Skip to content
Snippets Groups Projects
Select Git revision
  • 0026e00523a85b90a92a93ddf6660939ecef3e54
  • vme-testing default
  • ci-test
  • master
  • remoteproc
  • am625-sk-ov5640
  • pcal6534-upstreaming
  • lps22df-upstreaming
  • msc-upstreaming
  • imx8mp
  • iio/noa1305
  • vme-next
  • vme-next-4.14-rc4
  • v4.14-rc4
  • v4.14-rc3
  • v4.14-rc2
  • v4.14-rc1
  • v4.13
  • vme-next-4.13-rc7
  • v4.13-rc7
  • v4.13-rc6
  • v4.13-rc5
  • v4.13-rc4
  • v4.13-rc3
  • v4.13-rc2
  • v4.13-rc1
  • v4.12
  • v4.12-rc7
  • v4.12-rc6
  • v4.12-rc5
  • v4.12-rc4
  • v4.12-rc3
32 results

file.c

Blame
  • super.c 47.94 KiB
    // 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...\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);
    	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\n",
    			     sb->s_type->name, retval);
    		}
    	}
    
    	WRITE_ONCE(sb->s_flags, ((sb->s_flags & ~fc->sb_flags_mask) |
    				 (fc->sb_flags & fc->sb_flags_mask)));
    	/* 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\n");
    }
    
    void emergency_remount(void)
    {
    	struct work_struct *work;
    
    	work = kmalloc(sizeof(*work), GFP_ATOMIC);
    	if (work) {
    		INIT_WORK(work, do_emergency_remount);
    		schedule_work(work);
    	}
    }
    
    static void do_thaw_all_callback(struct super_block *sb)
    {
    	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\n");
    }
    
    /**
     * emergency_thaw_all -- forcibly thaw every frozen filesystem
     *
     * Used for emergency unfreeze of all filesystems via SysRq
     */
    void emergency_thaw_all(void)
    {
    	struct work_struct *work;
    
    	work = kmalloc(sizeof(*work), GFP_ATOMIC);
    	if (work) {
    		INIT_WORK(work, do_thaw_all);
    		schedule_work(work);
    	}
    }
    
    static DEFINE_IDA(unnamed_dev_ida);
    
    /**
     * get_anon_bdev - Allocate a block device for filesystems which don't have one.
     * @p: Pointer to a dev_t.
     *
     * Filesystems which don't use real block devices can call this function
     * to allocate a virtual block device.
     *
     * Context: Any context.  Frequently called while holding sb_lock.
     * Return: 0 on success, -EMFILE if there are no anonymous bdevs left
     * or -ENOMEM if memory allocation failed.
     */
    int get_anon_bdev(dev_t *p)
    {
    	int dev;
    
    	/*
    	 * Many userspace utilities consider an FSID of 0 invalid.
    	 * Always return at least 1 from get_anon_bdev.
    	 */
    	dev = ida_alloc_range(&unnamed_dev_ida, 1, (1 << MINORBITS) - 1,
    			GFP_ATOMIC);
    	if (dev == -ENOSPC)
    		dev = -EMFILE;
    	if (dev < 0)
    		return dev;
    
    	*p = MKDEV(0, dev);
    	return 0;
    }
    EXPORT_SYMBOL(get_anon_bdev);
    
    void free_anon_bdev(dev_t dev)
    {
    	ida_free(&unnamed_dev_ida, MINOR(dev));
    }
    EXPORT_SYMBOL(free_anon_bdev);
    
    int set_anon_super(struct super_block *s, void *data)
    {
    	return get_anon_bdev(&s->s_dev);
    }
    EXPORT_SYMBOL(set_anon_super);
    
    void kill_anon_super(struct super_block *sb)
    {
    	dev_t dev = sb->s_dev;
    	generic_shutdown_super(sb);
    	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);
    		warnf(fc, "%pg: Can't mount, blockdev is frozen", bdev);
    		blkdev_put(bdev, mode);
    		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\n",
    		       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)\n", fc->fs_type->name, sb->s_maxbytes);
    
    	return 0;
    }
    EXPORT_SYMBOL(vfs_get_tree);
    
    /*
     * Setup private BDI for given superblock. It gets automatically cleaned up
     * in generic_shutdown_super().
     */
    int super_setup_bdi_name(struct super_block *sb, char *fmt, ...)
    {
    	struct backing_dev_info *bdi;
    	int err;
    	va_list args;
    
    	bdi = bdi_alloc(NUMA_NO_NODE);
    	if (!bdi)
    		return -ENOMEM;
    
    	va_start(args, fmt);
    	err = bdi_register_va(bdi, fmt, args);
    	va_end(args);
    	if (err) {
    		bdi_put(bdi);
    		return err;
    	}
    	WARN_ON(sb->s_bdi != &noop_backing_dev_info);
    	sb->s_bdi = bdi;
    
    	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\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().
     */
    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\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;
    }
    
    int thaw_super(struct super_block *sb)
    {
    	down_write(&sb->s_umount);
    	return thaw_super_locked(sb);
    }
    EXPORT_SYMBOL(thaw_super);