// 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/cleancache.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);

  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 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);
  	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);
  }

  /* Free a superblock that has never been seen by anyone */
  static void destroy_unused_super(struct super_block *s)

  {

  	if (!s)
  		return;
  	up_write(&s->s_umount);

  	list_lru_destroy(&s->s_dentry_lru);
  	list_lru_destroy(&s->s_inode_lru);

  	security_sb_free(s);

  	put_user_ns(s->s_user_ns);

  	kfree(s->s_subtype);

  	free_prealloced_shrinker(&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_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);

  	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;

  	}

  	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_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->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;

  	if (prealloc_shrinker(&s->s_shrink))
  		goto fail;

  	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));
  		security_sb_free(s);

  		fscrypt_sb_free(s);

  		put_user_ns(s->s_user_ns);
  		kfree(s->s_subtype);
  		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.
   */

  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 & SB_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 & SB_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 &= ~SB_ACTIVE;

  		cgroup_writeback_umount();

  		/* evict all inodes with zero refcount */

  		evict_inodes(sb);

  		/* only nonzero refcount inodes can have marks */
  		fsnotify_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);

  		if (!list_empty(&sb->s_inodes)) {

  			printk("VFS: Busy inodes after unmount of %s. "
  			   "Self-destruct in 5 seconds.  Have a nice day...
  ",
  			   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);

  	if (sb->s_bdi != &noop_backing_dev_info) {

  		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
   *
   * Find or create a superblock using the parameters stored in the filesystem
   * context and the two callback functions.
   *
   * 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 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.
   */
  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;

  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;

  	strlcpy(s->s_id, s->s_type->name, sizeof(s->s_id));
  	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);
  	register_shrinker_prepared(&s->s_shrink);
  	return s;
  
  share_extant_sb:
  	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;
  }
  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;
  	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);

  	register_shrinker_prepared(&s->s_shrink);

  	return s;
  }

  EXPORT_SYMBOL(sget);
  
  void drop_super(struct super_block *sb)
  {
  	up_read(&sb->s_umount);
  	put_super(sb);
  }
  
  EXPORT_SYMBOL(drop_super);

  void drop_super_exclusive(struct super_block *sb)
  {
  	up_write(&sb->s_umount);
  	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 (hlist_unhashed(&sb->s_instances))
  			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) {

  		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 & SB_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 & SB_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);

  static struct super_block *__get_super(struct block_device *bdev, bool excl)

  {

  	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);

  			if (!excl)
  				down_read(&sb->s_umount);
  			else
  				down_write(&sb->s_umount);

  			/* still alive? */

  			if (sb->s_root && (sb->s_flags & SB_BORN))

  				return sb;

  			if (!excl)
  				up_read(&sb->s_umount);
  			else
  				up_write(&sb->s_umount);

  			/* nope, got unmounted */

  			spin_lock(&sb_lock);

  			__put_super(sb);
  			goto rescan;

  		}
  	}
  	spin_unlock(&sb_lock);
  	return NULL;
  }

  /**

   *	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)
  {
  	return __get_super(bdev, false);
  }
  EXPORT_SYMBOL(get_super);
  
  static struct super_block *__get_super_thawed(struct block_device *bdev,
  					      bool excl)

  {
  	while (1) {

  		struct super_block *s = __get_super(bdev, excl);

  		if (!s || s->s_writers.frozen == SB_UNFROZEN)

  			return s;

  		if (!excl)
  			up_read(&s->s_umount);
  		else
  			up_write(&s->s_umount);

  		wait_event(s->s_writers.wait_unfrozen,
  			   s->s_writers.frozen == SB_UNFROZEN);

  		put_super(s);
  	}
  }

  
  /**
   *	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)
  {
  	return __get_super_thawed(bdev, false);
  }

  EXPORT_SYMBOL(get_super_thawed);
  
  /**

   *	get_super_exclusive_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) and s_umount semaphore is held
   *	in exclusive mode. %NULL is returned if no match is found.
   */
  struct super_block *get_super_exclusive_thawed(struct block_device *bdev)
  {
  	return __get_super_thawed(bdev, true);
  }
  EXPORT_SYMBOL(get_super_exclusive_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 & SB_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;
  }

  /**

   * 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 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) && bdev_read_only(sb->s_bdev))
  			return -EACCES;

  #endif

  		remount_ro = (fc->sb_flags & SB_RDONLY) && !sb_rdonly(sb);
  	}

  	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 = !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->s_readonly_remount = 1;
  			smp_wmb();

  		} else {
  			retval = sb_prepare_remount_readonly(sb);
  			if (retval)
  				return retval;

  		}

  	}

  	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
  ",
  			     sb->s_type->name, retval);
  		}

  	}

  
  	WRITE_ONCE(sb->s_flags, ((sb->s_flags & ~fc->sb_flags_mask) |
  				 (fc->sb_flags & fc->sb_flags_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;

  }

  static void do_emergency_remount_callback(struct super_block *sb)

  {

  	down_write(&sb->s_umount);
  	if (sb->s_root && sb->s_bdev && (sb->s_flags & SB_BORN) &&
  	    !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);
  		}

  	}

  	up_write(&sb->s_umount);
  }
  
  static void do_emergency_remount(struct work_struct *work)
  {
  	__iterate_supers(do_emergency_remount_callback);

  	kfree(work);

  	printk("Emergency Remount complete
  ");
  }
  
  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)
  {
  	down_write(&sb->s_umount);

  	if (sb->s_root && sb->s_flags & SB_BORN) {

  		emergency_thaw_bdev(sb);
  		thaw_super_locked(sb);
  	} else {
  		up_write(&sb->s_umount);
  	}
  }
  
  static void do_thaw_all(struct work_struct *work)
  {
  	__iterate_supers(do_thaw_all_callback);
  	kfree(work);
  	printk(KERN_WARNING "Emergency Thaw complete
  ");
  }
  
  /**
   * 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);
  	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;
  }
  
  /**
   * vfs_get_super - Get a superblock with a search key set in s_fs_info.
   * @fc: The filesystem context holding the parameters
   * @keying: How to distinguish superblocks
   * @fill_super: Helper to initialise a new superblock
   *
   * Search for a superblock and create a new one if not found.  The search
   * criterion is controlled by @keying.  If the search fails, a new superblock
   * is created and @fill_super() is called to initialise it.
   *
   * @keying can take one of a number of values:
   *
   * (1) vfs_get_single_super - Only one superblock of this type may exist on the
   *     system.  This is typically used for special system filesystems.
   *
   * (2) vfs_get_keyed_super - Multiple superblocks may exist, but they must have
   *     distinct keys (where the key is in s_fs_info).  Searching for the same
   *     key again will turn up the superblock for that key.
   *
   * (3) vfs_get_independent_super - Multiple superblocks may exist and are
   *     unkeyed.  Each call will get a new superblock.
   *
   * A permissions check is made by sget_fc() unless we're getting a superblock
   * for a kernel-internal mount or a submount.
   */
  int vfs_get_super(struct fs_context *fc,
  		  enum vfs_get_super_keying keying,
  		  int (*fill_super)(struct super_block *sb,
  				    struct fs_context *fc))
  {
  	int (*test)(struct super_block *, struct fs_context *);
  	struct super_block *sb;

  	int err;

  
  	switch (keying) {
  	case vfs_get_single_super:

  	case vfs_get_single_reconf_super:

  		test = test_single_super;
  		break;
  	case vfs_get_keyed_super:
  		test = test_keyed_super;
  		break;
  	case vfs_get_independent_super:
  		test = NULL;
  		break;
  	default:
  		BUG();
  	}
  
  	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);
  	} else {
  		fc->root = dget(sb->s_root);
  		if (keying == vfs_get_single_reconf_super) {
  			err = reconfigure_super(fc);
  			if (err < 0) {
  				dput(fc->root);
  				fc->root = NULL;
  				goto error;
  			}
  		}

  	}

  	return 0;

  
  error:
  	deactivate_locked_super(sb);
  	return err;

  }
  EXPORT_SYMBOL(vfs_get_super);

  int get_tree_nodev(struct fs_context *fc,
  		  int (*fill_super)(struct super_block *sb,
  				    struct fs_context *fc))
  {
  	return vfs_get_super(fc, vfs_get_independent_super, 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, vfs_get_single_super, fill_super);
  }
  EXPORT_SYMBOL(get_tree_single);

  int get_tree_single_reconf(struct fs_context *fc,
  		  int (*fill_super)(struct super_block *sb,
  				    struct fs_context *fc))
  {
  	return vfs_get_super(fc, vfs_get_single_reconf_super, fill_super);
  }
  EXPORT_SYMBOL(get_tree_single_reconf);

  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, vfs_get_keyed_super, fill_super);
  }
  EXPORT_SYMBOL(get_tree_keyed);

  #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;

  	s->s_bdi = bdi_get(s->s_bdev->bd_bdi);

  	return 0;
  }

  static int set_bdev_super_fc(struct super_block *s, struct fs_context *fc)
  {
  	return set_bdev_super(s, fc->sget_key);
  }
  
  static int test_bdev_super_fc(struct super_block *s, struct fs_context *fc)
  {
  	return s->s_bdev == fc->sget_key;
  }
  
  /**
   * 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 block_device *bdev;
  	struct super_block *s;
  	fmode_t mode = FMODE_READ | FMODE_EXCL;
  	int error = 0;
  
  	if (!(fc->sb_flags & SB_RDONLY))
  		mode |= FMODE_WRITE;
  
  	if (!fc->source)
  		return invalf(fc, "No source specified");
  
  	bdev = blkdev_get_by_path(fc->source, mode, fc->fs_type);
  	if (IS_ERR(bdev)) {
  		errorf(fc, "%s: Can't open blockdev", fc->source);
  		return PTR_ERR(bdev);
  	}
  
  	/* Once the superblock is inserted into the list by sget_fc(), 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);

  		blkdev_put(bdev, mode);

  		warnf(fc, "%pg: Can't mount, blockdev is frozen", bdev);
  		return -EBUSY;
  	}
  
  	fc->sb_flags |= SB_NOSEC;
  	fc->sget_key = bdev;
  	s = sget_fc(fc, test_bdev_super_fc, set_bdev_super_fc);
  	mutex_unlock(&bdev->bd_fsfreeze_mutex);

  	if (IS_ERR(s)) {
  		blkdev_put(bdev, mode);

  		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", bdev);
  			deactivate_locked_super(s);
  			blkdev_put(bdev, mode);
  			return -EBUSY;
  		}
  
  		/*
  		 * 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, fc);
  		if (error) {
  			deactivate_locked_super(s);
  			return error;
  		}
  
  		s->s_flags |= SB_ACTIVE;
  		bdev->bd_super = s;
  	}
  
  	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 (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 & SB_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 | SB_NOSEC,

  		 bdev);

  	mutex_unlock(&bdev->bd_fsfreeze_mutex);

  	if (IS_ERR(s))

  		goto error_s;

  
  	if (s->s_root) {

  		if ((flags ^ s->s_flags) & SB_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 & SB_SILENT ? 1 : 0);

  		if (error) {

  			deactivate_locked_super(s);

  			goto error;

  		}

  		s->s_flags |= SB_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 & 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);

  static 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
  ",
  		       fc->fs_type->name);
  		/* 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);

  
  	/*
  	 * Write barrier is 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.
  	 */
  	smp_wmb();

  	sb->s_flags |= 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)
  ", 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(GFP_KERNEL);
  	if (!bdi)
  		return -ENOMEM;
  
  	bdi->name = sb->s_type->name;
  
  	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;

  
  	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);
  
  /*

   * 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 & SB_BORN)) {

  		up_write(&sb->s_umount);
  		return 0;	/* sic - it's "nothing to do" */
  	}

  	if (sb_rdonly(sb)) {

  		/* 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
  ");

  			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().
   */

  static int thaw_super_locked(struct super_block *sb)

  {
  	int error;

  	if (sb->s_writers.frozen != SB_FREEZE_COMPLETE) {

  		up_write(&sb->s_umount);
  		return -EINVAL;
  	}

  	if (sb_rdonly(sb)) {

  		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
  ");

  			lockdep_sb_freeze_release(sb);

  			up_write(&sb->s_umount);
  			return error;
  		}
  	}