);
 
 /**
  * vfs_get_tree - Get the mountable root
  * @fc: The superblock configuration context.
  *
  * The filesystem is invoked to get or create a superblock which can then later
  * be used for mounting.  The filesystem places a pointer to the root to be
  * used for mounting in @fc->root.
  */
 int vfs_get_tree(struct fs_context *fc)
 {
         struct super_block *sb;
         int error;
 
         if (fc->root)
                 return -EBUSY;
 
         /* Get the mountable root in fc->root, with a ref on the root and a ref
          * on the superblock.
          */
         error = fc->ops->get_tree(fc);
         if (error < 0)
                 return error;
 
         if (!fc->root) {
                 pr_err("Filesystem %s get_tree() didn't set fc->root, returned %i\n",
                        fc->fs_type->name, error);
                 /* We don't know what the locking state of the superblock is -
                  * if there is a superblock.
                  */
                 BUG();
         }
 
         sb = fc->root->d_sb;
         WARN_ON(!sb->s_bdi);
 
         /*
          * super_wake() contains a memory barrier which also care of
          * ordering for super_cache_count(). We place it before setting
          * SB_BORN as the data dependency between the two functions is
          * the superblock structure contents that we just set up, not
          * the SB_BORN flag.
          */
         super_wake(sb, SB_BORN);
 
         error = security_sb_set_mnt_opts(sb, fc->security, 0, NULL);
         if (unlikely(error)) {
                 fc_drop_locked(fc);
                 return error;
         }
 
         /*
          * filesystems should never set s_maxbytes larger than MAX_LFS_FILESIZE
          * but s_maxbytes was an unsigned long long for many releases. Throw
          * this warning for a little while to try and catch filesystems that
          * violate this rule.
          */
         WARN((sb->s_maxbytes < 0), "%s set sb->s_maxbytes to "
                 "negative value (%lld)\n", fc->fs_type->name, sb->s_maxbytes);
 
         return 0;
 }
 EXPORT_SYMBOL(vfs_get_tree);
 
 /*
  * Setup private BDI for given superblock. It gets automatically cleaned up
  * in generic_shutdown_super().
  */
 int super_setup_bdi_name(struct super_block *sb, char *fmt, ...)
 {
         struct backing_dev_info *bdi;
         int err;
         va_list args;
 
         bdi = bdi_alloc(NUMA_NO_NODE);
         if (!bdi)
                 return -ENOMEM;
 
         va_start(args, fmt);
         err = bdi_register_va(bdi, fmt, args);
         va_end(args);
         if (err) {
                 bdi_put(bdi);
                 return err;
         }
         WARN_ON(sb->s_bdi != &noop_backing_dev_info);
         sb->s_bdi = bdi;
         sb->s_iflags |= SB_I_PERSB_BDI;
 
         return 0;
 }
 EXPORT_SYMBOL(super_setup_bdi_name);
 
 /*
  * Setup private BDI for given superblock. I gets automatically cleaned up
  * in generic_shutdown_super().
  */
 int super_setup_bdi(struct super_block *sb)
 {
         static atomic_long_t bdi_seq = ATOMIC_LONG_INIT(0);
 
         return super_setup_bdi_name(sb, "%.28s-%ld", sb->s_type->name,
                                     atomic_long_inc_return(&bdi_seq));
 }
 EXPORT_SYMBOL(super_setup_bdi);
 
 /**
  * sb_wait_write - wait until all writers to given file system finish
  * @sb: the super for which we wait
  * @level: type of writers we wait for (normal vs page fault)
  *
  * This function waits until there are no writers of given type to given file
  * system.
  */
 static void sb_wait_write(struct super_block *sb, int level)
 {
         percpu_down_write(sb->s_writers.rw_sem + level-1);
 }
 
 /*
  * We are going to return to userspace and forget about these locks, the
  * ownership goes to the caller of thaw_super() which does unlock().
  */
 static void lockdep_sb_freeze_release(struct super_block *sb)
 {
         int level;
 
         for (level = SB_FREEZE_LEVELS - 1; level >= 0; level--)
                 percpu_rwsem_release(sb->s_writers.rw_sem + level, 0, _THIS_IP_);
 }
 
 /*
  * Tell lockdep we are holding these locks before we call ->unfreeze_fs(sb).
  */
 static void lockdep_sb_freeze_acquire(struct super_block *sb)
 {
         int level;
 
         for (level = 0; level < SB_FREEZE_LEVELS; ++level)
                 percpu_rwsem_acquire(sb->s_writers.rw_sem + level, 0, _THIS_IP_);
 }
 
 static void sb_freeze_unlock(struct super_block *sb, int level)
 {
         for (level--; level >= 0; level--)
                 percpu_up_write(sb->s_writers.rw_sem + level);
 }
 
 static int wait_for_partially_frozen(struct super_block *sb)
 {
         int ret = 0;
 
         do {
                 unsigned short old = sb->s_writers.frozen;
 
                 up_write(&sb->s_umount);
                 ret = wait_var_event_killable(&sb->s_writers.frozen,
                                                sb->s_writers.frozen != old);
                 down_write(&sb->s_umount);
         } while (ret == 0 &&
                  sb->s_writers.frozen != SB_UNFROZEN &&
                  sb->s_writers.frozen != SB_FREEZE_COMPLETE);
 
         return ret;
 }
 
 #define FREEZE_HOLDERS (FREEZE_HOLDER_KERNEL | FREEZE_HOLDER_USERSPACE)
 #define FREEZE_FLAGS (FREEZE_HOLDERS | FREEZE_MAY_NEST)
 
 static inline int freeze_inc(struct super_block *sb, enum freeze_holder who)
 {
         WARN_ON_ONCE((who & ~FREEZE_FLAGS));
         WARN_ON_ONCE(hweight32(who & FREEZE_HOLDERS) > 1);
 
         if (who & FREEZE_HOLDER_KERNEL)
                 ++sb->s_writers.freeze_kcount;
         if (who & FREEZE_HOLDER_USERSPACE)
                 ++sb->s_writers.freeze_ucount;
         return sb->s_writers.freeze_kcount + sb->s_writers.freeze_ucount;
 }
 
 static inline int freeze_dec(struct super_block *sb, enum freeze_holder who)
 {
         WARN_ON_ONCE((who & ~FREEZE_FLAGS));
         WARN_ON_ONCE(hweight32(who & FREEZE_HOLDERS) > 1);
 
         if ((who & FREEZE_HOLDER_KERNEL) && sb->s_writers.freeze_kcount)
                 --sb->s_writers.freeze_kcount;
         if ((who & FREEZE_HOLDER_USERSPACE) && sb->s_writers.freeze_ucount)
                 --sb->s_writers.freeze_ucount;
         return sb->s_writers.freeze_kcount + sb->s_writers.freeze_ucount;
 }
 
 static inline bool may_freeze(struct super_block *sb, enum freeze_holder who)
 {
         WARN_ON_ONCE((who & ~FREEZE_FLAGS));
         WARN_ON_ONCE(hweight32(who & FREEZE_HOLDERS) > 1);
 
         if (who & FREEZE_HOLDER_KERNEL)
                 return (who & FREEZE_MAY_NEST) ||
                        sb->s_writers.freeze_kcount == 0;
         if (who & FREEZE_HOLDER_USERSPACE)
                 return (who & FREEZE_MAY_NEST) ||
                        sb->s_writers.freeze_ucount == 0;
         return false;
 }
 
 /**
  * freeze_super - lock the filesystem and force it into a consistent state
  * @sb: the super to lock
  * @who: context that wants to freeze
  *
  * Syncs the super to make sure the filesystem is consistent and calls the fs's
  * freeze_fs.  Subsequent calls to this without first thawing the fs may return
  * -EBUSY.
  *
  * @who should be:
  * * %FREEZE_HOLDER_USERSPACE if userspace wants to freeze the fs;
  * * %FREEZE_HOLDER_KERNEL if the kernel wants to freeze the fs.
  * * %FREEZE_MAY_NEST whether nesting freeze and thaw requests is allowed.
  *
  * The @who argument distinguishes between the kernel and userspace trying to
  * freeze the filesystem.  Although there cannot be multiple kernel freezes or
  * multiple userspace freezes in effect at any given time, the kernel and
  * userspace can both hold a filesystem frozen.  The filesystem remains frozen
  * until there are no kernel or userspace freezes in effect.
  *
  * A filesystem may hold multiple devices and thus a filesystems may be
  * frozen through the block layer via multiple block devices. In this
  * case the request is marked as being allowed to nest by passing
  * FREEZE_MAY_NEST. The filesystem remains frozen until all block
  * devices are unfrozen. If multiple freezes are attempted without
  * FREEZE_MAY_NEST -EBUSY will be returned.
  *
  * During this function, sb->s_writers.frozen goes through these values:
  *
  * SB_UNFROZEN: File system is normal, all writes progress as usual.
  *
  * SB_FREEZE_WRITE: The file system is in the process of being frozen.  New
  * writes should be blocked, though page faults are still allowed. We wait for
  * all writes to complete and then proceed to the next stage.
  *
  * SB_FREEZE_PAGEFAULT: Freezing continues. Now also page faults are blocked
  * but internal fs threads can still modify the filesystem (although they
  * should not dirty new pages or inodes), writeback can run etc. After waiting
  * for all running page faults we sync the filesystem which will clean all
  * dirty pages and inodes (no new dirty pages or inodes can be created when
  * sync is running).
  *
  * SB_FREEZE_FS: The file system is frozen. Now all internal sources of fs
  * modification are blocked (e.g. XFS preallocation truncation on inode
  * reclaim). This is usually implemented by blocking new transactions for
  * filesystems that have them and need this additional guard. After all
  * internal writers are finished we call ->freeze_fs() to finish filesystem
  * freezing. Then we transition to SB_FREEZE_COMPLETE state. This state is
  * mostly auxiliary for filesystems to verify they do not modify frozen fs.
  *
  * sb->s_writers.frozen is protected by sb->s_umount.
  *
  * Return: If the freeze was successful zero is returned. If the freeze
  *         failed a negative error code is returned.
  */
 int freeze_super(struct super_block *sb, enum freeze_holder who)
 {
         int ret;
 
         if (!super_lock_excl(sb)) {
                 WARN_ON_ONCE("Dying superblock while freezing!");
                 return -EINVAL;
         }
         atomic_inc(&sb->s_active);
 
 retry:
         if (sb->s_writers.frozen == SB_FREEZE_COMPLETE) {
                 if (may_freeze(sb, who))
                         ret = !!WARN_ON_ONCE(freeze_inc(sb, who) == 1);
                 else
                         ret = -EBUSY;
                 /* All freezers share a single active reference. */
                 deactivate_locked_super(sb);
                 return ret;
         }
 
         if (sb->s_writers.frozen != SB_UNFROZEN) {
                 ret = wait_for_partially_frozen(sb);
                 if (ret) {
                         deactivate_locked_super(sb);
                         return ret;
                 }
 
                 goto retry;
         }
 
         if (sb_rdonly(sb)) {
                 /* Nothing to do really... */
                 WARN_ON_ONCE(freeze_inc(sb, who) > 1);
                 sb->s_writers.frozen = SB_FREEZE_COMPLETE;
                 wake_up_var(&sb->s_writers.frozen);
                 super_unlock_excl(sb);
                 return 0;
         }
 
         sb->s_writers.frozen = SB_FREEZE_WRITE;
         /* Release s_umount to preserve sb_start_write -> s_umount ordering */
         super_unlock_excl(sb);
         sb_wait_write(sb, SB_FREEZE_WRITE);
         __super_lock_excl(sb);
 
         /* Now we go and block page faults... */
         sb->s_writers.frozen = SB_FREEZE_PAGEFAULT;
         sb_wait_write(sb, SB_FREEZE_PAGEFAULT);
 
         /* All writers are done so after syncing there won't be dirty data */
         ret = sync_filesystem(sb);
         if (ret) {
                 sb->s_writers.frozen = SB_UNFROZEN;
                 sb_freeze_unlock(sb, SB_FREEZE_PAGEFAULT);
                 wake_up_var(&sb->s_writers.frozen);
                 deactivate_locked_super(sb);
                 return ret;
         }
 
         /* Now wait for internal filesystem counter */
         sb->s_writers.frozen = SB_FREEZE_FS;
         sb_wait_write(sb, SB_FREEZE_FS);
 
         if (sb->s_op->freeze_fs) {
                 ret = sb->s_op->freeze_fs(sb);
                 if (ret) {
                         printk(KERN_ERR
                                 "VFS:Filesystem freeze failed\n");
                         sb->s_writers.frozen = SB_UNFROZEN;
                         sb_freeze_unlock(sb, SB_FREEZE_FS);
                         wake_up_var(&sb->s_writers.frozen);
                         deactivate_locked_super(sb);
                         return ret;
                 }
         }
         /*
          * For debugging purposes so that fs can warn if it sees write activity
          * when frozen is set to SB_FREEZE_COMPLETE, and for thaw_super().
          */
         WARN_ON_ONCE(freeze_inc(sb, who) > 1);
         sb->s_writers.frozen = SB_FREEZE_COMPLETE;
         wake_up_var(&sb->s_writers.frozen);
         lockdep_sb_freeze_release(sb);
         super_unlock_excl(sb);
         return 0;
 }
 EXPORT_SYMBOL(freeze_super);
 
 /*
  * Undoes the effect of a freeze_super_locked call.  If the filesystem is
  * frozen both by userspace and the kernel, a thaw call from either source
  * removes that state without releasing the other state or unlocking the
  * filesystem.
  */
 static int thaw_super_locked(struct super_block *sb, enum freeze_holder who)
 {
         int error = -EINVAL;
 
         if (sb->s_writers.frozen != SB_FREEZE_COMPLETE)
                 goto out_unlock;
 
         /*
          * All freezers share a single active reference.
          * So just unlock in case there are any left.
          */
         if (freeze_dec(sb, who))
                 goto out_unlock;
 
         if (sb_rdonly(sb)) {
                 sb->s_writers.frozen = SB_UNFROZEN;
                 wake_up_var(&sb->s_writers.frozen);
                 goto out_deactivate;
         }
 
         lockdep_sb_freeze_acquire(sb);
 
         if (sb->s_op->unfreeze_fs) {
                 error = sb->s_op->unfreeze_fs(sb);
                 if (error) {
                         pr_err("VFS: Filesystem thaw failed\n");
                         freeze_inc(sb, who);
                         lockdep_sb_freeze_release(sb);
                         goto out_unlock;
                 }
         }
 
         sb->s_writers.frozen = SB_UNFROZEN;
         wake_up_var(&sb->s_writers.frozen);
         sb_freeze_unlock(sb, SB_FREEZE_FS);
 out_deactivate:
         deactivate_locked_super(sb);
         return 0;
 
 out_unlock:
         super_unlock_excl(sb);
         return error;
 }
 
 /**
  * thaw_super -- unlock filesystem
  * @sb: the super to thaw
  * @who: context that wants to freeze
  *
  * Unlocks the filesystem and marks it writeable again after freeze_super()
  * if there are no remaining freezes on the filesystem.
  *
  * @who should be:
  * * %FREEZE_HOLDER_USERSPACE if userspace wants to thaw the fs;
  * * %FREEZE_HOLDER_KERNEL if the kernel wants to thaw the fs.
  * * %FREEZE_MAY_NEST whether nesting freeze and thaw requests is allowed
  *
  * A filesystem may hold multiple devices and thus a filesystems may
  * have been frozen through the block layer via multiple block devices.
  * The filesystem remains frozen until all block devices are unfrozen.
  */
 int thaw_super(struct super_block *sb, enum freeze_holder who)
 {
         if (!super_lock_excl(sb)) {
                 WARN_ON_ONCE("Dying superblock while thawing!");
                 return -EINVAL;
         }
         return thaw_super_locked(sb, who);
 }
 EXPORT_SYMBOL(thaw_super);
 
 /*
  * Create workqueue for deferred direct IO completions. We allocate the
  * workqueue when it's first needed. This avoids creating workqueue for
  * filesystems that don't need it and also allows us to create the workqueue
  * late enough so the we can include s_id in the name of the workqueue.
  */
 int sb_init_dio_done_wq(struct super_block *sb)
 {
         struct workqueue_struct *old;
         struct workqueue_struct *wq = alloc_workqueue("dio/%s",
                                                       WQ_MEM_RECLAIM, 0,
                                                       sb->s_id);
         if (!wq)
                 return -ENOMEM;
         /*
          * This has to be atomic as more DIOs can race to create the workqueue
          */
         old = cmpxchg(&sb->s_dio_done_wq, NULL, wq);
         /* Someone created workqueue before us? Free ours... */
         if (old)
                 destroy_workqueue(wq);
         return 0;
 }
 EXPORT_SYMBOL_GPL(sb_init_dio_done_wq);
 

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