LSM: lift parsing LSM options into the caller of ->sb_kern_mount()

[linux.git] / fs / super.c

// SPDX-License-Identifier: GPL-2.0
/*
 *  linux/fs/super.c
 *
 *  Copyright (C) 1991, 1992  Linus Torvalds
 *
 *  super.c contains code to handle: - mount structures
 *                                   - super-block tables
 *                                   - filesystem drivers list
 *                                   - mount system call
 *                                   - umount system call
 *                                   - ustat system call
 *
 * GK 2/5/95  -  Changed to support mounting the root fs via NFS
 *
 *  Added kerneld support: Jacques Gelinas and Bjorn Ekwall
 *  Added change_root: Werner Almesberger & Hans Lermen, Feb '96
 *  Added options to /proc/mounts:
 *    Torbjörn Lindh (torbjorn.lindh@gopta.se), April 14, 1996.
 *  Added devfs support: Richard Gooch <rgooch@atnf.csiro.au>, 13-JAN-1998
 *  Heavily rewritten for 'one fs - one tree' dcache architecture. AV, Mar 2000
 */

#include <linux/export.h>
#include <linux/slab.h>
#include <linux/blkdev.h>
#include <linux/mount.h>
#include <linux/security.h>
#include <linux/writeback.h>            /* for the emergency remount stuff */
#include <linux/idr.h>
#include <linux/mutex.h>
#include <linux/backing-dev.h>
#include <linux/rculist_bl.h>
#include <linux/cleancache.h>
#include <linux/fsnotify.h>
#include <linux/lockdep.h>
#include <linux/user_namespace.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->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);
                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;

                fsnotify_sb_delete(sb);
                cgroup_writeback_umount();

                evict_inodes(sb);

                if (sb->s_dio_done_wq) {
                        destroy_workqueue(sb->s_dio_done_wq);
                        sb->s_dio_done_wq = NULL;
                }

                if (sop->put_super)
                        sop->put_super(sb);

                if (!list_empty(&sb->s_inodes)) {
                        printk("VFS: Busy inodes after unmount of %s. "
                           "Self-destruct in 5 seconds.  Have a nice day...\n",
                           sb->s_id);
                }
        }
        spin_lock(&sb_lock);
        /* should be initialized for __put_super_and_need_restart() */
        hlist_del_init(&sb->s_instances);
        spin_unlock(&sb_lock);
        up_write(&sb->s_umount);
        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);

/**
 *      sget_userns -   find or create a superblock
 *      @type:  filesystem type superblock should belong to
 *      @test:  comparison callback
 *      @set:   setup callback
 *      @flags: mount flags
 *      @user_ns: User namespace for the super_block
 *      @data:  argument to each of them
 */
struct super_block *sget_userns(struct file_system_type *type,
                        int (*test)(struct super_block *,void *),
                        int (*set)(struct super_block *,void *),
                        int flags, struct user_namespace *user_ns,
                        void *data)
{
        struct super_block *s = NULL;
        struct super_block *old;
        int err;

        if (!(flags & (SB_KERNMOUNT|SB_SUBMOUNT)) &&
            !(type->fs_flags & FS_USERNS_MOUNT) &&
            !capable(CAP_SYS_ADMIN))
                return ERR_PTR(-EPERM);
retry:
        spin_lock(&sb_lock);
        if (test) {
                hlist_for_each_entry(old, &type->fs_supers, s_instances) {
                        if (!test(old, data))
                                continue;
                        if (user_ns != old->s_user_ns) {
                                spin_unlock(&sb_lock);
                                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_userns);

/**
 *      sget    -       find or create a superblock
 *      @type:    filesystem type superblock should belong to
 *      @test:    comparison callback
 *      @set:     setup callback
 *      @flags:   mount flags
 *      @data:    argument to each of them
 */
struct super_block *sget(struct file_system_type *type,
                        int (*test)(struct super_block *,void *),
                        int (*set)(struct super_block *,void *),
                        int flags,
                        void *data)
{
        struct user_namespace *user_ns = current_user_ns();

        /* We don't yet pass the user namespace of the parent
         * mount through to here so always use &init_user_ns
         * until that changes.
         */
        if (flags & SB_SUBMOUNT)
                user_ns = &init_user_ns;

        /* Ensure the requestor has permissions over the target filesystem */
        if (!(flags & (SB_KERNMOUNT|SB_SUBMOUNT)) && !ns_capable(user_ns, CAP_SYS_ADMIN))
                return ERR_PTR(-EPERM);

        return sget_userns(type, test, set, flags, user_ns, data);
}

EXPORT_SYMBOL(sget);

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

EXPORT_SYMBOL(drop_super);

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;
}

/**
 *      do_remount_sb - asks filesystem to change mount options.
 *      @sb:    superblock in question
 *      @sb_flags: revised superblock flags
 *      @data:  the rest of options
 *      @force: whether or not to force the change
 *
 *      Alters the mount options of a mounted file system.
 */
int do_remount_sb(struct super_block *sb, int sb_flags, void *data, int force)
{
        int retval;
        int remount_ro;

        if (sb->s_writers.frozen != SB_UNFROZEN)
                return -EBUSY;

#ifdef CONFIG_BLOCK
        if (!(sb_flags & SB_RDONLY) && bdev_read_only(sb->s_bdev))
                return -EACCES;
#endif

        remount_ro = (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_flags & SB_RDONLY) && !sb_rdonly(sb);
                }
        }
        shrink_dcache_sb(sb);

        /* If we are remounting RDONLY and current sb is read/write,
           make sure there are no rw files opened */
        if (remount_ro) {
                if (force) {
                        sb->s_readonly_remount = 1;
                        smp_wmb();
                } else {
                        retval = sb_prepare_remount_readonly(sb);
                        if (retval)
                                return retval;
                }
        }

        if (sb->s_op->remount_fs) {
                retval = sb->s_op->remount_fs(sb, &sb_flags, data);
                if (retval) {
                        if (!force)
                                goto cancel_readonly;
                        /* If forced remount, go ahead despite any errors */
                        WARN(1, "forced remount of a %s fs returned %i\n",
                             sb->s_type->name, retval);
                }
        }
        sb->s_flags = (sb->s_flags & ~MS_RMT_MASK) | (sb_flags & MS_RMT_MASK);
        /* Needs to be ordered wrt mnt_is_readonly() */
        smp_wmb();
        sb->s_readonly_remount = 0;

        /*
         * Some filesystems modify their metadata via some other path than the
         * bdev buffer cache (eg. use a private mapping, or directories in
         * pagecache, etc). Also file data modifications go via their own
         * mappings. So If we try to mount readonly then copy the filesystem
         * from bdev, we could get stale data, so invalidate it to give a best
         * effort at coherency.
         */
        if (remount_ro && sb->s_bdev)
                invalidate_bdev(sb->s_bdev);
        return 0;

cancel_readonly:
        sb->s_readonly_remount = 0;
        return retval;
}

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)) {
                /*
                 * What lock protects sb->s_flags??
                 */
                do_remount_sb(sb, SB_RDONLY, NULL, 1);
        }
        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);

static int ns_test_super(struct super_block *sb, void *data)
{
        return sb->s_fs_info == data;
}

static int ns_set_super(struct super_block *sb, void *data)
{
        sb->s_fs_info = data;
        return set_anon_super(sb, NULL);
}

struct dentry *mount_ns(struct file_system_type *fs_type,
        int flags, void *data, void *ns, struct user_namespace *user_ns,
        int (*fill_super)(struct super_block *, void *, int))
{
        struct super_block *sb;

        /* Don't allow mounting unless the caller has CAP_SYS_ADMIN
         * over the namespace.
         */
        if (!(flags & SB_KERNMOUNT) && !ns_capable(user_ns, CAP_SYS_ADMIN))
                return ERR_PTR(-EPERM);

        sb = sget_userns(fs_type, ns_test_super, ns_set_super, flags,
                         user_ns, ns);
        if (IS_ERR(sb))
                return ERR_CAST(sb);

        if (!sb->s_root) {
                int err;
                err = fill_super(sb, data, flags & SB_SILENT ? 1 : 0);
                if (err) {
                        deactivate_locked_super(sb);
                        return ERR_PTR(err);
                }

                sb->s_flags |= SB_ACTIVE;
        }

        return dget(sb->s_root);
}

EXPORT_SYMBOL(mount_ns);

#ifdef CONFIG_BLOCK
static int set_bdev_super(struct super_block *s, void *data)
{
        s->s_bdev = data;
        s->s_dev = s->s_bdev->bd_dev;
        s->s_bdi = bdi_get(s->s_bdev->bd_bdi);

        return 0;
}

static int test_bdev_super(struct super_block *s, void *data)
{
        return (void *)s->s_bdev == data;
}

struct dentry *mount_bdev(struct file_system_type *fs_type,
        int flags, const char *dev_name, void *data,
        int (*fill_super)(struct super_block *, void *, int))
{
        struct block_device *bdev;
        struct super_block *s;
        fmode_t mode = FMODE_READ | FMODE_EXCL;
        int error = 0;

        if (!(flags & 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 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) {
                        deactivate_locked_super(s);
                        return ERR_PTR(error);
                }
                s->s_flags |= SB_ACTIVE;
        } else {
                do_remount_sb(s, flags, data, 0);
        }
        return dget(s->s_root);
}
EXPORT_SYMBOL(mount_single);

struct dentry *
mount_fs(struct file_system_type *type, int flags, const char *name, void *data)
{
        struct dentry *root;
        struct super_block *sb;
        int error = -ENOMEM;
        struct security_mnt_opts opts;

        security_init_mnt_opts(&opts);

        if (data && !(type->fs_flags & FS_BINARY_MOUNTDATA)) {
                char *secdata = alloc_secdata();
                if (!secdata)
                        return ERR_PTR(-ENOMEM);

                error = security_sb_copy_data(data, secdata);
                if (error) {
                        free_secdata(secdata);
                        return ERR_PTR(error);
                }

                error = security_sb_parse_opts_str(secdata, &opts);
                free_secdata(secdata);
                if (error)
                        return ERR_PTR(error);
        }

        root = type->mount(type, flags, name, data);
        if (IS_ERR(root)) {
                error = PTR_ERR(root);
                goto out_free_secdata;
        }
        sb = root->d_sb;
        BUG_ON(!sb);
        WARN_ON(!sb->s_bdi);

        /*
         * Write barrier is for super_cache_count(). We place it before setting
         * 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_kern_mount(sb, flags, &opts);
        if (error)
                goto out_sb;

        /*
         * filesystems should never set s_maxbytes larger than MAX_LFS_FILESIZE
         * but s_maxbytes was an unsigned long long for many releases. Throw
         * this warning for a little while to try and catch filesystems that
         * violate this rule.
         */
        WARN((sb->s_maxbytes < 0), "%s set sb->s_maxbytes to "
                "negative value (%lld)\n", type->name, sb->s_maxbytes);

        up_write(&sb->s_umount);
        security_free_mnt_opts(&opts);
        return root;
out_sb:
        dput(root);
        deactivate_locked_super(sb);
out_free_secdata:
        security_free_mnt_opts(&opts);
        return ERR_PTR(error);
}

/*
 * 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\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);