text: /*
   *  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/cleancache.h>
  #include <linux/fsnotify.h>
  #include <linux/lockdep.h>
  #include <linux/user_namespace.h>
  #include "internal.h"
  
  
  static LIST_HEAD(super_blocks);
  static DEFINE_SPINLOCK(sb_lock);
  
  static char *sb_writers_name[SB_FREEZE_LEVELS] = {
  	"sb_writers",
  	"sb_pagefaults",
  	"sb_internal",
  };
  
  /*
   * 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_rwsem. 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 = container_of(shrink, struct super_block, s_shrink);
  
  	/*
  	 * 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 (!trylock_super(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);
  	}
  
  	up_read(&sb->s_umount);
  	return freed;
  }
  
  static unsigned long super_cache_count(struct shrinker *shrink,
  				       struct shrink_control *sc)
  {
  	struct super_block *sb;
  	long	total_objects = 0;
  
  	sb = container_of(shrink, struct super_block, s_shrink);
  
  	/*
  	 * We don't call trylock_super() 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.  trylock_super() uses a MS_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 & MS_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);
  
  	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);
  	int i;
  
  	for (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);
  }
  
  /**
   *	destroy_super	-	frees a superblock
   *	@s: superblock to free
   *
   *	Frees a superblock.
   */
  static void destroy_super(struct super_block *s)
  {
  	list_lru_destroy(&s->s_dentry_lru);
  	list_lru_destroy(&s->s_inode_lru);
  	security_sb_free(s);
  	WARN_ON(!list_empty(&s->s_mounts));
  	put_user_ns(s->s_user_ns);
  	kfree(s->s_subtype);
  	kfree(s->s_options);
  	call_rcu(&s->rcu, destroy_super_rcu);
  }
  
  /**
   *	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_USER);
  	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);
  
  	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;
  	}
  	init_waitqueue_head(&s->s_writers.wait_unfrozen);
  	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_anon);
  	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);
  
  	if (list_lru_init_memcg(&s->s_dentry_lru))
  		goto fail;
  	if (list_lru_init_memcg(&s->s_inode_lru))
  		goto fail;
  
  	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);
  	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);
  	mutex_init(&s->s_dquot.dqio_mutex);
  	mutex_init(&s->s_dquot.dqonoff_mutex);
  	s->s_maxbytes = MAX_NON_LFS;
  	s->s_op = &default_op;
  	s->s_time_gran = 1000000000;
  	s->cleancache_poolid = CLEANCACHE_NO_POOL;
  
  	s->s_shrink.seeks = DEFAULT_SEEKS;
  	s->s_shrink.scan_objects = super_cache_scan;
  	s->s_shrink.count_objects = super_cache_count;
  	s->s_shrink.batch = 1024;
  	s->s_shrink.flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE;
  	return s;
  
  fail:
  	destroy_super(s);
  	return NULL;
  }
  
  /* Superblock refcounting  */
  
  /*
   * Drop a superblock's refcount.  The caller must hold sb_lock.
   */
  static void __put_super(struct super_block *sb)
  {
  	if (!--sb->s_count) {
  		list_del_init(&sb->s_list);
  		destroy_super(sb);
  	}
  }
  
  /**
   *	put_super	-	drop a temporary reference to superblock
   *	@sb: superblock in question
   *
   *	Drops a temporary reference, frees superblock if there's no
   *	references left.
   */
  static void put_super(struct super_block *sb)
  {
  	spin_lock(&sb_lock);
  	__put_super(sb);
  	spin_unlock(&sb_lock);
  }
  
  
  /**
   *	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)) {
  		cleancache_invalidate_fs(s);
  		unregister_shrinker(&s->s_shrink);
  		fs->kill_sb(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 {
  		up_write(&s->s_umount);
  	}
  }
  
  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)) {
  		down_write(&s->s_umount);
  		deactivate_locked_super(s);
  	}
  }
  
  EXPORT_SYMBOL(deactivate_super);
  
  /**
   *	grab_super - acquire an active reference
   *	@s: reference we are trying to make active
   *
   *	Tries to acquire an active reference.  grab_super() is used when we
   * 	had just found a superblock in super_blocks or fs_type->fs_supers
   *	and want to turn it into a full-blown active reference.  grab_super()
   *	is called with sb_lock held and drops it.  Returns 1 in case of
   *	success, 0 if we had failed (superblock contents was already dead or
   *	dying when grab_super() had been called).  Note that this is only
   *	called for superblocks not in rundown mode (== ones still on ->fs_supers
   *	of their type), so increment of ->s_count is OK here.
   */
  static int grab_super(struct super_block *s) __releases(sb_lock)
  {
  	s->s_count++;
  	spin_unlock(&sb_lock);
  	down_write(&s->s_umount);
  	if ((s->s_flags & MS_BORN) && atomic_inc_not_zero(&s->s_active)) {
  		put_super(s);
  		return 1;
  	}
  	up_write(&s->s_umount);
  	put_super(s);
  	return 0;
  }
  
  /*
   *	trylock_super - 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 trylock_super(struct super_block *sb)
  {
  	if (down_read_trylock(&sb->s_umount)) {
  		if (!hlist_unhashed(&sb->s_instances) &&
  		    sb->s_root && (sb->s_flags & MS_BORN))
  			return true;
  		up_read(&sb->s_umount);
  	}
  
  	return false;
  }
  
  /**
   *	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 &= ~MS_ACTIVE;
  
  		fsnotify_unmount_inodes(sb);
  		cgroup_writeback_umount();
  
  		evict_inodes(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);
  
  		if (!list_empty(&sb->s_inodes)) {
  			printk("VFS: Busy inodes after unmount of %s. "
  			   "Self-destruct in 5 seconds.  Have a nice day...\n",
  			   sb->s_id);
  		}
  	}
  	spin_lock(&sb_lock);
  	/* should be initialized for __put_super_and_need_restart() */
  	hlist_del_init(&sb->s_instances);
  	spin_unlock(&sb_lock);
  	up_write(&sb->s_umount);
  }
  
  EXPORT_SYMBOL(generic_shutdown_super);
  
  /**
   *	sget_userns -	find or create a superblock
   *	@type:	filesystem type superblock should belong to
   *	@test:	comparison callback
   *	@set:	setup callback
   *	@flags:	mount flags
   *	@user_ns: User namespace for the super_block
   *	@data:	argument to each of them
   */
  struct super_block *sget_userns(struct file_system_type *type,
  			int (*test)(struct super_block *,void *),
  			int (*set)(struct super_block *,void *),
  			int flags, struct user_namespace *user_ns,
  			void *data)
  {
  	struct super_block *s = NULL;
  	struct super_block *old;
  	int err;
  
  	if (!(flags & (MS_KERNMOUNT|MS_SUBMOUNT)) &&
  	    !(type->fs_flags & FS_USERNS_MOUNT) &&
  	    !capable(CAP_SYS_ADMIN))
  		return ERR_PTR(-EPERM);
  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);
  				if (s) {
  					up_write(&s->s_umount);
  					destroy_super(s);
  				}
  				return ERR_PTR(-EBUSY);
  			}
  			if (!grab_super(old))
  				goto retry;
  			if (s) {
  				up_write(&s->s_umount);
  				destroy_super(s);
  				s = NULL;
  			}
  			return old;
  		}
  	}
  	if (!s) {
  		spin_unlock(&sb_lock);
  		s = alloc_super(type, (flags & ~MS_SUBMOUNT), user_ns);
  		if (!s)
  			return ERR_PTR(-ENOMEM);
  		goto retry;
  	}
  		
  	err = set(s, data);
  	if (err) {
  		spin_unlock(&sb_lock);
  		up_write(&s->s_umount);
  		destroy_super(s);
  		return ERR_PTR(err);
  	}
  	s->s_type = type;
  	strlcpy(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);
  	err = register_shrinker(&s->s_shrink);
  	if (err) {
  		deactivate_locked_super(s);
  		s = ERR_PTR(err);
  	}
  	return s;
  }
  
  EXPORT_SYMBOL(sget_userns);
  
  /**
   *	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();
  
  	/* 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 & MS_SUBMOUNT)
  		user_ns = &init_user_ns;
  
  	/* Ensure the requestor has permissions over the target filesystem */
  	if (!(flags & (MS_KERNMOUNT|MS_SUBMOUNT)) && !ns_capable(user_ns, CAP_SYS_ADMIN))
  		return ERR_PTR(-EPERM);
  
  	return sget_userns(type, test, set, flags, user_ns, data);
  }
  
  EXPORT_SYMBOL(sget);
  
  void drop_super(struct super_block *sb)
  {
  	up_read(&sb->s_umount);
  	put_super(sb);
  }
  
  EXPORT_SYMBOL(drop_super);
  
  /**
   *	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) {
  		if (hlist_unhashed(&sb->s_instances))
  			continue;
  		sb->s_count++;
  		spin_unlock(&sb_lock);
  
  		down_read(&sb->s_umount);
  		if (sb->s_root && (sb->s_flags & MS_BORN))
  			f(sb, arg);
  		up_read(&sb->s_umount);
  
  		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) {
  		sb->s_count++;
  		spin_unlock(&sb_lock);
  
  		down_read(&sb->s_umount);
  		if (sb->s_root && (sb->s_flags & MS_BORN))
  			f(sb, arg);
  		up_read(&sb->s_umount);
  
  		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);
  
  /**
   *	get_super - get the superblock of a device
   *	@bdev: device to get the superblock for
   *	
   *	Scans the superblock list and finds the superblock of the file system
   *	mounted on the device given. %NULL is returned if no match is found.
   */
  
  struct super_block *get_super(struct block_device *bdev)
  {
  	struct super_block *sb;
  
  	if (!bdev)
  		return NULL;
  
  	spin_lock(&sb_lock);
  rescan:
  	list_for_each_entry(sb, &super_blocks, s_list) {
  		if (hlist_unhashed(&sb->s_instances))
  			continue;
  		if (sb->s_bdev == bdev) {
  			sb->s_count++;
  			spin_unlock(&sb_lock);
  			down_read(&sb->s_umount);
  			/* still alive? */
  			if (sb->s_root && (sb->s_flags & MS_BORN))
  				return sb;
  			up_read(&sb->s_umount);
  			/* nope, got unmounted */
  			spin_lock(&sb_lock);
  			__put_super(sb);
  			goto rescan;
  		}
  	}
  	spin_unlock(&sb_lock);
  	return NULL;
  }
  
  EXPORT_SYMBOL(get_super);
  
  /**
   *	get_super_thawed - get thawed superblock of a device
   *	@bdev: device to get the superblock for
   *
   *	Scans the superblock list and finds the superblock of the file system
   *	mounted on the device. The superblock is returned once it is thawed
   *	(or immediately if it was not frozen). %NULL is returned if no match
   *	is found.
   */
  struct super_block *get_super_thawed(struct block_device *bdev)
  {
  	while (1) {
  		struct super_block *s = get_super(bdev);
  		if (!s || s->s_writers.frozen == SB_UNFROZEN)
  			return s;
  		up_read(&s->s_umount);
  		wait_event(s->s_writers.wait_unfrozen,
  			   s->s_writers.frozen == SB_UNFROZEN);
  		put_super(s);
  	}
  }
  EXPORT_SYMBOL(get_super_thawed);
  
  /**
   * get_active_super - get an active reference to the superblock of a device
   * @bdev: device to get the superblock for
   *
   * Scans the superblock list and finds the superblock of the file system
   * mounted on the device given.  Returns the superblock with an active
   * reference or %NULL if none was found.
   */
  struct super_block *get_active_super(struct block_device *bdev)
  {
  	struct super_block *sb;
  
  	if (!bdev)
  		return NULL;
  
  restart:
  	spin_lock(&sb_lock);
  	list_for_each_entry(sb, &super_blocks, s_list) {
  		if (hlist_unhashed(&sb->s_instances))
  			continue;
  		if (sb->s_bdev == bdev) {
  			if (!grab_super(sb))
  				goto restart;
  			up_write(&sb->s_umount);
  			return sb;
  		}
  	}
  	spin_unlock(&sb_lock);
  	return NULL;
  }
   
  struct super_block *user_get_super(dev_t dev)
  {
  	struct super_block *sb;
  
  	spin_lock(&sb_lock);
  rescan:
  	list_for_each_entry(sb, &super_blocks, s_list) {
  		if (hlist_unhashed(&sb->s_instances))
  			continue;
  		if (sb->s_dev ==  dev) {
  			sb->s_count++;
  			spin_unlock(&sb_lock);
  			down_read(&sb->s_umount);
  			/* still alive? */
  			if (sb->s_root && (sb->s_flags & MS_BORN))
  				return sb;
  			up_read(&sb->s_umount);
  			/* nope, got unmounted */
  			spin_lock(&sb_lock);
  			__put_super(sb);
  			goto rescan;
  		}
  	}
  	spin_unlock(&sb_lock);
  	return NULL;
  }
  
  /**
   *	do_remount_sb2 - asks filesystem to change mount options.
   *	@mnt:   mount we are looking at
   *	@sb:	superblock in question
   *	@flags:	numeric part of options
   *	@data:	the rest of options
   *      @force: whether or not to force the change
   *
   *	Alters the mount options of a mounted file system.
   */
  int do_remount_sb2(struct vfsmount *mnt, struct super_block *sb, int flags, void *data, int force)
  {
  	int retval;
  	int remount_ro;
  
  	if (sb->s_writers.frozen != SB_UNFROZEN)
  		return -EBUSY;
  
  #ifdef CONFIG_BLOCK
  	if (!(flags & MS_RDONLY) && bdev_read_only(sb->s_bdev))
  		return -EACCES;
  #endif
  
  	remount_ro = (flags & MS_RDONLY) && !(sb->s_flags & MS_RDONLY);
  
  	if (remount_ro) {
  		if (!hlist_empty(&sb->s_pins)) {
  			up_write(&sb->s_umount);
  			group_pin_kill(&sb->s_pins);
  			down_write(&sb->s_umount);
  			if (!sb->s_root)
  				return 0;
  			if (sb->s_writers.frozen != SB_UNFROZEN)
  				return -EBUSY;
  			remount_ro = (flags & MS_RDONLY) && !(sb->s_flags & MS_RDONLY);
  		}
  	}
  	shrink_dcache_sb(sb);
  
  	/* If we are remounting RDONLY and current sb is read/write,
  	   make sure there are no rw files opened */
  	if (remount_ro) {
  		if (force) {
  			sb->s_readonly_remount = 1;
  			smp_wmb();
  		} else {
  			retval = sb_prepare_remount_readonly(sb);
  			if (retval)
  				return retval;
  		}
  	}
  
  	if (mnt && sb->s_op->remount_fs2) {
  		retval = sb->s_op->remount_fs2(mnt, sb, &flags, data);
  		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);
  		}
  	} else if (sb->s_op->remount_fs) {
  		retval = sb->s_op->remount_fs(sb, &flags, data);
  		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);
  		}
  	}
  	sb->s_flags = (sb->s_flags & ~MS_RMT_MASK) | (flags & MS_RMT_MASK);
  	/* Needs to be ordered wrt mnt_is_readonly() */
  	smp_wmb();
  	sb->s_readonly_remount = 0;
  
  	/*
  	 * 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->s_readonly_remount = 0;
  	return retval;
  }
  
  int do_remount_sb(struct super_block *sb, int flags, void *data, int force)
  {
  	return do_remount_sb2(NULL, sb, flags, data, force);
  }
  
  static void do_emergency_remount(struct work_struct *work)
  {
  	struct super_block *sb, *p = NULL;
  
  	spin_lock(&sb_lock);
  	list_for_each_entry_reverse(sb, &super_blocks, s_list) {
  		if (hlist_unhashed(&sb->s_instances))
  			continue;
  		sb->s_count++;
  		spin_unlock(&sb_lock);
  		down_write(&sb->s_umount);
  		if (sb->s_root && sb->s_bdev && (sb->s_flags & MS_BORN) &&
  		    !(sb->s_flags & MS_RDONLY)) {
  			/*
  			 * What lock protects sb->s_flags??
  			 */
  			do_remount_sb(sb, MS_RDONLY, NULL, 1);
  		}
  		up_write(&sb->s_umount);
  		spin_lock(&sb_lock);
  		if (p)
  			__put_super(p);
  		p = sb;
  	}
  	if (p)
  		__put_super(p);
  	spin_unlock(&sb_lock);
  	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);
  	}
  }
  
  /*
   * Unnamed block devices are dummy devices used by virtual
   * filesystems which don't use real block-devices.  -- jrs
   */
  
  static DEFINE_IDA(unnamed_dev_ida);
  static DEFINE_SPINLOCK(unnamed_dev_lock);/* protects the above */
  /* Many userspace utilities consider an FSID of 0 invalid.
   * Always return at least 1 from get_anon_bdev.
   */
  static int unnamed_dev_start = 1;
  
  int get_anon_bdev(dev_t *p)
  {
  	int dev;
  	int error;
  
   retry:
  	if (ida_pre_get(&unnamed_dev_ida, GFP_ATOMIC) == 0)
  		return -ENOMEM;
  	spin_lock(&unnamed_dev_lock);
  	error = ida_get_new_above(&unnamed_dev_ida, unnamed_dev_start, &dev);
  	if (!error)
  		unnamed_dev_start = dev + 1;
  	spin_unlock(&unnamed_dev_lock);
  	if (error == -EAGAIN)
  		/* We raced and lost with another CPU. */
  		goto retry;
  	else if (error)
  		return -EAGAIN;
  
  	if (dev >= (1 << MINORBITS)) {
  		spin_lock(&unnamed_dev_lock);
  		ida_remove(&unnamed_dev_ida, dev);
  		if (unnamed_dev_start > dev)
  			unnamed_dev_start = dev;
  		spin_unlock(&unnamed_dev_lock);
  		return -EMFILE;
  	}
  	*p = MKDEV(0, dev & MINORMASK);
  	return 0;
  }
  EXPORT_SYMBOL(get_anon_bdev);
  
  void free_anon_bdev(dev_t dev)
  {
  	int slot = MINOR(dev);
  	spin_lock(&unnamed_dev_lock);
  	ida_remove(&unnamed_dev_ida, slot);
  	if (slot < unnamed_dev_start)
  		unnamed_dev_start = slot;
  	spin_unlock(&unnamed_dev_lock);
  }
  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);
  	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);
  
  static int ns_test_super(struct super_block *sb, void *data)
  {
  	return sb->s_fs_info == data;
  }
  
  static int ns_set_super(struct super_block *sb, void *data)
  {
  	sb->s_fs_info = data;
  	return set_anon_super(sb, NULL);
  }
  
  struct dentry *mount_ns(struct file_system_type *fs_type,
  	int flags, void *data, void *ns, struct user_namespace *user_ns,
  	int (*fill_super)(struct super_block *, void *, int))
  {
  	struct super_block *sb;
  
  	/* Don't allow mounting unless the caller has CAP_SYS_ADMIN
  	 * over the namespace.
  	 */
  	if (!(flags & MS_KERNMOUNT) && !ns_capable(user_ns, CAP_SYS_ADMIN))
  		return ERR_PTR(-EPERM);
  
  	sb = sget_userns(fs_type, ns_test_super, ns_set_super, flags,
  			 user_ns, ns);
  	if (IS_ERR(sb))
  		return ERR_CAST(sb);
  
  	if (!sb->s_root) {
  		int err;
  		err = fill_super(sb, data, flags & MS_SILENT ? 1 : 0);
  		if (err) {
  			deactivate_locked_super(sb);
  			return ERR_PTR(err);
  		}
  
  		sb->s_flags |= MS_ACTIVE;
  	}
  
  	return dget(sb->s_root);
  }
  
  EXPORT_SYMBOL(mount_ns);
  
  #ifdef CONFIG_BLOCK
  static int set_bdev_super(struct super_block *s, void *data)
  {
  	s->s_bdev = data;
  	s->s_dev = s->s_bdev->bd_dev;
  
  	/*
  	 * We set the bdi here to the queue backing, file systems can
  	 * overwrite this in ->fill_super()
  	 */
  	s->s_bdi = bdev_get_queue(s->s_bdev)->backing_dev_info;
  	return 0;
  }
  
  static int test_bdev_super(struct super_block *s, void *data)
  {
  	return (void *)s->s_bdev == 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 block_device *bdev;
  	struct super_block *s;
  	fmode_t mode = FMODE_READ | FMODE_EXCL;
  	int error = 0;
  
  	if (!(flags & MS_RDONLY))
  		mode |= FMODE_WRITE;
  
  	bdev = blkdev_get_by_path(dev_name, mode, fs_type);
  	if (IS_ERR(bdev))
  		return ERR_CAST(bdev);
  
  	/*
  	 * once the super is inserted into the list by sget, s_umount
  	 * will protect the lockfs code from trying to start a snapshot
  	 * while we are mounting
  	 */
  	mutex_lock(&bdev->bd_fsfreeze_mutex);
  	if (bdev->bd_fsfreeze_count > 0) {
  		mutex_unlock(&bdev->bd_fsfreeze_mutex);
  		error = -EBUSY;
  		goto error_bdev;
  	}
  	s = sget(fs_type, test_bdev_super, set_bdev_super, flags | MS_NOSEC,
  		 bdev);
  	mutex_unlock(&bdev->bd_fsfreeze_mutex);
  	if (IS_ERR(s))
  		goto error_s;
  
  	if (s->s_root) {
  		if ((flags ^ s->s_flags) & MS_RDONLY) {
  			deactivate_locked_super(s);
  			error = -EBUSY;
  			goto error_bdev;
  		}
  
  		/*
  		 * s_umount nests inside bd_mutex during
  		 * __invalidate_device().  blkdev_put() acquires
  		 * bd_mutex and can't be called under s_umount.  Drop
  		 * s_umount temporarily.  This is safe as we're
  		 * holding an active reference.
  		 */
  		up_write(&s->s_umount);
  		blkdev_put(bdev, mode);
  		down_write(&s->s_umount);
  	} else {
  		s->s_mode = mode;
  		snprintf(s->s_id, sizeof(s->s_id), "%pg", bdev);
  		sb_set_blocksize(s, block_size(bdev));
  		error = fill_super(s, data, flags & MS_SILENT ? 1 : 0);
  		if (error) {
  			deactivate_locked_super(s);
  			goto error;
  		}
  
  		s->s_flags |= MS_ACTIVE;
  		bdev->bd_super = s;
  	}
  
  	return dget(s->s_root);
  
  error_s:
  	error = PTR_ERR(s);
  error_bdev:
  	blkdev_put(bdev, mode);
  error:
  	return ERR_PTR(error);
  }
  EXPORT_SYMBOL(mount_bdev);
  
  void kill_block_super(struct super_block *sb)
  {
  	struct block_device *bdev = sb->s_bdev;
  	fmode_t mode = sb->s_mode;
  
  	bdev->bd_super = NULL;
  	generic_shutdown_super(sb);
  	sync_blockdev(bdev);
  	WARN_ON_ONCE(!(mode & FMODE_EXCL));
  	blkdev_put(bdev, mode | FMODE_EXCL);
  }
  
  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 & MS_SILENT ? 1 : 0);
  	if (error) {
  		deactivate_locked_super(s);
  		return ERR_PTR(error);
  	}
  	s->s_flags |= MS_ACTIVE;
  	return dget(s->s_root);
  }
  EXPORT_SYMBOL(mount_nodev);
  
  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 & MS_SILENT ? 1 : 0);
  		if (error) {
  			deactivate_locked_super(s);
  			return ERR_PTR(error);
  		}
  		s->s_flags |= MS_ACTIVE;
  	} else {
  		do_remount_sb(s, flags, data, 0);
  	}
  	return dget(s->s_root);
  }
  EXPORT_SYMBOL(mount_single);
  
  struct dentry *
  mount_fs(struct file_system_type *type, int flags, const char *name, struct vfsmount *mnt, void *data)
  {
  	struct dentry *root;
  	struct super_block *sb;
  	char *secdata = NULL;
  	int error = -ENOMEM;
  
  	if (data && !(type->fs_flags & FS_BINARY_MOUNTDATA)) {
  		secdata = alloc_secdata();
  		if (!secdata)
  			goto out;
  
  		error = security_sb_copy_data(data, secdata);
  		if (error)
  			goto out_free_secdata;
  	}
  
  	if (type->mount2)
  		root = type->mount2(mnt, type, flags, name, data);
  	else
  		root = type->mount(type, flags, name, data);
  	if (IS_ERR(root)) {
  		error = PTR_ERR(root);
  		goto out_free_secdata;
  	}
  	sb = root->d_sb;
  	BUG_ON(!sb);
  	WARN_ON(!sb->s_bdi);
  
  	/*
  	 * Write barrier is for super_cache_count(). We place it before setting
  	 * MS_BORN as the data dependency between the two functions is the
  	 * superblock structure contents that we just set up, not the MS_BORN
  	 * flag.
  	 */
  	smp_wmb();
  	sb->s_flags |= MS_BORN;
  
  	error = security_sb_kern_mount(sb, flags, secdata);
  	if (error)
  		goto out_sb;
  
  	/*
  	 * 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", type->name, sb->s_maxbytes);
  
  	up_write(&sb->s_umount);
  	free_secdata(secdata);
  	return root;
  out_sb:
  	dput(root);
  	deactivate_locked_super(sb);
  out_free_secdata:
  	free_secdata(secdata);
  out:
  	return ERR_PTR(error);
  }
  
  /*
   * This is an internal function, please use sb_end_{write,pagefault,intwrite}
   * instead.
   */
  void __sb_end_write(struct super_block *sb, int level)
  {
  	percpu_up_read(sb->s_writers.rw_sem + level-1);
  }
  EXPORT_SYMBOL(__sb_end_write);
  
  /*
   * This is an internal function, please use sb_start_{write,pagefault,intwrite}
   * instead.
   */
  int __sb_start_write(struct super_block *sb, int level, bool wait)
  {
  	bool force_trylock = false;
  	int ret = 1;
  
  #ifdef CONFIG_LOCKDEP
  	/*
  	 * We want lockdep to tell us about possible deadlocks with freezing
  	 * but it's it bit tricky to properly instrument it. Getting a freeze
  	 * protection works as getting a read lock but there are subtle
  	 * problems. XFS for example gets freeze protection on internal level
  	 * twice in some cases, which is OK only because we already hold a
  	 * freeze protection also on higher level. Due to these cases we have
  	 * to use wait == F (trylock mode) which must not fail.
  	 */
  	if (wait) {
  		int i;
  
  		for (i = 0; i < level - 1; i++)
  			if (percpu_rwsem_is_held(sb->s_writers.rw_sem + i)) {
  				force_trylock = true;
  				break;
  			}
  	}
  #endif
  	if (wait && !force_trylock)
  		percpu_down_read(sb->s_writers.rw_sem + level-1);
  	else
  		ret = percpu_down_read_trylock(sb->s_writers.rw_sem + level-1);
  
  	WARN_ON(force_trylock && !ret);
  	return ret;
  }
  EXPORT_SYMBOL(__sb_start_write);
  
  /**
   * 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 = SB_FREEZE_LEVELS - 1; level >= 0; level--)
  		percpu_up_write(sb->s_writers.rw_sem + level);
  }
  
  /**
   * freeze_super - lock the filesystem and force it into a consistent state
   * @sb: the super to lock
   *
   * 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 will return
   * -EBUSY.
   *
   * 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.
   */
  int freeze_super(struct super_block *sb)
  {
  	int ret;
  
  	atomic_inc(&sb->s_active);
  	down_write(&sb->s_umount);
  	if (sb->s_writers.frozen != SB_UNFROZEN) {
  		deactivate_locked_super(sb);
  		return -EBUSY;
  	}
  
  	if (!(sb->s_flags & MS_BORN)) {
  		up_write(&sb->s_umount);
  		return 0;	/* sic - it's "nothing to do" */
  	}
  
  	if (sb->s_flags & MS_RDONLY) {
  		/* Nothing to do really... */
  		sb->s_writers.frozen = SB_FREEZE_COMPLETE;
  		up_write(&sb->s_umount);
  		return 0;
  	}
  
  	sb->s_writers.frozen = SB_FREEZE_WRITE;
  	/* Release s_umount to preserve sb_start_write -> s_umount ordering */
  	up_write(&sb->s_umount);
  	sb_wait_write(sb, SB_FREEZE_WRITE);
  	down_write(&sb->s_umount);
  
  	/* 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 */
  	sync_filesystem(sb);
  
  	/* 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);
  			wake_up(&sb->s_writers.wait_unfrozen);
  			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().
  	 */
  	sb->s_writers.frozen = SB_FREEZE_COMPLETE;
  	lockdep_sb_freeze_release(sb);
  	up_write(&sb->s_umount);
  	return 0;
  }
  EXPORT_SYMBOL(freeze_super);
  
  /**
   * thaw_super -- unlock filesystem
   * @sb: the super to thaw
   *
   * Unlocks the filesystem and marks it writeable again after freeze_super().
   */
  int thaw_super(struct super_block *sb)
  {
  	int error;
  
  	down_write(&sb->s_umount);
  	if (sb->s_writers.frozen != SB_FREEZE_COMPLETE) {
  		up_write(&sb->s_umount);
  		return -EINVAL;
  	}
  
  	if (sb->s_flags & MS_RDONLY) {
  		sb->s_writers.frozen = SB_UNFROZEN;
  		goto out;
  	}
  
  	lockdep_sb_freeze_acquire(sb);
  
  	if (sb->s_op->unfreeze_fs) {
  		error = sb->s_op->unfreeze_fs(sb);
  		if (error) {
  			printk(KERN_ERR
  				"VFS:Filesystem thaw failed\n");
  			lockdep_sb_freeze_release(sb);
  			up_write(&sb->s_umount);
  			return error;
  		}
  	}
  
  	sb->s_writers.frozen = SB_UNFROZEN;
  	sb_freeze_unlock(sb);
  out:
  	wake_up(&sb->s_writers.wait_unfrozen);
  	deactivate_locked_super(sb);
  	return 0;
  }
  EXPORT_SYMBOL(thaw_super);
  