[linux-block.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/fscrypt.h>
#include <linux/fsnotify.h>
#include <linux/lockdep.h>
#include <linux/user_namespace.h>
#include <linux/fs_context.h>
#include <uapi/linux/mount.h>
#include "internal.h"

static int thaw_super_locked(struct super_block *sb, enum freeze_holder who,
                             const void *freeze_owner);

static LIST_HEAD(super_blocks);
static DEFINE_SPINLOCK(sb_lock);

static char *sb_writers_name[SB_FREEZE_LEVELS] = {
        "sb_writers",
        "sb_pagefaults",
        "sb_internal",
};

static inline void __super_lock(struct super_block *sb, bool excl)
{
        if (excl)
                down_write(&sb->s_umount);
        else
                down_read(&sb->s_umount);
}

static inline void super_unlock(struct super_block *sb, bool excl)
{
        if (excl)
                up_write(&sb->s_umount);
        else
                up_read(&sb->s_umount);
}

static inline void __super_lock_excl(struct super_block *sb)
{
        __super_lock(sb, true);
}

static inline void super_unlock_excl(struct super_block *sb)
{
        super_unlock(sb, true);
}

static inline void super_unlock_shared(struct super_block *sb)
{
        super_unlock(sb, false);
}

static bool super_flags(const struct super_block *sb, unsigned int flags)
{
        /*
         * Pairs with smp_store_release() in super_wake() and ensures
         * that we see @flags after we're woken.
         */
        return smp_load_acquire(&sb->s_flags) & flags;
}

/**
 * super_lock - wait for superblock to become ready and lock it
 * @sb: superblock to wait for
 * @excl: whether exclusive access is required
 *
 * If the superblock has neither passed through vfs_get_tree() or
 * generic_shutdown_super() yet wait for it to happen. Either superblock
 * creation will succeed and SB_BORN is set by vfs_get_tree() or we're
 * woken and we'll see SB_DYING.
 *
 * The caller must have acquired a temporary reference on @sb->s_count.
 *
 * Return: The function returns true if SB_BORN was set and with
 *         s_umount held. The function returns false if SB_DYING was
 *         set and without s_umount held.
 */
static __must_check bool super_lock(struct super_block *sb, bool excl)
{
        lockdep_assert_not_held(&sb->s_umount);

        /* wait until the superblock is ready or dying */
        wait_var_event(&sb->s_flags, super_flags(sb, SB_BORN | SB_DYING));

        /* Don't pointlessly acquire s_umount. */
        if (super_flags(sb, SB_DYING))
                return false;

        __super_lock(sb, excl);

        /*
         * Has gone through generic_shutdown_super() in the meantime.
         * @sb->s_root is NULL and @sb->s_active is 0. No one needs to
         * grab a reference to this. Tell them so.
         */
        if (sb->s_flags & SB_DYING) {
                super_unlock(sb, excl);
                return false;
        }

        WARN_ON_ONCE(!(sb->s_flags & SB_BORN));
        return true;
}

/* wait and try to acquire read-side of @sb->s_umount */
static inline bool super_lock_shared(struct super_block *sb)
{
        return super_lock(sb, false);
}

/* wait and try to acquire write-side of @sb->s_umount */
static inline bool super_lock_excl(struct super_block *sb)
{
        return super_lock(sb, true);
}

/* wake waiters */
#define SUPER_WAKE_FLAGS (SB_BORN | SB_DYING | SB_DEAD)
static void super_wake(struct super_block *sb, unsigned int flag)
{
        WARN_ON_ONCE((flag & ~SUPER_WAKE_FLAGS));
        WARN_ON_ONCE(hweight32(flag & SUPER_WAKE_FLAGS) > 1);

        /*
         * Pairs with smp_load_acquire() in super_lock() to make sure
         * all initializations in the superblock are seen by the user
         * seeing SB_BORN sent.
         */
        smp_store_release(&sb->s_flags, sb->s_flags | flag);
        /*
         * Pairs with the barrier in prepare_to_wait_event() to make sure
         * ___wait_var_event() either sees SB_BORN set or
         * waitqueue_active() check in wake_up_var() sees the waiter.
         */
        smp_mb();
        wake_up_var(&sb->s_flags);
}

/*
 * One thing we have to be careful of with a per-sb shrinker is that we don't
 * drop the last active reference to the superblock from within the shrinker.
 * If that happens we could trigger unregistering the shrinker from within the
 * shrinker path and that leads to deadlock on the shrinker_mutex. Hence we
 * take a passive reference to the superblock to avoid this from occurring.
 */
static unsigned long super_cache_scan(struct shrinker *shrink,
                                      struct shrink_control *sc)
{
        struct super_block *sb;
        long    fs_objects = 0;
        long    total_objects;
        long    freed = 0;
        long    dentries;
        long    inodes;

        sb = shrink->private_data;

        /*
         * Deadlock avoidance.  We may hold various FS locks, and we don't want
         * to recurse into the FS that called us in clear_inode() and friends..
         */
        if (!(sc->gfp_mask & __GFP_FS))
                return SHRINK_STOP;

        if (!super_trylock_shared(sb))
                return SHRINK_STOP;

        if (sb->s_op->nr_cached_objects)
                fs_objects = sb->s_op->nr_cached_objects(sb, sc);

        inodes = list_lru_shrink_count(&sb->s_inode_lru, sc);
        dentries = list_lru_shrink_count(&sb->s_dentry_lru, sc);
        total_objects = dentries + inodes + fs_objects;
        if (!total_objects)
                total_objects = 1;

        /* proportion the scan between the caches */
        dentries = mult_frac(sc->nr_to_scan, dentries, total_objects);
        inodes = mult_frac(sc->nr_to_scan, inodes, total_objects);
        fs_objects = mult_frac(sc->nr_to_scan, fs_objects, total_objects);

        /*
         * prune the dcache first as the icache is pinned by it, then
         * prune the icache, followed by the filesystem specific caches
         *
         * Ensure that we always scan at least one object - memcg kmem
         * accounting uses this to fully empty the caches.
         */
        sc->nr_to_scan = dentries + 1;
        freed = prune_dcache_sb(sb, sc);
        sc->nr_to_scan = inodes + 1;
        freed += prune_icache_sb(sb, sc);

        if (fs_objects) {
                sc->nr_to_scan = fs_objects + 1;
                freed += sb->s_op->free_cached_objects(sb, sc);
        }

        super_unlock_shared(sb);
        return freed;
}

static unsigned long super_cache_count(struct shrinker *shrink,
                                       struct shrink_control *sc)
{
        struct super_block *sb;
        long    total_objects = 0;

        sb = shrink->private_data;

        /*
         * We don't call super_trylock_shared() here as it is a scalability
         * bottleneck, so we're exposed to partial setup state. The shrinker
         * rwsem does not protect filesystem operations backing
         * list_lru_shrink_count() or s_op->nr_cached_objects(). Counts can
         * change between super_cache_count and super_cache_scan, so we really
         * don't need locks here.
         *
         * However, if we are currently mounting the superblock, the underlying
         * filesystem might be in a state of partial construction and hence it
         * is dangerous to access it.  super_trylock_shared() uses a SB_BORN check
         * to avoid this situation, so do the same here. The memory barrier is
         * matched with the one in mount_fs() as we don't hold locks here.
         */
        if (!(sb->s_flags & SB_BORN))
                return 0;
        smp_rmb();

        if (sb->s_op && sb->s_op->nr_cached_objects)
                total_objects = sb->s_op->nr_cached_objects(sb, sc);

        total_objects += list_lru_shrink_count(&sb->s_dentry_lru, sc);
        total_objects += list_lru_shrink_count(&sb->s_inode_lru, sc);

        if (!total_objects)
                return SHRINK_EMPTY;

        total_objects = vfs_pressure_ratio(total_objects);
        return total_objects;
}

static void destroy_super_work(struct work_struct *work)
{
        struct super_block *s = container_of(work, struct super_block,
                                                        destroy_work);
        fsnotify_sb_free(s);
        security_sb_free(s);
        put_user_ns(s->s_user_ns);
        kfree(s->s_subtype);
        for (int i = 0; i < SB_FREEZE_LEVELS; i++)
                percpu_free_rwsem(&s->s_writers.rw_sem[i]);
        kfree(s);
}

static void destroy_super_rcu(struct rcu_head *head)
{
        struct super_block *s = container_of(head, struct super_block, rcu);
        INIT_WORK(&s->destroy_work, destroy_super_work);
        schedule_work(&s->destroy_work);
}

/* Free a superblock that has never been seen by anyone */
static void destroy_unused_super(struct super_block *s)
{
        if (!s)
                return;
        super_unlock_excl(s);
        list_lru_destroy(&s->s_dentry_lru);
        list_lru_destroy(&s->s_inode_lru);
        shrinker_free(s->s_shrink);
        /* no delays needed */
        destroy_super_work(&s->destroy_work);
}

/**
 *      alloc_super     -       create new superblock
 *      @type:  filesystem type superblock should belong to
 *      @flags: the mount flags
 *      @user_ns: User namespace for the super_block
 *
 *      Allocates and initializes a new &struct super_block.  alloc_super()
 *      returns a pointer new superblock or %NULL if allocation had failed.
 */
static struct super_block *alloc_super(struct file_system_type *type, int flags,
                                       struct user_namespace *user_ns)
{
        struct super_block *s = kzalloc(sizeof(struct super_block), GFP_KERNEL);
        static const struct super_operations default_op;
        int i;

        if (!s)
                return NULL;

        INIT_LIST_HEAD(&s->s_mounts);
        s->s_user_ns = get_user_ns(user_ns);
        init_rwsem(&s->s_umount);
        lockdep_set_class(&s->s_umount, &type->s_umount_key);
        /*
         * sget() can have s_umount recursion.
         *
         * When it cannot find a suitable sb, it allocates a new
         * one (this one), and tries again to find a suitable old
         * one.
         *
         * In case that succeeds, it will acquire the s_umount
         * lock of the old one. Since these are clearly distrinct
         * locks, and this object isn't exposed yet, there's no
         * risk of deadlocks.
         *
         * Annotate this by putting this lock in a different
         * subclass.
         */
        down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING);

        if (security_sb_alloc(s))
                goto fail;

        for (i = 0; i < SB_FREEZE_LEVELS; i++) {
                if (__percpu_init_rwsem(&s->s_writers.rw_sem[i],
                                        sb_writers_name[i],
                                        &type->s_writers_key[i]))
                        goto fail;
        }
        s->s_bdi = &noop_backing_dev_info;
        s->s_flags = flags;
        if (s->s_user_ns != &init_user_ns)
                s->s_iflags |= SB_I_NODEV;
        INIT_HLIST_NODE(&s->s_instances);
        INIT_HLIST_BL_HEAD(&s->s_roots);
        mutex_init(&s->s_sync_lock);
        INIT_LIST_HEAD(&s->s_inodes);
        spin_lock_init(&s->s_inode_list_lock);
        INIT_LIST_HEAD(&s->s_inodes_wb);
        spin_lock_init(&s->s_inode_wblist_lock);

        s->s_count = 1;
        atomic_set(&s->s_active, 1);
        mutex_init(&s->s_vfs_rename_mutex);
        lockdep_set_class(&s->s_vfs_rename_mutex, &type->s_vfs_rename_key);
        init_rwsem(&s->s_dquot.dqio_sem);
        s->s_maxbytes = MAX_NON_LFS;
        s->s_op = &default_op;
        s->s_time_gran = 1000000000;
        s->s_time_min = TIME64_MIN;
        s->s_time_max = TIME64_MAX;

        s->s_shrink = shrinker_alloc(SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE,
                                     "sb-%s", type->name);
        if (!s->s_shrink)
                goto fail;

        s->s_shrink->scan_objects = super_cache_scan;
        s->s_shrink->count_objects = super_cache_count;
        s->s_shrink->batch = 1024;
        s->s_shrink->private_data = s;

        if (list_lru_init_memcg(&s->s_dentry_lru, s->s_shrink))
                goto fail;
        if (list_lru_init_memcg(&s->s_inode_lru, s->s_shrink))
                goto fail;
        return s;

fail:
        destroy_unused_super(s);
        return NULL;
}

/* Superblock refcounting  */

/*
 * Drop a superblock's refcount.  The caller must hold sb_lock.
 */
static void __put_super(struct super_block *s)
{
        if (!--s->s_count) {
                list_del_init(&s->s_list);
                WARN_ON(s->s_dentry_lru.node);
                WARN_ON(s->s_inode_lru.node);
                WARN_ON(!list_empty(&s->s_mounts));
                call_rcu(&s->rcu, destroy_super_rcu);
        }
}

/**
 *      put_super       -       drop a temporary reference to superblock
 *      @sb: superblock in question
 *
 *      Drops a temporary reference, frees superblock if there's no
 *      references left.
 */
void put_super(struct super_block *sb)
{
        spin_lock(&sb_lock);
        __put_super(sb);
        spin_unlock(&sb_lock);
}

static void kill_super_notify(struct super_block *sb)
{
        lockdep_assert_not_held(&sb->s_umount);

        /* already notified earlier */
        if (sb->s_flags & SB_DEAD)
                return;

        /*
         * Remove it from @fs_supers so it isn't found by new
         * sget{_fc}() walkers anymore. Any concurrent mounter still
         * managing to grab a temporary reference is guaranteed to
         * already see SB_DYING and will wait until we notify them about
         * SB_DEAD.
         */
        spin_lock(&sb_lock);
        hlist_del_init(&sb->s_instances);
        spin_unlock(&sb_lock);

        /*
         * Let concurrent mounts know that this thing is really dead.
         * We don't need @sb->s_umount here as every concurrent caller
         * will see SB_DYING and either discard the superblock or wait
         * for SB_DEAD.
         */
        super_wake(sb, SB_DEAD);
}

/**
 *      deactivate_locked_super -       drop an active reference to superblock
 *      @s: superblock to deactivate
 *
 *      Drops an active reference to superblock, converting it into a temporary
 *      one if there is no other active references left.  In that case we
 *      tell fs driver to shut it down and drop the temporary reference we
 *      had just acquired.
 *
 *      Caller holds exclusive lock on superblock; that lock is released.
 */
void deactivate_locked_super(struct super_block *s)
{
        struct file_system_type *fs = s->s_type;
        if (atomic_dec_and_test(&s->s_active)) {
                shrinker_free(s->s_shrink);
                fs->kill_sb(s);

                kill_super_notify(s);

                /*
                 * Since list_lru_destroy() may sleep, we cannot call it from
                 * put_super(), where we hold the sb_lock. Therefore we destroy
                 * the lru lists right now.
                 */
                list_lru_destroy(&s->s_dentry_lru);
                list_lru_destroy(&s->s_inode_lru);

                put_filesystem(fs);
                put_super(s);
        } else {
                super_unlock_excl(s);
        }
}

EXPORT_SYMBOL(deactivate_locked_super);

/**
 *      deactivate_super        -       drop an active reference to superblock
 *      @s: superblock to deactivate
 *
 *      Variant of deactivate_locked_super(), except that superblock is *not*
 *      locked by caller.  If we are going to drop the final active reference,
 *      lock will be acquired prior to that.
 */
void deactivate_super(struct super_block *s)
{
        if (!atomic_add_unless(&s->s_active, -1, 1)) {
                __super_lock_excl(s);
                deactivate_locked_super(s);
        }
}

EXPORT_SYMBOL(deactivate_super);

/**
 * grab_super - acquire an active reference to a superblock
 * @sb: superblock to acquire
 *
 * Acquire a temporary reference on a superblock and try to trade it for
 * an active reference. This is used in sget{_fc}() to wait for a
 * superblock to either become SB_BORN or for it to pass through
 * sb->kill() and be marked as SB_DEAD.
 *
 * Return: This returns true if an active reference could be acquired,
 *         false if not.
 */
static bool grab_super(struct super_block *sb)
{
        bool locked;

        sb->s_count++;
        spin_unlock(&sb_lock);
        locked = super_lock_excl(sb);
        if (locked) {
                if (atomic_inc_not_zero(&sb->s_active)) {
                        put_super(sb);
                        return true;
                }
                super_unlock_excl(sb);
        }
        wait_var_event(&sb->s_flags, super_flags(sb, SB_DEAD));
        put_super(sb);
        return false;
}

/*
 *      super_trylock_shared - try to grab ->s_umount shared
 *      @sb: reference we are trying to grab
 *
 *      Try to prevent fs shutdown.  This is used in places where we
 *      cannot take an active reference but we need to ensure that the
 *      filesystem is not shut down while we are working on it. It returns
 *      false if we cannot acquire s_umount or if we lose the race and
 *      filesystem already got into shutdown, and returns true with the s_umount
 *      lock held in read mode in case of success. On successful return,
 *      the caller must drop the s_umount lock when done.
 *
 *      Note that unlike get_super() et.al. this one does *not* bump ->s_count.
 *      The reason why it's safe is that we are OK with doing trylock instead
 *      of down_read().  There's a couple of places that are OK with that, but
 *      it's very much not a general-purpose interface.
 */
bool super_trylock_shared(struct super_block *sb)
{
        if (down_read_trylock(&sb->s_umount)) {
                if (!(sb->s_flags & SB_DYING) && sb->s_root &&
                    (sb->s_flags & SB_BORN))
                        return true;
                super_unlock_shared(sb);
        }

        return false;
}

/**
 *      retire_super    -       prevents superblock from being reused
 *      @sb: superblock to retire
 *
 *      The function marks superblock to be ignored in superblock test, which
 *      prevents it from being reused for any new mounts.  If the superblock has
 *      a private bdi, it also unregisters it, but doesn't reduce the refcount
 *      of the superblock to prevent potential races.  The refcount is reduced
 *      by generic_shutdown_super().  The function can not be called
 *      concurrently with generic_shutdown_super().  It is safe to call the
 *      function multiple times, subsequent calls have no effect.
 *
 *      The marker will affect the re-use only for block-device-based
 *      superblocks.  Other superblocks will still get marked if this function
 *      is used, but that will not affect their reusability.
 */
void retire_super(struct super_block *sb)
{
        WARN_ON(!sb->s_bdev);
        __super_lock_excl(sb);
        if (sb->s_iflags & SB_I_PERSB_BDI) {
                bdi_unregister(sb->s_bdi);
                sb->s_iflags &= ~SB_I_PERSB_BDI;
        }
        sb->s_iflags |= SB_I_RETIRED;
        super_unlock_excl(sb);
}
EXPORT_SYMBOL(retire_super);

/**
 *      generic_shutdown_super  -       common helper for ->kill_sb()
 *      @sb: superblock to kill
 *
 *      generic_shutdown_super() does all fs-independent work on superblock
 *      shutdown.  Typical ->kill_sb() should pick all fs-specific objects
 *      that need destruction out of superblock, call generic_shutdown_super()
 *      and release aforementioned objects.  Note: dentries and inodes _are_
 *      taken care of and do not need specific handling.
 *
 *      Upon calling this function, the filesystem may no longer alter or
 *      rearrange the set of dentries belonging to this super_block, nor may it
 *      change the attachments of dentries to inodes.
 */
void generic_shutdown_super(struct super_block *sb)
{
        const struct super_operations *sop = sb->s_op;

        if (sb->s_root) {
                shrink_dcache_for_umount(sb);
                sync_filesystem(sb);
                sb->s_flags &= ~SB_ACTIVE;

                cgroup_writeback_umount(sb);

                /* Evict all inodes with zero refcount. */
                evict_inodes(sb);

                /*
                 * Clean up and evict any inodes that still have references due
                 * to fsnotify or the security policy.
                 */
                fsnotify_sb_delete(sb);
                security_sb_delete(sb);

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

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

                /*
                 * Now that all potentially-encrypted inodes have been evicted,
                 * the fscrypt keyring can be destroyed.
                 */
                fscrypt_destroy_keyring(sb);

                if (CHECK_DATA_CORRUPTION(!list_empty(&sb->s_inodes), NULL,
                                "VFS: Busy inodes after unmount of %s (%s)",
                                sb->s_id, sb->s_type->name)) {
                        /*
                         * Adding a proper bailout path here would be hard, but
                         * we can at least make it more likely that a later
                         * iput_final() or such crashes cleanly.
                         */
                        struct inode *inode;

                        spin_lock(&sb->s_inode_list_lock);
                        list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
                                inode->i_op = VFS_PTR_POISON;
                                inode->i_sb = VFS_PTR_POISON;
                                inode->i_mapping = VFS_PTR_POISON;
                        }
                        spin_unlock(&sb->s_inode_list_lock);
                }
        }
        /*
         * Broadcast to everyone that grabbed a temporary reference to this
         * superblock before we removed it from @fs_supers that the superblock
         * is dying. Every walker of @fs_supers outside of sget{_fc}() will now
         * discard this superblock and treat it as dead.
         *
         * We leave the superblock on @fs_supers so it can be found by
         * sget{_fc}() until we passed sb->kill_sb().
         */
        super_wake(sb, SB_DYING);
        super_unlock_excl(sb);
        if (sb->s_bdi != &noop_backing_dev_info) {
                if (sb->s_iflags & SB_I_PERSB_BDI)
                        bdi_unregister(sb->s_bdi);
                bdi_put(sb->s_bdi);
                sb->s_bdi = &noop_backing_dev_info;
        }
}

EXPORT_SYMBOL(generic_shutdown_super);

bool mount_capable(struct fs_context *fc)
{
        if (!(fc->fs_type->fs_flags & FS_USERNS_MOUNT))
                return capable(CAP_SYS_ADMIN);
        else
                return ns_capable(fc->user_ns, CAP_SYS_ADMIN);
}

/**
 * sget_fc - Find or create a superblock
 * @fc: Filesystem context.
 * @test: Comparison callback
 * @set: Setup callback
 *
 * Create a new superblock or find an existing one.
 *
 * The @test callback is used to find a matching existing superblock.
 * Whether or not the requested parameters in @fc are taken into account
 * is specific to the @test callback that is used. They may even be
 * completely ignored.
 *
 * If an extant superblock is matched, it will be returned unless:
 *
 * (1) the namespace the filesystem context @fc and the extant
 *     superblock's namespace differ
 *
 * (2) the filesystem context @fc has requested that reusing an extant
 *     superblock is not allowed
 *
 * In both cases EBUSY will be returned.
 *
 * If no match is made, a new superblock will be allocated and basic
 * initialisation will be performed (s_type, s_fs_info and s_id will be
 * set and the @set callback will be invoked), the superblock will be
 * published and it will be returned in a partially constructed state
 * with SB_BORN and SB_ACTIVE as yet unset.
 *
 * Return: On success, an extant or newly created superblock is
 *         returned. On failure an error pointer is returned.
 */
struct super_block *sget_fc(struct fs_context *fc,
                            int (*test)(struct super_block *, struct fs_context *),
                            int (*set)(struct super_block *, struct fs_context *))
{
        struct super_block *s = NULL;
        struct super_block *old;
        struct user_namespace *user_ns = fc->global ? &init_user_ns : fc->user_ns;
        int err;

        /*
         * Never allow s_user_ns != &init_user_ns when FS_USERNS_MOUNT is
         * not set, as the filesystem is likely unprepared to handle it.
         * This can happen when fsconfig() is called from init_user_ns with
         * an fs_fd opened in another user namespace.
         */
        if (user_ns != &init_user_ns && !(fc->fs_type->fs_flags & FS_USERNS_MOUNT)) {
                errorfc(fc, "VFS: Mounting from non-initial user namespace is not allowed");
                return ERR_PTR(-EPERM);
        }

retry:
        spin_lock(&sb_lock);
        if (test) {
                hlist_for_each_entry(old, &fc->fs_type->fs_supers, s_instances) {
                        if (test(old, fc))
                                goto share_extant_sb;
                }
        }
        if (!s) {
                spin_unlock(&sb_lock);
                s = alloc_super(fc->fs_type, fc->sb_flags, user_ns);
                if (!s)
                        return ERR_PTR(-ENOMEM);
                goto retry;
        }

        s->s_fs_info = fc->s_fs_info;
        err = set(s, fc);
        if (err) {
                s->s_fs_info = NULL;
                spin_unlock(&sb_lock);
                destroy_unused_super(s);
                return ERR_PTR(err);
        }
        fc->s_fs_info = NULL;
        s->s_type = fc->fs_type;
        s->s_iflags |= fc->s_iflags;
        strscpy(s->s_id, s->s_type->name, sizeof(s->s_id));
        /*
         * Make the superblock visible on @super_blocks and @fs_supers.
         * It's in a nascent state and users should wait on SB_BORN or
         * SB_DYING to be set.
         */
        list_add_tail(&s->s_list, &super_blocks);
        hlist_add_head(&s->s_instances, &s->s_type->fs_supers);
        spin_unlock(&sb_lock);
        get_filesystem(s->s_type);
        shrinker_register(s->s_shrink);
        return s;

share_extant_sb:
        if (user_ns != old->s_user_ns || fc->exclusive) {
                spin_unlock(&sb_lock);
                destroy_unused_super(s);
                if (fc->exclusive)
                        warnfc(fc, "reusing existing filesystem not allowed");
                else
                        warnfc(fc, "reusing existing filesystem in another namespace not allowed");
                return ERR_PTR(-EBUSY);
        }
        if (!grab_super(old))
                goto retry;
        destroy_unused_super(s);
        return old;
}
EXPORT_SYMBOL(sget_fc);

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

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, user_ns);
                if (!s)
                        return ERR_PTR(-ENOMEM);
                goto retry;
        }

        err = set(s, data);
        if (err) {
                spin_unlock(&sb_lock);
                destroy_unused_super(s);
                return ERR_PTR(err);
        }
        s->s_type = type;
        strscpy(s->s_id, type->name, sizeof(s->s_id));
        list_add_tail(&s->s_list, &super_blocks);
        hlist_add_head(&s->s_instances, &type->fs_supers);
        spin_unlock(&sb_lock);
        get_filesystem(type);
        shrinker_register(s->s_shrink);
        return s;
}
EXPORT_SYMBOL(sget);

void drop_super(struct super_block *sb)
{
        super_unlock_shared(sb);
        put_super(sb);
}

EXPORT_SYMBOL(drop_super);

void drop_super_exclusive(struct super_block *sb)
{
        super_unlock_excl(sb);
        put_super(sb);
}
EXPORT_SYMBOL(drop_super_exclusive);

enum super_iter_flags_t {
        SUPER_ITER_EXCL         = (1U << 0),
        SUPER_ITER_UNLOCKED     = (1U << 1),
        SUPER_ITER_REVERSE      = (1U << 2),
};

static inline struct super_block *first_super(enum super_iter_flags_t flags)
{
        if (flags & SUPER_ITER_REVERSE)
                return list_last_entry(&super_blocks, struct super_block, s_list);
        return list_first_entry(&super_blocks, struct super_block, s_list);
}

static inline struct super_block *next_super(struct super_block *sb,
                                             enum super_iter_flags_t flags)
{
        if (flags & SUPER_ITER_REVERSE)
                return list_prev_entry(sb, s_list);
        return list_next_entry(sb, s_list);
}

static void __iterate_supers(void (*f)(struct super_block *, void *), void *arg,
                             enum super_iter_flags_t flags)
{
        struct super_block *sb, *p = NULL;
        bool excl = flags & SUPER_ITER_EXCL;

        guard(spinlock)(&sb_lock);

        for (sb = first_super(flags);
             !list_entry_is_head(sb, &super_blocks, s_list);
             sb = next_super(sb, flags)) {
                if (super_flags(sb, SB_DYING))
                        continue;
                sb->s_count++;
                spin_unlock(&sb_lock);

                if (flags & SUPER_ITER_UNLOCKED) {
                        f(sb, arg);
                } else if (super_lock(sb, excl)) {
                        f(sb, arg);
                        super_unlock(sb, excl);
                }

                spin_lock(&sb_lock);
                if (p)
                        __put_super(p);
                p = sb;
        }
        if (p)
                __put_super(p);
}

void iterate_supers(void (*f)(struct super_block *, void *), void *arg)
{
        __iterate_supers(f, arg, 0);
}

/**
 *      iterate_supers_type - call function for superblocks of given type
 *      @type: fs type
 *      @f: function to call
 *      @arg: argument to pass to it
 *
 *      Scans the superblock list and calls given function, passing it
 *      locked superblock and given argument.
 */
void iterate_supers_type(struct file_system_type *type,
        void (*f)(struct super_block *, void *), void *arg)
{
        struct super_block *sb, *p = NULL;

        spin_lock(&sb_lock);
        hlist_for_each_entry(sb, &type->fs_supers, s_instances) {
                bool locked;

                if (super_flags(sb, SB_DYING))
                        continue;

                sb->s_count++;
                spin_unlock(&sb_lock);

                locked = super_lock_shared(sb);
                if (locked) {
                        f(sb, arg);
                        super_unlock_shared(sb);
                }

                spin_lock(&sb_lock);
                if (p)
                        __put_super(p);
                p = sb;
        }
        if (p)
                __put_super(p);
        spin_unlock(&sb_lock);
}

EXPORT_SYMBOL(iterate_supers_type);

struct super_block *user_get_super(dev_t dev, bool excl)
{
        struct super_block *sb;

        spin_lock(&sb_lock);
        list_for_each_entry(sb, &super_blocks, s_list) {
                bool locked;

                if (sb->s_dev != dev)
                        continue;

                sb->s_count++;
                spin_unlock(&sb_lock);

                locked = super_lock(sb, excl);
                if (locked)
                        return sb;

                spin_lock(&sb_lock);
                __put_super(sb);
                break;
        }
        spin_unlock(&sb_lock);
        return NULL;
}

/**
 * reconfigure_super - asks filesystem to change superblock parameters
 * @fc: The superblock and configuration
 *
 * Alters the configuration parameters of a live superblock.
 */
int reconfigure_super(struct fs_context *fc)
{
        struct super_block *sb = fc->root->d_sb;
        int retval;
        bool remount_ro = false;
        bool remount_rw = false;
        bool force = fc->sb_flags & SB_FORCE;

        if (fc->sb_flags_mask & ~MS_RMT_MASK)
                return -EINVAL;
        if (sb->s_writers.frozen != SB_UNFROZEN)
                return -EBUSY;

        retval = security_sb_remount(sb, fc->security);
        if (retval)
                return retval;

        if (fc->sb_flags_mask & SB_RDONLY) {
#ifdef CONFIG_BLOCK
                if (!(fc->sb_flags & SB_RDONLY) && sb->s_bdev &&
                    bdev_read_only(sb->s_bdev))
                        return -EACCES;
#endif
                remount_rw = !(fc->sb_flags & SB_RDONLY) && sb_rdonly(sb);
                remount_ro = (fc->sb_flags & SB_RDONLY) && !sb_rdonly(sb);
        }

        if (remount_ro) {
                if (!hlist_empty(&sb->s_pins)) {
                        super_unlock_excl(sb);
                        group_pin_kill(&sb->s_pins);
                        __super_lock_excl(sb);
                        if (!sb->s_root)
                                return 0;
                        if (sb->s_writers.frozen != SB_UNFROZEN)
                                return -EBUSY;
                        remount_ro = !sb_rdonly(sb);
                }
        }
        shrink_dcache_sb(sb);

        /* If we are reconfiguring to RDONLY and current sb is read/write,
         * make sure there are no files open for writing.
         */
        if (remount_ro) {
                if (force) {
                        sb_start_ro_state_change(sb);
                } else {
                        retval = sb_prepare_remount_readonly(sb);
                        if (retval)
                                return retval;
                }
        } else if (remount_rw) {
                /*
                 * Protect filesystem's reconfigure code from writes from
                 * userspace until reconfigure finishes.
                 */
                sb_start_ro_state_change(sb);
        }

        if (fc->ops->reconfigure) {
                retval = fc->ops->reconfigure(fc);
                if (retval) {
                        if (!force)
                                goto cancel_readonly;
                        /* If forced remount, go ahead despite any errors */
                        WARN(1, "forced remount of a %s fs returned %i\n",
                             sb->s_type->name, retval);
                }
        }

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

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

cancel_readonly:
        sb_end_ro_state_change(sb);
        return retval;
}

static void do_emergency_remount_callback(struct super_block *sb, void *unused)
{
        if (sb->s_bdev && !sb_rdonly(sb)) {
                struct fs_context *fc;

                fc = fs_context_for_reconfigure(sb->s_root,
                                        SB_RDONLY | SB_FORCE, SB_RDONLY);
                if (!IS_ERR(fc)) {
                        if (parse_monolithic_mount_data(fc, NULL) == 0)
                                (void)reconfigure_super(fc);
                        put_fs_context(fc);
                }
        }
}

static void do_emergency_remount(struct work_struct *work)
{
        __iterate_supers(do_emergency_remount_callback, NULL,
                         SUPER_ITER_EXCL | SUPER_ITER_REVERSE);
        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, void *unused)
{
        if (IS_ENABLED(CONFIG_BLOCK))
                while (sb->s_bdev && !bdev_thaw(sb->s_bdev))
                        pr_warn("Emergency Thaw on %pg\n", sb->s_bdev);
        thaw_super_locked(sb, FREEZE_HOLDER_USERSPACE, NULL);
        return;
}

static void do_thaw_all(struct work_struct *work)
{
        __iterate_supers(do_thaw_all_callback, NULL, SUPER_ITER_EXCL);
        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 inline bool get_active_super(struct super_block *sb)
{
        bool active = false;

        if (super_lock_excl(sb)) {
                active = atomic_inc_not_zero(&sb->s_active);
                super_unlock_excl(sb);
        }
        return active;
}

static const char *filesystems_freeze_ptr = "filesystems_freeze";

static void filesystems_freeze_callback(struct super_block *sb, void *unused)
{
        if (!sb->s_op->freeze_fs && !sb->s_op->freeze_super)
                return;

        if (!get_active_super(sb))
                return;

        if (sb->s_op->freeze_super)
                sb->s_op->freeze_super(sb, FREEZE_EXCL | FREEZE_HOLDER_KERNEL,
                                       filesystems_freeze_ptr);
        else
                freeze_super(sb, FREEZE_EXCL | FREEZE_HOLDER_KERNEL,
                             filesystems_freeze_ptr);

        deactivate_super(sb);
}

void filesystems_freeze(void)
{
        __iterate_supers(filesystems_freeze_callback, NULL,
                         SUPER_ITER_UNLOCKED | SUPER_ITER_REVERSE);
}

static void filesystems_thaw_callback(struct super_block *sb, void *unused)
{
        if (!sb->s_op->freeze_fs && !sb->s_op->freeze_super)
                return;

        if (!get_active_super(sb))
                return;

        if (sb->s_op->thaw_super)
                sb->s_op->thaw_super(sb, FREEZE_EXCL | FREEZE_HOLDER_KERNEL,
                                     filesystems_freeze_ptr);
        else
                thaw_super(sb, FREEZE_EXCL | FREEZE_HOLDER_KERNEL,
                           filesystems_freeze_ptr);

        deactivate_super(sb);
}

void filesystems_thaw(void)
{
        __iterate_supers(filesystems_thaw_callback, NULL, SUPER_ITER_UNLOCKED);
}

static DEFINE_IDA(unnamed_dev_ida);

/**
 * get_anon_bdev - Allocate a block device for filesystems which don't have one.
 * @p: Pointer to a dev_t.
 *
 * Filesystems which don't use real block devices can call this function
 * to allocate a virtual block device.
 *
 * Context: Any context.  Frequently called while holding sb_lock.
 * Return: 0 on success, -EMFILE if there are no anonymous bdevs left
 * or -ENOMEM if memory allocation failed.
 */
int get_anon_bdev(dev_t *p)
{
        int dev;

        /*
         * Many userspace utilities consider an FSID of 0 invalid.
         * Always return at least 1 from get_anon_bdev.
         */
        dev = ida_alloc_range(&unnamed_dev_ida, 1, (1 << MINORBITS) - 1,
                        GFP_ATOMIC);
        if (dev == -ENOSPC)
                dev = -EMFILE;
        if (dev < 0)
                return dev;

        *p = MKDEV(0, dev);
        return 0;
}
EXPORT_SYMBOL(get_anon_bdev);

void free_anon_bdev(dev_t dev)
{
        ida_free(&unnamed_dev_ida, MINOR(dev));
}
EXPORT_SYMBOL(free_anon_bdev);

int set_anon_super(struct super_block *s, void *data)
{
        return get_anon_bdev(&s->s_dev);
}
EXPORT_SYMBOL(set_anon_super);

void kill_anon_super(struct super_block *sb)
{
        dev_t dev = sb->s_dev;
        generic_shutdown_super(sb);
        kill_super_notify(sb);
        free_anon_bdev(dev);
}
EXPORT_SYMBOL(kill_anon_super);

void kill_litter_super(struct super_block *sb)
{
        if (sb->s_root)
                d_genocide(sb->s_root);
        kill_anon_super(sb);
}
EXPORT_SYMBOL(kill_litter_super);

int set_anon_super_fc(struct super_block *sb, struct fs_context *fc)
{
        return set_anon_super(sb, NULL);
}
EXPORT_SYMBOL(set_anon_super_fc);

static int test_keyed_super(struct super_block *sb, struct fs_context *fc)
{
        return sb->s_fs_info == fc->s_fs_info;
}

static int test_single_super(struct super_block *s, struct fs_context *fc)
{
        return 1;
}

static int vfs_get_super(struct fs_context *fc,
                int (*test)(struct super_block *, struct fs_context *),
                int (*fill_super)(struct super_block *sb,
                                  struct fs_context *fc))
{
        struct super_block *sb;
        int err;

        sb = sget_fc(fc, test, set_anon_super_fc);
        if (IS_ERR(sb))
                return PTR_ERR(sb);

        if (!sb->s_root) {
                err = fill_super(sb, fc);
                if (err)
                        goto error;

                sb->s_flags |= SB_ACTIVE;
        }

        fc->root = dget(sb->s_root);
        return 0;

error:
        deactivate_locked_super(sb);
        return err;
}

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

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

int get_tree_keyed(struct fs_context *fc,
                  int (*fill_super)(struct super_block *sb,
                                    struct fs_context *fc),
                void *key)
{
        fc->s_fs_info = key;
        return vfs_get_super(fc, test_keyed_super, fill_super);
}
EXPORT_SYMBOL(get_tree_keyed);

static int set_bdev_super(struct super_block *s, void *data)
{
        s->s_dev = *(dev_t *)data;
        return 0;
}

static int super_s_dev_set(struct super_block *s, struct fs_context *fc)
{
        return set_bdev_super(s, fc->sget_key);
}

static int super_s_dev_test(struct super_block *s, struct fs_context *fc)
{
        return !(s->s_iflags & SB_I_RETIRED) &&
                s->s_dev == *(dev_t *)fc->sget_key;
}

/**
 * sget_dev - Find or create a superblock by device number
 * @fc: Filesystem context.
 * @dev: device number
 *
 * Find or create a superblock using the provided device number that
 * will be stored in fc->sget_key.
 *
 * If an extant superblock is matched, then that will be returned with
 * an elevated reference count that the caller must transfer or discard.
 *
 * If no match is made, a new superblock will be allocated and basic
 * initialisation will be performed (s_type, s_fs_info, s_id, s_dev will
 * be set). The superblock will be published and it will be returned in
 * a partially constructed state with SB_BORN and SB_ACTIVE as yet
 * unset.
 *
 * Return: an existing or newly created superblock on success, an error
 *         pointer on failure.
 */
struct super_block *sget_dev(struct fs_context *fc, dev_t dev)
{
        fc->sget_key = &dev;
        return sget_fc(fc, super_s_dev_test, super_s_dev_set);
}
EXPORT_SYMBOL(sget_dev);

#ifdef CONFIG_BLOCK
/*
 * Lock the superblock that is holder of the bdev. Returns the superblock
 * pointer if we successfully locked the superblock and it is alive. Otherwise
 * we return NULL and just unlock bdev->bd_holder_lock.
 *
 * The function must be called with bdev->bd_holder_lock and releases it.
 */
static struct super_block *bdev_super_lock(struct block_device *bdev, bool excl)
        __releases(&bdev->bd_holder_lock)
{
        struct super_block *sb = bdev->bd_holder;
        bool locked;

        lockdep_assert_held(&bdev->bd_holder_lock);
        lockdep_assert_not_held(&sb->s_umount);
        lockdep_assert_not_held(&bdev->bd_disk->open_mutex);

        /* Make sure sb doesn't go away from under us */
        spin_lock(&sb_lock);
        sb->s_count++;
        spin_unlock(&sb_lock);

        mutex_unlock(&bdev->bd_holder_lock);

        locked = super_lock(sb, excl);

        /*
         * If the superblock wasn't already SB_DYING then we hold
         * s_umount and can safely drop our temporary reference.
         */
        put_super(sb);

        if (!locked)
                return NULL;

        if (!sb->s_root || !(sb->s_flags & SB_ACTIVE)) {
                super_unlock(sb, excl);
                return NULL;
        }

        return sb;
}

static void fs_bdev_mark_dead(struct block_device *bdev, bool surprise)
{
        struct super_block *sb;

        sb = bdev_super_lock(bdev, false);
        if (!sb)
                return;

        if (!surprise)
                sync_filesystem(sb);
        shrink_dcache_sb(sb);
        evict_inodes(sb);
        if (sb->s_op->shutdown)
                sb->s_op->shutdown(sb);

        super_unlock_shared(sb);
}

static void fs_bdev_sync(struct block_device *bdev)
{
        struct super_block *sb;

        sb = bdev_super_lock(bdev, false);
        if (!sb)
                return;

        sync_filesystem(sb);
        super_unlock_shared(sb);
}

static struct super_block *get_bdev_super(struct block_device *bdev)
{
        bool active = false;
        struct super_block *sb;

        sb = bdev_super_lock(bdev, true);
        if (sb) {
                active = atomic_inc_not_zero(&sb->s_active);
                super_unlock_excl(sb);
        }
        if (!active)
                return NULL;
        return sb;
}

/**
 * fs_bdev_freeze - freeze owning filesystem of block device
 * @bdev: block device
 *
 * Freeze the filesystem that owns this block device if it is still
 * active.
 *
 * A filesystem that owns multiple block devices may be frozen from each
 * block device and won't be unfrozen until all block devices are
 * unfrozen. Each block device can only freeze the filesystem once as we
 * nest freezes for block devices in the block layer.
 *
 * Return: If the freeze was successful zero is returned. If the freeze
 *         failed a negative error code is returned.
 */
static int fs_bdev_freeze(struct block_device *bdev)
{
        struct super_block *sb;
        int error = 0;

        lockdep_assert_held(&bdev->bd_fsfreeze_mutex);

        sb = get_bdev_super(bdev);
        if (!sb)
                return -EINVAL;

        if (sb->s_op->freeze_super)
                error = sb->s_op->freeze_super(sb,
                                FREEZE_MAY_NEST | FREEZE_HOLDER_USERSPACE, NULL);
        else
                error = freeze_super(sb,
                                FREEZE_MAY_NEST | FREEZE_HOLDER_USERSPACE, NULL);
        if (!error)
                error = sync_blockdev(bdev);
        deactivate_super(sb);
        return error;
}

/**
 * fs_bdev_thaw - thaw owning filesystem of block device
 * @bdev: block device
 *
 * Thaw the filesystem that owns this block device.
 *
 * A filesystem that owns multiple block devices may be frozen from each
 * block device and won't be unfrozen until all block devices are
 * unfrozen. Each block device can only freeze the filesystem once as we
 * nest freezes for block devices in the block layer.
 *
 * Return: If the thaw was successful zero is returned. If the thaw
 *         failed a negative error code is returned. If this function
 *         returns zero it doesn't mean that the filesystem is unfrozen
 *         as it may have been frozen multiple times (kernel may hold a
 *         freeze or might be frozen from other block devices).
 */
static int fs_bdev_thaw(struct block_device *bdev)
{
        struct super_block *sb;
        int error;

        lockdep_assert_held(&bdev->bd_fsfreeze_mutex);

        /*
         * The block device may have been frozen before it was claimed by a
         * filesystem. Concurrently another process might try to mount that
         * frozen block device and has temporarily claimed the block device for
         * that purpose causing a concurrent fs_bdev_thaw() to end up here. The
         * mounter is already about to abort mounting because they still saw an
         * elevanted bdev->bd_fsfreeze_count so get_bdev_super() will return
         * NULL in that case.
         */
        sb = get_bdev_super(bdev);
        if (!sb)
                return -EINVAL;

        if (sb->s_op->thaw_super)
                error = sb->s_op->thaw_super(sb,
                                FREEZE_MAY_NEST | FREEZE_HOLDER_USERSPACE, NULL);
        else
                error = thaw_super(sb,
                                FREEZE_MAY_NEST | FREEZE_HOLDER_USERSPACE, NULL);
        deactivate_super(sb);
        return error;
}

const struct blk_holder_ops fs_holder_ops = {
        .mark_dead              = fs_bdev_mark_dead,
        .sync                   = fs_bdev_sync,
        .freeze                 = fs_bdev_freeze,
        .thaw                   = fs_bdev_thaw,
};
EXPORT_SYMBOL_GPL(fs_holder_ops);

int setup_bdev_super(struct super_block *sb, int sb_flags,
                struct fs_context *fc)
{
        blk_mode_t mode = sb_open_mode(sb_flags);
        struct file *bdev_file;
        struct block_device *bdev;

        bdev_file = bdev_file_open_by_dev(sb->s_dev, mode, sb, &fs_holder_ops);
        if (IS_ERR(bdev_file)) {
                if (fc)
                        errorf(fc, "%s: Can't open blockdev", fc->source);
                return PTR_ERR(bdev_file);
        }
        bdev = file_bdev(bdev_file);

        /*
         * This really should be in blkdev_get_by_dev, but right now can't due
         * to legacy issues that require us to allow opening a block device node
         * writable from userspace even for a read-only block device.
         */
        if ((mode & BLK_OPEN_WRITE) && bdev_read_only(bdev)) {
                bdev_fput(bdev_file);
                return -EACCES;
        }

        /*
         * It is enough to check bdev was not frozen before we set
         * s_bdev as freezing will wait until SB_BORN is set.
         */
        if (atomic_read(&bdev->bd_fsfreeze_count) > 0) {
                if (fc)
                        warnf(fc, "%pg: Can't mount, blockdev is frozen", bdev);
                bdev_fput(bdev_file);
                return -EBUSY;
        }
        spin_lock(&sb_lock);
        sb->s_bdev_file = bdev_file;
        sb->s_bdev = bdev;
        sb->s_bdi = bdi_get(bdev->bd_disk->bdi);
        if (bdev_stable_writes(bdev))
                sb->s_iflags |= SB_I_STABLE_WRITES;
        spin_unlock(&sb_lock);

        snprintf(sb->s_id, sizeof(sb->s_id), "%pg", bdev);
        shrinker_debugfs_rename(sb->s_shrink, "sb-%s:%s", sb->s_type->name,
                                sb->s_id);
        sb_set_blocksize(sb, block_size(bdev));
        return 0;
}
EXPORT_SYMBOL_GPL(setup_bdev_super);

/**
 * get_tree_bdev_flags - Get a superblock based on a single block device
 * @fc: The filesystem context holding the parameters
 * @fill_super: Helper to initialise a new superblock
 * @flags: GET_TREE_BDEV_* flags
 */
int get_tree_bdev_flags(struct fs_context *fc,
                int (*fill_super)(struct super_block *sb,
                                  struct fs_context *fc), unsigned int flags)
{
        struct super_block *s;
        int error = 0;
        dev_t dev;

        if (!fc->source)
                return invalf(fc, "No source specified");

        error = lookup_bdev(fc->source, &dev);
        if (error) {
                if (!(flags & GET_TREE_BDEV_QUIET_LOOKUP))
                        errorf(fc, "%s: Can't lookup blockdev", fc->source);
                return error;
        }
        fc->sb_flags |= SB_NOSEC;
        s = sget_dev(fc, dev);
        if (IS_ERR(s))
                return PTR_ERR(s);

        if (s->s_root) {
                /* Don't summarily change the RO/RW state. */
                if ((fc->sb_flags ^ s->s_flags) & SB_RDONLY) {
                        warnf(fc, "%pg: Can't mount, would change RO state", s->s_bdev);
                        deactivate_locked_super(s);
                        return -EBUSY;
                }
        } else {
                error = setup_bdev_super(s, fc->sb_flags, fc);
                if (!error)
                        error = fill_super(s, fc);
                if (error) {
                        deactivate_locked_super(s);
                        return error;
                }
                s->s_flags |= SB_ACTIVE;
        }

        BUG_ON(fc->root);
        fc->root = dget(s->s_root);
        return 0;
}
EXPORT_SYMBOL_GPL(get_tree_bdev_flags);

/**
 * 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 *))
{
        return get_tree_bdev_flags(fc, fill_super, 0);
}
EXPORT_SYMBOL(get_tree_bdev);

static int test_bdev_super(struct super_block *s, void *data)
{
        return !(s->s_iflags & SB_I_RETIRED) && s->s_dev == *(dev_t *)data;
}

struct dentry *mount_bdev(struct file_system_type *fs_type,
        int flags, const char *dev_name, void *data,
        int (*fill_super)(struct super_block *, void *, int))
{
        struct super_block *s;
        int error;
        dev_t dev;

        error = lookup_bdev(dev_name, &dev);
        if (error)
                return ERR_PTR(error);

        flags |= SB_NOSEC;
        s = sget(fs_type, test_bdev_super, set_bdev_super, flags, &dev);
        if (IS_ERR(s))
                return ERR_CAST(s);

        if (s->s_root) {
                if ((flags ^ s->s_flags) & SB_RDONLY) {
                        deactivate_locked_super(s);
                        return ERR_PTR(-EBUSY);
                }
        } else {
                error = setup_bdev_super(s, flags, NULL);
                if (!error)
                        error = fill_super(s, data, flags & SB_SILENT ? 1 : 0);
                if (error) {
                        deactivate_locked_super(s);
                        return ERR_PTR(error);
                }

                s->s_flags |= SB_ACTIVE;
        }

        return dget(s->s_root);
}
EXPORT_SYMBOL(mount_bdev);

void kill_block_super(struct super_block *sb)
{
        struct block_device *bdev = sb->s_bdev;

        generic_shutdown_super(sb);
        if (bdev) {
                sync_blockdev(bdev);
                bdev_fput(sb->s_bdev_file);
        }
}

EXPORT_SYMBOL(kill_block_super);
#endif

struct dentry *mount_nodev(struct file_system_type *fs_type,
        int flags, void *data,
        int (*fill_super)(struct super_block *, void *, int))
{
        int error;
        struct super_block *s = sget(fs_type, NULL, set_anon_super, flags, NULL);

        if (IS_ERR(s))
                return ERR_CAST(s);

        error = fill_super(s, data, flags & SB_SILENT ? 1 : 0);
        if (error) {
                deactivate_locked_super(s);
                return ERR_PTR(error);
        }
        s->s_flags |= SB_ACTIVE;
        return dget(s->s_root);
}
EXPORT_SYMBOL(mount_nodev);

/**
 * vfs_get_tree - Get the mountable root
 * @fc: The superblock configuration context.
 *
 * The filesystem is invoked to get or create a superblock which can then later
 * be used for mounting.  The filesystem places a pointer to the root to be
 * used for mounting in @fc->root.
 */
int vfs_get_tree(struct fs_context *fc)
{
        struct super_block *sb;
        int error;

        if (fc->root)
                return -EBUSY;

        /* Get the mountable root in fc->root, with a ref on the root and a ref
         * on the superblock.
         */
        error = fc->ops->get_tree(fc);
        if (error < 0)
                return error;

        if (!fc->root) {
                pr_err("Filesystem %s get_tree() didn't set fc->root, returned %i\n",
                       fc->fs_type->name, error);
                /* We don't know what the locking state of the superblock is -
                 * if there is a superblock.
                 */
                BUG();
        }

        sb = fc->root->d_sb;
        WARN_ON(!sb->s_bdi);

        /*
         * super_wake() contains a memory barrier which also care of
         * ordering for super_cache_count(). We place it before setting
         * SB_BORN as the data dependency between the two functions is
         * the superblock structure contents that we just set up, not
         * the SB_BORN flag.
         */
        super_wake(sb, SB_BORN);

        error = security_sb_set_mnt_opts(sb, fc->security, 0, NULL);
        if (unlikely(error)) {
                fc_drop_locked(fc);
                return error;
        }

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

        return 0;
}
EXPORT_SYMBOL(vfs_get_tree);

/*
 * Setup private BDI for given superblock. It gets automatically cleaned up
 * in generic_shutdown_super().
 */
int super_setup_bdi_name(struct super_block *sb, char *fmt, ...)
{
        struct backing_dev_info *bdi;
        int err;
        va_list args;

        bdi = bdi_alloc(NUMA_NO_NODE);
        if (!bdi)
                return -ENOMEM;

        va_start(args, fmt);
        err = bdi_register_va(bdi, fmt, args);
        va_end(args);
        if (err) {
                bdi_put(bdi);
                return err;
        }
        WARN_ON(sb->s_bdi != &noop_backing_dev_info);
        sb->s_bdi = bdi;
        sb->s_iflags |= SB_I_PERSB_BDI;

        return 0;
}
EXPORT_SYMBOL(super_setup_bdi_name);

/*
 * Setup private BDI for given superblock. I gets automatically cleaned up
 * in generic_shutdown_super().
 */
int super_setup_bdi(struct super_block *sb)
{
        static atomic_long_t bdi_seq = ATOMIC_LONG_INIT(0);

        return super_setup_bdi_name(sb, "%.28s-%ld", sb->s_type->name,
                                    atomic_long_inc_return(&bdi_seq));
}
EXPORT_SYMBOL(super_setup_bdi);

/**
 * sb_wait_write - wait until all writers to given file system finish
 * @sb: the super for which we wait
 * @level: type of writers we wait for (normal vs page fault)
 *
 * This function waits until there are no writers of given type to given file
 * system.
 */
static void sb_wait_write(struct super_block *sb, int level)
{
        percpu_down_write(sb->s_writers.rw_sem + level-1);
}

/*
 * We are going to return to userspace and forget about these locks, the
 * ownership goes to the caller of thaw_super() which does unlock().
 */
static void lockdep_sb_freeze_release(struct super_block *sb)
{
        int level;

        for (level = SB_FREEZE_LEVELS - 1; level >= 0; level--)
                percpu_rwsem_release(sb->s_writers.rw_sem + level, _THIS_IP_);
}

/*
 * Tell lockdep we are holding these locks before we call ->unfreeze_fs(sb).
 */
static void lockdep_sb_freeze_acquire(struct super_block *sb)
{
        int level;

        for (level = 0; level < SB_FREEZE_LEVELS; ++level)
                percpu_rwsem_acquire(sb->s_writers.rw_sem + level, 0, _THIS_IP_);
}

static void sb_freeze_unlock(struct super_block *sb, int level)
{
        for (level--; level >= 0; level--)
                percpu_up_write(sb->s_writers.rw_sem + level);
}

static int wait_for_partially_frozen(struct super_block *sb)
{
        int ret = 0;

        do {
                unsigned short old = sb->s_writers.frozen;

                up_write(&sb->s_umount);
                ret = wait_var_event_killable(&sb->s_writers.frozen,
                                               sb->s_writers.frozen != old);
                down_write(&sb->s_umount);
        } while (ret == 0 &&
                 sb->s_writers.frozen != SB_UNFROZEN &&
                 sb->s_writers.frozen != SB_FREEZE_COMPLETE);

        return ret;
}

#define FREEZE_HOLDERS (FREEZE_HOLDER_KERNEL | FREEZE_HOLDER_USERSPACE)
#define FREEZE_FLAGS (FREEZE_HOLDERS | FREEZE_MAY_NEST | FREEZE_EXCL)

static inline int freeze_inc(struct super_block *sb, enum freeze_holder who)
{
        WARN_ON_ONCE((who & ~FREEZE_FLAGS));
        WARN_ON_ONCE(hweight32(who & FREEZE_HOLDERS) > 1);

        if (who & FREEZE_HOLDER_KERNEL)
                ++sb->s_writers.freeze_kcount;
        if (who & FREEZE_HOLDER_USERSPACE)
                ++sb->s_writers.freeze_ucount;
        return sb->s_writers.freeze_kcount + sb->s_writers.freeze_ucount;
}

static inline int freeze_dec(struct super_block *sb, enum freeze_holder who)
{
        WARN_ON_ONCE((who & ~FREEZE_FLAGS));
        WARN_ON_ONCE(hweight32(who & FREEZE_HOLDERS) > 1);

        if ((who & FREEZE_HOLDER_KERNEL) && sb->s_writers.freeze_kcount)
                --sb->s_writers.freeze_kcount;
        if ((who & FREEZE_HOLDER_USERSPACE) && sb->s_writers.freeze_ucount)
                --sb->s_writers.freeze_ucount;
        return sb->s_writers.freeze_kcount + sb->s_writers.freeze_ucount;
}

static inline bool may_freeze(struct super_block *sb, enum freeze_holder who,
                              const void *freeze_owner)
{
        lockdep_assert_held(&sb->s_umount);

        WARN_ON_ONCE((who & ~FREEZE_FLAGS));
        WARN_ON_ONCE(hweight32(who & FREEZE_HOLDERS) > 1);

        if (who & FREEZE_EXCL) {
                if (WARN_ON_ONCE(!(who & FREEZE_HOLDER_KERNEL)))
                        return false;
                if (WARN_ON_ONCE(who & ~(FREEZE_EXCL | FREEZE_HOLDER_KERNEL)))
                        return false;
                if (WARN_ON_ONCE(!freeze_owner))
                        return false;
                /* This freeze already has a specific owner. */
                if (sb->s_writers.freeze_owner)
                        return false;
                /*
                 * This is already frozen multiple times so we're just
                 * going to take a reference count and mark the freeze as
                 * being owned by the caller.
                 */
                if (sb->s_writers.freeze_kcount + sb->s_writers.freeze_ucount)
                        sb->s_writers.freeze_owner = freeze_owner;
                return true;
        }

        if (who & FREEZE_HOLDER_KERNEL)
                return (who & FREEZE_MAY_NEST) ||
                       sb->s_writers.freeze_kcount == 0;
        if (who & FREEZE_HOLDER_USERSPACE)
                return (who & FREEZE_MAY_NEST) ||
                       sb->s_writers.freeze_ucount == 0;
        return false;
}

static inline bool may_unfreeze(struct super_block *sb, enum freeze_holder who,
                                const void *freeze_owner)
{
        lockdep_assert_held(&sb->s_umount);

        WARN_ON_ONCE((who & ~FREEZE_FLAGS));
        WARN_ON_ONCE(hweight32(who & FREEZE_HOLDERS) > 1);

        if (who & FREEZE_EXCL) {
                if (WARN_ON_ONCE(!(who & FREEZE_HOLDER_KERNEL)))
                        return false;
                if (WARN_ON_ONCE(who & ~(FREEZE_EXCL | FREEZE_HOLDER_KERNEL)))
                        return false;
                if (WARN_ON_ONCE(!freeze_owner))
                        return false;
                if (WARN_ON_ONCE(sb->s_writers.freeze_kcount == 0))
                        return false;
                /* This isn't exclusively frozen. */
                if (!sb->s_writers.freeze_owner)
                        return false;
                /* This isn't exclusively frozen by us. */
                if (sb->s_writers.freeze_owner != freeze_owner)
                        return false;
                /*
                 * This is still frozen multiple times so we're just
                 * going to drop our reference count and undo our
                 * exclusive freeze.
                 */
                if ((sb->s_writers.freeze_kcount + sb->s_writers.freeze_ucount) > 1)
                        sb->s_writers.freeze_owner = NULL;
                return true;
        }

        if (who & FREEZE_HOLDER_KERNEL) {
                /*
                 * Someone's trying to steal the reference belonging to
                 * @sb->s_writers.freeze_owner.
                 */
                if (sb->s_writers.freeze_kcount == 1 &&
                    sb->s_writers.freeze_owner)
                        return false;
                return sb->s_writers.freeze_kcount > 0;
        }

        if (who & FREEZE_HOLDER_USERSPACE)
                return sb->s_writers.freeze_ucount > 0;

        return false;
}

/**
 * freeze_super - lock the filesystem and force it into a consistent state
 * @sb: the super to lock
 * @who: context that wants to freeze
 * @freeze_owner: owner of the freeze
 *
 * Syncs the super to make sure the filesystem is consistent and calls the fs's
 * freeze_fs.  Subsequent calls to this without first thawing the fs may return
 * -EBUSY.
 *
 * @who should be:
 * * %FREEZE_HOLDER_USERSPACE if userspace wants to freeze the fs;
 * * %FREEZE_HOLDER_KERNEL if the kernel wants to freeze the fs.
 * * %FREEZE_MAY_NEST whether nesting freeze and thaw requests is allowed.
 *
 * The @who argument distinguishes between the kernel and userspace trying to
 * freeze the filesystem.  Although there cannot be multiple kernel freezes or
 * multiple userspace freezes in effect at any given time, the kernel and
 * userspace can both hold a filesystem frozen.  The filesystem remains frozen
 * until there are no kernel or userspace freezes in effect.
 *
 * A filesystem may hold multiple devices and thus a filesystems may be
 * frozen through the block layer via multiple block devices. In this
 * case the request is marked as being allowed to nest by passing
 * FREEZE_MAY_NEST. The filesystem remains frozen until all block
 * devices are unfrozen. If multiple freezes are attempted without
 * FREEZE_MAY_NEST -EBUSY will be returned.
 *
 * During this function, sb->s_writers.frozen goes through these values:
 *
 * SB_UNFROZEN: File system is normal, all writes progress as usual.
 *
 * SB_FREEZE_WRITE: The file system is in the process of being frozen.  New
 * writes should be blocked, though page faults are still allowed. We wait for
 * all writes to complete and then proceed to the next stage.
 *
 * SB_FREEZE_PAGEFAULT: Freezing continues. Now also page faults are blocked
 * but internal fs threads can still modify the filesystem (although they
 * should not dirty new pages or inodes), writeback can run etc. After waiting
 * for all running page faults we sync the filesystem which will clean all
 * dirty pages and inodes (no new dirty pages or inodes can be created when
 * sync is running).
 *
 * SB_FREEZE_FS: The file system is frozen. Now all internal sources of fs
 * modification are blocked (e.g. XFS preallocation truncation on inode
 * reclaim). This is usually implemented by blocking new transactions for
 * filesystems that have them and need this additional guard. After all
 * internal writers are finished we call ->freeze_fs() to finish filesystem
 * freezing. Then we transition to SB_FREEZE_COMPLETE state. This state is
 * mostly auxiliary for filesystems to verify they do not modify frozen fs.
 *
 * sb->s_writers.frozen is protected by sb->s_umount.
 *
 * Return: If the freeze was successful zero is returned. If the freeze
 *         failed a negative error code is returned.
 */
int freeze_super(struct super_block *sb, enum freeze_holder who, const void *freeze_owner)
{
        int ret;

        if (!super_lock_excl(sb)) {
                WARN_ON_ONCE("Dying superblock while freezing!");
                return -EINVAL;
        }
        atomic_inc(&sb->s_active);

retry:
        if (sb->s_writers.frozen == SB_FREEZE_COMPLETE) {
                if (may_freeze(sb, who, freeze_owner))
                        ret = !!WARN_ON_ONCE(freeze_inc(sb, who) == 1);
                else
                        ret = -EBUSY;
                /* All freezers share a single active reference. */
                deactivate_locked_super(sb);
                return ret;
        }

        if (sb->s_writers.frozen != SB_UNFROZEN) {
                ret = wait_for_partially_frozen(sb);
                if (ret) {
                        deactivate_locked_super(sb);
                        return ret;
                }

                goto retry;
        }

        if (sb_rdonly(sb)) {
                /* Nothing to do really... */
                WARN_ON_ONCE(freeze_inc(sb, who) > 1);
                sb->s_writers.freeze_owner = freeze_owner;
                sb->s_writers.frozen = SB_FREEZE_COMPLETE;
                wake_up_var(&sb->s_writers.frozen);
                super_unlock_excl(sb);
                return 0;
        }

        sb->s_writers.frozen = SB_FREEZE_WRITE;
        /* Release s_umount to preserve sb_start_write -> s_umount ordering */
        super_unlock_excl(sb);
        sb_wait_write(sb, SB_FREEZE_WRITE);
        __super_lock_excl(sb);

        /* Now we go and block page faults... */
        sb->s_writers.frozen = SB_FREEZE_PAGEFAULT;
        sb_wait_write(sb, SB_FREEZE_PAGEFAULT);

        /* All writers are done so after syncing there won't be dirty data */
        ret = sync_filesystem(sb);
        if (ret) {
                sb->s_writers.frozen = SB_UNFROZEN;
                sb_freeze_unlock(sb, SB_FREEZE_PAGEFAULT);
                wake_up_var(&sb->s_writers.frozen);
                deactivate_locked_super(sb);
                return ret;
        }

        /* Now wait for internal filesystem counter */
        sb->s_writers.frozen = SB_FREEZE_FS;
        sb_wait_write(sb, SB_FREEZE_FS);

        if (sb->s_op->freeze_fs) {
                ret = sb->s_op->freeze_fs(sb);
                if (ret) {
                        printk(KERN_ERR
                                "VFS:Filesystem freeze failed\n");
                        sb->s_writers.frozen = SB_UNFROZEN;
                        sb_freeze_unlock(sb, SB_FREEZE_FS);
                        wake_up_var(&sb->s_writers.frozen);
                        deactivate_locked_super(sb);
                        return ret;
                }
        }
        /*
         * For debugging purposes so that fs can warn if it sees write activity
         * when frozen is set to SB_FREEZE_COMPLETE, and for thaw_super().
         */
        WARN_ON_ONCE(freeze_inc(sb, who) > 1);
        sb->s_writers.freeze_owner = freeze_owner;
        sb->s_writers.frozen = SB_FREEZE_COMPLETE;
        wake_up_var(&sb->s_writers.frozen);
        lockdep_sb_freeze_release(sb);
        super_unlock_excl(sb);
        return 0;
}
EXPORT_SYMBOL(freeze_super);

/*
 * Undoes the effect of a freeze_super_locked call.  If the filesystem is
 * frozen both by userspace and the kernel, a thaw call from either source
 * removes that state without releasing the other state or unlocking the
 * filesystem.
 */
static int thaw_super_locked(struct super_block *sb, enum freeze_holder who,
                             const void *freeze_owner)
{
        int error = -EINVAL;

        if (sb->s_writers.frozen != SB_FREEZE_COMPLETE)
                goto out_unlock;

        if (!may_unfreeze(sb, who, freeze_owner))
                goto out_unlock;

        /*
         * All freezers share a single active reference.
         * So just unlock in case there are any left.
         */
        if (freeze_dec(sb, who))
                goto out_unlock;

        if (sb_rdonly(sb)) {
                sb->s_writers.frozen = SB_UNFROZEN;
                sb->s_writers.freeze_owner = NULL;
                wake_up_var(&sb->s_writers.frozen);
                goto out_deactivate;
        }

        lockdep_sb_freeze_acquire(sb);

        if (sb->s_op->unfreeze_fs) {
                error = sb->s_op->unfreeze_fs(sb);
                if (error) {
                        pr_err("VFS: Filesystem thaw failed\n");
                        freeze_inc(sb, who);
                        lockdep_sb_freeze_release(sb);
                        goto out_unlock;
                }
        }

        sb->s_writers.frozen = SB_UNFROZEN;
        sb->s_writers.freeze_owner = NULL;
        wake_up_var(&sb->s_writers.frozen);
        sb_freeze_unlock(sb, SB_FREEZE_FS);
out_deactivate:
        deactivate_locked_super(sb);
        return 0;

out_unlock:
        super_unlock_excl(sb);
        return error;
}

/**
 * thaw_super -- unlock filesystem
 * @sb: the super to thaw
 * @who: context that wants to freeze
 * @freeze_owner: owner of the freeze
 *
 * Unlocks the filesystem and marks it writeable again after freeze_super()
 * if there are no remaining freezes on the filesystem.
 *
 * @who should be:
 * * %FREEZE_HOLDER_USERSPACE if userspace wants to thaw the fs;
 * * %FREEZE_HOLDER_KERNEL if the kernel wants to thaw the fs.
 * * %FREEZE_MAY_NEST whether nesting freeze and thaw requests is allowed
 *
 * A filesystem may hold multiple devices and thus a filesystems may
 * have been frozen through the block layer via multiple block devices.
 * The filesystem remains frozen until all block devices are unfrozen.
 */
int thaw_super(struct super_block *sb, enum freeze_holder who,
               const void *freeze_owner)
{
        if (!super_lock_excl(sb)) {
                WARN_ON_ONCE("Dying superblock while thawing!");
                return -EINVAL;
        }
        return thaw_super_locked(sb, who, freeze_owner);
}
EXPORT_SYMBOL(thaw_super);

/*
 * Create workqueue for deferred direct IO completions. We allocate the
 * workqueue when it's first needed. This avoids creating workqueue for
 * filesystems that don't need it and also allows us to create the workqueue
 * late enough so the we can include s_id in the name of the workqueue.
 */
int sb_init_dio_done_wq(struct super_block *sb)
{
        struct workqueue_struct *old;
        struct workqueue_struct *wq = alloc_workqueue("dio/%s",
                                                      WQ_MEM_RECLAIM, 0,
                                                      sb->s_id);
        if (!wq)
                return -ENOMEM;
        /*
         * This has to be atomic as more DIOs can race to create the workqueue
         */
        old = cmpxchg(&sb->s_dio_done_wq, NULL, wq);
        /* Someone created workqueue before us? Free ours... */
        if (old)
                destroy_workqueue(wq);
        return 0;
}
EXPORT_SYMBOL_GPL(sb_init_dio_done_wq);