super.c

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/*
 *  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 ([email protected]), April 14, 1996.
 *  Added devfs support: Richard Gooch <[email protected]>, 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/acct.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 "internal.h"


LIST_HEAD(super_blocks);
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 int prune_super(struct shrinker *shrink, struct shrink_control *sc)
{
    struct super_block *sb;
    int fs_objects = 0;
    int total_objects;

    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->nr_to_scan && !(sc->gfp_mask & __GFP_FS))
        return -1;

    if (!grab_super_passive(sb))
        return -1;

    if (sb->s_op && sb->s_op->nr_cached_objects)
        fs_objects = sb->s_op->nr_cached_objects(sb);

    total_objects = sb->s_nr_dentry_unused +
            sb->s_nr_inodes_unused + fs_objects + 1;

    if (sc->nr_to_scan) {
        int dentries;
        int inodes;

        /* proportion the scan between the caches */
        dentries = (sc->nr_to_scan * sb->s_nr_dentry_unused) /
                            total_objects;
        inodes = (sc->nr_to_scan * sb->s_nr_inodes_unused) /
                            total_objects;
        if (fs_objects)
            fs_objects = (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
         */
        prune_dcache_sb(sb, dentries);
        prune_icache_sb(sb, inodes);

        if (fs_objects && sb->s_op->free_cached_objects) {
            sb->s_op->free_cached_objects(sb, fs_objects);
            fs_objects = sb->s_op->nr_cached_objects(sb);
        }
        total_objects = sb->s_nr_dentry_unused +
                sb->s_nr_inodes_unused + fs_objects;
    }

    total_objects = (total_objects / 100) * sysctl_vfs_cache_pressure;
    drop_super(sb);
    return total_objects;
}

static int init_sb_writers(struct super_block *s, struct file_system_type *type)
{
    int err;
    int i;

    for (i = 0; i < SB_FREEZE_LEVELS; i++) {
        err = percpu_counter_init(&s->s_writers.counter[i], 0);
        if (err < 0)
            goto err_out;
        lockdep_init_map(&s->s_writers.lock_map[i], sb_writers_name[i],
                 &type->s_writers_key[i], 0);
    }
    init_waitqueue_head(&s->s_writers.wait);
    init_waitqueue_head(&s->s_writers.wait_unfrozen);
    return 0;
err_out:
    while (--i >= 0)
        percpu_counter_destroy(&s->s_writers.counter[i]);
    return err;
}

static void destroy_sb_writers(struct super_block *s)
{
    int i;

    for (i = 0; i < SB_FREEZE_LEVELS; i++)
        percpu_counter_destroy(&s->s_writers.counter[i]);
}

static struct super_block *alloc_super(struct file_system_type *type, int flags)
{
    struct super_block *s = kzalloc(sizeof(struct super_block),  GFP_USER);
    static const struct super_operations default_op;

    if (s) {
        if (security_sb_alloc(s)) {
            /*
             * We cannot call security_sb_free() without
             * security_sb_alloc() succeeding. So bail out manually
             */
            kfree(s);
            s = NULL;
            goto out;
        }
#ifdef CONFIG_SMP
        s->s_files = alloc_percpu(struct list_head);
        if (!s->s_files)
            goto err_out;
        else {
            int i;

            for_each_possible_cpu(i)
                INIT_LIST_HEAD(per_cpu_ptr(s->s_files, i));
        }
#else
        INIT_LIST_HEAD(&s->s_files);
#endif
        if (init_sb_writers(s, type))
            goto err_out;
        s->s_flags = flags;
        s->s_bdi = &default_backing_dev_info;
        INIT_HLIST_NODE(&s->s_instances);
        INIT_HLIST_BL_HEAD(&s->s_anon);
        INIT_LIST_HEAD(&s->s_inodes);
        INIT_LIST_HEAD(&s->s_dentry_lru);
        INIT_LIST_HEAD(&s->s_inode_lru);
        spin_lock_init(&s->s_inode_lru_lock);
        INIT_LIST_HEAD(&s->s_mounts);
        init_rwsem(&s->s_umount);
        lockdep_set_class(&s->s_umount, &type->s_umount_key);
        /*
         * sget() can have s_umount recursion.
         *
         * When it cannot find a suitable sb, it allocates a new
         * one (this one), and tries again to find a suitable old
         * one.
         *
         * In case that succeeds, it will acquire the s_umount
         * lock of the old one. Since these are clearly distrinct
         * locks, and this object isn't exposed yet, there's no
         * risk of deadlocks.
         *
         * Annotate this by putting this lock in a different
         * subclass.
         */
        down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING);
        s->s_count = 1;
        atomic_set(&s->s_active, 1);
        mutex_init(&s->s_vfs_rename_mutex);
        lockdep_set_class(&s->s_vfs_rename_mutex, &type->s_vfs_rename_key);
        mutex_init(&s->s_dquot.dqio_mutex);
        mutex_init(&s->s_dquot.dqonoff_mutex);
        init_rwsem(&s->s_dquot.dqptr_sem);
        s->s_maxbytes = MAX_NON_LFS;
        s->s_op = &default_op;
        s->s_time_gran = 1000000000;
        s->cleancache_poolid = -1;

        s->s_shrink.seeks = DEFAULT_SEEKS;
        s->s_shrink.shrink = prune_super;
        s->s_shrink.batch = 1024;
    }
out:
    return s;
err_out:
    security_sb_free(s);
#ifdef CONFIG_SMP
    if (s->s_files)
        free_percpu(s->s_files);
#endif
    destroy_sb_writers(s);
    kfree(s);
    s = NULL;
    goto out;
}

static inline void destroy_super(struct super_block *s)
{
#ifdef CONFIG_SMP
    free_percpu(s->s_files);
#endif
    destroy_sb_writers(s);
    security_sb_free(s);
    WARN_ON(!list_empty(&s->s_mounts));
    kfree(s->s_subtype);
    kfree(s->s_options);
    kfree(s);
}

/* Superblock refcounting  */

/*
 * Drop a superblock's refcount.  The caller must hold sb_lock.
 */
static void __put_super(struct super_block *sb)
{
    if (!--sb->s_count) {
        list_del_init(&sb->s_list);
        destroy_super(sb);
    }
}

static void put_super(struct super_block *sb)
{
    spin_lock(&sb_lock);
    __put_super(sb);
    spin_unlock(&sb_lock);
}


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);
        fs->kill_sb(s);

        /* caches are now gone, we can safely kill the shrinker now */
        unregister_shrinker(&s->s_shrink);
        put_filesystem(fs);
        put_super(s);
    } else {
        up_write(&s->s_umount);
    }
}

EXPORT_SYMBOL(deactivate_locked_super);

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);

static int grab_super(struct super_block *s) __releases(sb_lock)
{
    if (atomic_inc_not_zero(&s->s_active)) {
        spin_unlock(&sb_lock);
        return 1;
    }
    /* it's going away */
    s->s_count++;
    spin_unlock(&sb_lock);
    /* wait for it to die */
    down_write(&s->s_umount);
    up_write(&s->s_umount);
    put_super(s);
    return 0;
}

/*
 *  grab_super_passive - acquire a passive reference
 *  @sb: reference we are trying to grab
 *
 *  Tries to acquire a passive reference. This is used in places where we
 *  cannot take an active reference but we need to ensure that the
 *  superblock does not go away while we are working on it. It returns
 *  false if a reference was not gained, and returns true with the s_umount
 *  lock held in read mode if a reference is gained. On successful return,
 *  the caller must drop the s_umount lock and the passive reference when
 *  done.
 */
bool grab_super_passive(struct super_block *sb)
{
    spin_lock(&sb_lock);
    if (hlist_unhashed(&sb->s_instances)) {
        spin_unlock(&sb_lock);
        return false;
    }

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

    if (down_read_trylock(&sb->s_umount)) {
        if (sb->s_root && (sb->s_flags & MS_BORN))
            return true;
        up_read(&sb->s_umount);
    }

    put_super(sb);
    return false;
}

void generic_shutdown_super(struct super_block *sb)
{
    const struct super_operations *sop = sb->s_op;

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

        fsnotify_unmount_inodes(&sb->s_inodes);

        evict_inodes(sb);

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

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

EXPORT_SYMBOL(generic_shutdown_super);

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 super_block *s = NULL;
    struct hlist_node *node;
    struct super_block *old;
    int err;

retry:
    spin_lock(&sb_lock);
    if (test) {
        hlist_for_each_entry(old, node, &type->fs_supers, s_instances) {
            if (!test(old, data))
                continue;
            if (!grab_super(old))
                goto retry;
            if (s) {
                up_write(&s->s_umount);
                destroy_super(s);
                s = NULL;
            }
            down_write(&old->s_umount);
            if (unlikely(!(old->s_flags & MS_BORN))) {
                deactivate_locked_super(old);
                goto retry;
            }
            return old;
        }
    }
    if (!s) {
        spin_unlock(&sb_lock);
        s = alloc_super(type, flags);
        if (!s)
            return ERR_PTR(-ENOMEM);
        goto retry;
    }
        
    err = set(s, data);
    if (err) {
        spin_unlock(&sb_lock);
        up_write(&s->s_umount);
        destroy_super(s);
        return ERR_PTR(err);
    }
    s->s_type = type;
    strlcpy(s->s_id, type->name, sizeof(s->s_id));
    list_add_tail(&s->s_list, &super_blocks);
    hlist_add_head(&s->s_instances, &type->fs_supers);
    spin_unlock(&sb_lock);
    get_filesystem(type);
    register_shrinker(&s->s_shrink);
    return s;
}

EXPORT_SYMBOL(sget);

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

EXPORT_SYMBOL(drop_super);

void iterate_supers(void (*f)(struct super_block *, void *), void *arg)
{
    struct super_block *sb, *p = NULL;

    spin_lock(&sb_lock);
    list_for_each_entry(sb, &super_blocks, s_list) {
        if (hlist_unhashed(&sb->s_instances))
            continue;
        sb->s_count++;
        spin_unlock(&sb_lock);

        down_read(&sb->s_umount);
        if (sb->s_root && (sb->s_flags & MS_BORN))
            f(sb, arg);
        up_read(&sb->s_umount);

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

void iterate_supers_type(struct file_system_type *type,
    void (*f)(struct super_block *, void *), void *arg)
{
    struct super_block *sb, *p = NULL;
    struct hlist_node *node;

    spin_lock(&sb_lock);
    hlist_for_each_entry(sb, node, &type->fs_supers, s_instances) {
        sb->s_count++;
        spin_unlock(&sb_lock);

        down_read(&sb->s_umount);
        if (sb->s_root && (sb->s_flags & MS_BORN))
            f(sb, arg);
        up_read(&sb->s_umount);

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

EXPORT_SYMBOL(iterate_supers_type);

struct super_block *get_super(struct block_device *bdev)
{
    struct super_block *sb;

    if (!bdev)
        return NULL;

    spin_lock(&sb_lock);
rescan:
    list_for_each_entry(sb, &super_blocks, s_list) {
        if (hlist_unhashed(&sb->s_instances))
            continue;
        if (sb->s_bdev == bdev) {
            sb->s_count++;
            spin_unlock(&sb_lock);
            down_read(&sb->s_umount);
            /* still alive? */
            if (sb->s_root && (sb->s_flags & MS_BORN))
                return sb;
            up_read(&sb->s_umount);
            /* nope, got unmounted */
            spin_lock(&sb_lock);
            __put_super(sb);
            goto rescan;
        }
    }
    spin_unlock(&sb_lock);
    return NULL;
}

EXPORT_SYMBOL(get_super);

struct super_block *get_super_thawed(struct block_device *bdev)
{
    while (1) {
        struct super_block *s = get_super(bdev);
        if (!s || s->s_writers.frozen == SB_UNFROZEN)
            return s;
        up_read(&s->s_umount);
        wait_event(s->s_writers.wait_unfrozen,
               s->s_writers.frozen == SB_UNFROZEN);
        put_super(s);
    }
}
EXPORT_SYMBOL(get_super_thawed);

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)) /* drops sb_lock */
                return sb;
            else
                goto restart;
        }
    }
    spin_unlock(&sb_lock);
    return NULL;
}
 
struct super_block *user_get_super(dev_t dev)
{
    struct super_block *sb;

    spin_lock(&sb_lock);
rescan:
    list_for_each_entry(sb, &super_blocks, s_list) {
        if (hlist_unhashed(&sb->s_instances))
            continue;
        if (sb->s_dev ==  dev) {
            sb->s_count++;
            spin_unlock(&sb_lock);
            down_read(&sb->s_umount);
            /* still alive? */
            if (sb->s_root && (sb->s_flags & MS_BORN))
                return sb;
            up_read(&sb->s_umount);
            /* nope, got unmounted */
            spin_lock(&sb_lock);
            __put_super(sb);
            goto rescan;
        }
    }
    spin_unlock(&sb_lock);
    return NULL;
}

int do_remount_sb(struct super_block *sb, int flags, void *data, int force)
{
    int retval;
    int remount_ro;

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

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

    if (flags & MS_RDONLY)
        acct_auto_close(sb);
    shrink_dcache_sb(sb);
    sync_filesystem(sb);

    remount_ro = (flags & MS_RDONLY) && !(sb->s_flags & MS_RDONLY);

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

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

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

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

static void do_emergency_remount(struct work_struct *work)
{
    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_write(&sb->s_umount);
        if (sb->s_root && sb->s_bdev && (sb->s_flags & MS_BORN) &&
            !(sb->s_flags & MS_RDONLY)) {
            /*
             * What lock protects sb->s_flags??
             */
            do_remount_sb(sb, MS_RDONLY, NULL, 1);
        }
        up_write(&sb->s_umount);
        spin_lock(&sb_lock);
        if (p)
            __put_super(p);
        p = sb;
    }
    if (p)
        __put_super(p);
    spin_unlock(&sb_lock);
    kfree(work);
    printk("Emergency Remount complete\n");
}

void emergency_remount(void)
{
    struct work_struct *work;

    work = kmalloc(sizeof(*work), GFP_ATOMIC);
    if (work) {
        INIT_WORK(work, do_emergency_remount);
        schedule_work(work);
    }
}

/*
 * Unnamed block devices are dummy devices used by virtual
 * filesystems which don't use real block-devices.  -- jrs
 */

static DEFINE_IDA(unnamed_dev_ida);
static DEFINE_SPINLOCK(unnamed_dev_lock);/* protects the above */
static int unnamed_dev_start = 0; /* don't bother trying below it */

int get_anon_bdev(dev_t *p)
{
    int dev;
    int error;

 retry:
    if (ida_pre_get(&unnamed_dev_ida, GFP_ATOMIC) == 0)
        return -ENOMEM;
    spin_lock(&unnamed_dev_lock);
    error = ida_get_new_above(&unnamed_dev_ida, unnamed_dev_start, &dev);
    if (!error)
        unnamed_dev_start = dev + 1;
    spin_unlock(&unnamed_dev_lock);
    if (error == -EAGAIN)
        /* We raced and lost with another CPU. */
        goto retry;
    else if (error)
        return -EAGAIN;

    if ((dev & MAX_IDR_MASK) == (1 << MINORBITS)) {
        spin_lock(&unnamed_dev_lock);
        ida_remove(&unnamed_dev_ida, dev);
        if (unnamed_dev_start > dev)
            unnamed_dev_start = dev;
        spin_unlock(&unnamed_dev_lock);
        return -EMFILE;
    }
    *p = MKDEV(0, dev & MINORMASK);
    return 0;
}
EXPORT_SYMBOL(get_anon_bdev);

void free_anon_bdev(dev_t dev)
{
    int slot = MINOR(dev);
    spin_lock(&unnamed_dev_lock);
    ida_remove(&unnamed_dev_ida, slot);
    if (slot < unnamed_dev_start)
        unnamed_dev_start = slot;
    spin_unlock(&unnamed_dev_lock);
}
EXPORT_SYMBOL(free_anon_bdev);

int set_anon_super(struct super_block *s, void *data)
{
    int error = get_anon_bdev(&s->s_dev);
    if (!error)
        s->s_bdi = &noop_backing_dev_info;
    return error;
}

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, int (*fill_super)(struct super_block *, void *, int))
{
    struct super_block *sb;

    sb = sget(fs_type, ns_test_super, ns_set_super, flags, data);
    if (IS_ERR(sb))
        return ERR_CAST(sb);

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

        sb->s_flags |= MS_ACTIVE;
    }

    return dget(sb->s_root);
}

EXPORT_SYMBOL(mount_ns);

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

    /*
     * We set the bdi here to the queue backing, file systems can
     * overwrite this in ->fill_super()
     */
    s->s_bdi = &bdev_get_queue(s->s_bdev)->backing_dev_info;
    return 0;
}

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

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

    if (!(flags & MS_RDONLY))
        mode |= FMODE_WRITE;

    bdev = blkdev_get_by_path(dev_name, mode, fs_type);
    if (IS_ERR(bdev))
        return ERR_CAST(bdev);

    /*
     * once the super is inserted into the list by sget, s_umount
     * will protect the lockfs code from trying to start a snapshot
     * while we are mounting
     */
    mutex_lock(&bdev->bd_fsfreeze_mutex);
    if (bdev->bd_fsfreeze_count > 0) {
        mutex_unlock(&bdev->bd_fsfreeze_mutex);
        error = -EBUSY;
        goto error_bdev;
    }
    s = sget(fs_type, test_bdev_super, set_bdev_super, flags | MS_NOSEC,
         bdev);
    mutex_unlock(&bdev->bd_fsfreeze_mutex);
    if (IS_ERR(s))
        goto error_s;

    if (s->s_root) {
        if ((flags ^ s->s_flags) & MS_RDONLY) {
            deactivate_locked_super(s);
            error = -EBUSY;
            goto error_bdev;
        }

        /*
         * s_umount nests inside bd_mutex during
         * __invalidate_device().  blkdev_put() acquires
         * bd_mutex and can't be called under s_umount.  Drop
         * s_umount temporarily.  This is safe as we're
         * holding an active reference.
         */
        up_write(&s->s_umount);
        blkdev_put(bdev, mode);
        down_write(&s->s_umount);
    } else {
        char b[BDEVNAME_SIZE];

        s->s_mode = mode;
        strlcpy(s->s_id, bdevname(bdev, b), sizeof(s->s_id));
        sb_set_blocksize(s, block_size(bdev));
        error = fill_super(s, data, flags & MS_SILENT ? 1 : 0);
        if (error) {
            deactivate_locked_super(s);
            goto error;
        }

        s->s_flags |= MS_ACTIVE;
        bdev->bd_super = s;
    }

    return dget(s->s_root);

error_s:
    error = PTR_ERR(s);
error_bdev:
    blkdev_put(bdev, mode);
error:
    return ERR_PTR(error);
}
EXPORT_SYMBOL(mount_bdev);

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

    bdev->bd_super = NULL;
    generic_shutdown_super(sb);
    sync_blockdev(bdev);
    WARN_ON_ONCE(!(mode & FMODE_EXCL));
    blkdev_put(bdev, mode | FMODE_EXCL);
}

EXPORT_SYMBOL(kill_block_super);
#endif

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

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

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

static int compare_single(struct super_block *s, void *p)
{
    return 1;
}

struct dentry *mount_single(struct file_system_type *fs_type,
    int flags, void *data,
    int (*fill_super)(struct super_block *, void *, int))
{
    struct super_block *s;
    int error;

    s = sget(fs_type, compare_single, set_anon_super, flags, NULL);
    if (IS_ERR(s))
        return ERR_CAST(s);
    if (!s->s_root) {
        error = fill_super(s, data, flags & MS_SILENT ? 1 : 0);
        if (error) {
            deactivate_locked_super(s);
            return ERR_PTR(error);
        }
        s->s_flags |= MS_ACTIVE;
    } else {
        do_remount_sb(s, flags, data, 0);
    }
    return dget(s->s_root);
}
EXPORT_SYMBOL(mount_single);

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

    if (data && !(type->fs_flags & FS_BINARY_MOUNTDATA)) {
        secdata = alloc_secdata();
        if (!secdata)
            goto out;

        error = security_sb_copy_data(data, secdata);
        if (error)
            goto out_free_secdata;
    }

    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);
    WARN_ON(sb->s_bdi == &default_backing_dev_info);
    sb->s_flags |= MS_BORN;

    error = security_sb_kern_mount(sb, flags, secdata);
    if (error)
        goto out_sb;

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

    up_write(&sb->s_umount);
    free_secdata(secdata);
    return root;
out_sb:
    dput(root);
    deactivate_locked_super(sb);
out_free_secdata:
    free_secdata(secdata);
out:
    return ERR_PTR(error);
}

/*
 * This is an internal function, please use sb_end_{write,pagefault,intwrite}
 * instead.
 */
void __sb_end_write(struct super_block *sb, int level)
{
    percpu_counter_dec(&sb->s_writers.counter[level-1]);
    /*
     * Make sure s_writers are updated before we wake up waiters in
     * freeze_super().
     */
    smp_mb();
    if (waitqueue_active(&sb->s_writers.wait))
        wake_up(&sb->s_writers.wait);
    rwsem_release(&sb->s_writers.lock_map[level-1], 1, _RET_IP_);
}
EXPORT_SYMBOL(__sb_end_write);

#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 tell lockdep we are doing trylock when we
 * already hold a freeze protection for a higher freeze level.
 */
static void acquire_freeze_lock(struct super_block *sb, int level, bool trylock,
                unsigned long ip)
{
    int i;

    if (!trylock) {
        for (i = 0; i < level - 1; i++)
            if (lock_is_held(&sb->s_writers.lock_map[i])) {
                trylock = true;
                break;
            }
    }
    rwsem_acquire_read(&sb->s_writers.lock_map[level-1], 0, trylock, ip);
}
#endif

/*
 * 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)
{
retry:
    if (unlikely(sb->s_writers.frozen >= level)) {
        if (!wait)
            return 0;
        wait_event(sb->s_writers.wait_unfrozen,
               sb->s_writers.frozen < level);
    }

#ifdef CONFIG_LOCKDEP
    acquire_freeze_lock(sb, level, !wait, _RET_IP_);
#endif
    percpu_counter_inc(&sb->s_writers.counter[level-1]);
    /*
     * Make sure counter is updated before we check for frozen.
     * freeze_super() first sets frozen and then checks the counter.
     */
    smp_mb();
    if (unlikely(sb->s_writers.frozen >= level)) {
        __sb_end_write(sb, level);
        goto retry;
    }
    return 1;
}
EXPORT_SYMBOL(__sb_start_write);

static void sb_wait_write(struct super_block *sb, int level)
{
    s64 writers;

    /*
     * We just cycle-through lockdep here so that it does not complain
     * about returning with lock to userspace
     */
    rwsem_acquire(&sb->s_writers.lock_map[level-1], 0, 0, _THIS_IP_);
    rwsem_release(&sb->s_writers.lock_map[level-1], 1, _THIS_IP_);

    do {
        DEFINE_WAIT(wait);

        /*
         * We use a barrier in prepare_to_wait() to separate setting
         * of frozen and checking of the counter
         */
        prepare_to_wait(&sb->s_writers.wait, &wait,
                TASK_UNINTERRUPTIBLE);

        writers = percpu_counter_sum(&sb->s_writers.counter[level-1]);
        if (writers)
            schedule();

        finish_wait(&sb->s_writers.wait, &wait);
    } while (writers);
}

int freeze_super(struct super_block *sb)
{
    int ret;

    atomic_inc(&sb->s_active);
    down_write(&sb->s_umount);
    if (sb->s_writers.frozen != SB_UNFROZEN) {
        deactivate_locked_super(sb);
        return -EBUSY;
    }

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

    if (sb->s_flags & MS_RDONLY) {
        /* Nothing to do really... */
        sb->s_writers.frozen = SB_FREEZE_COMPLETE;
        up_write(&sb->s_umount);
        return 0;
    }

    /* From now on, no new normal writers can start */
    sb->s_writers.frozen = SB_FREEZE_WRITE;
    smp_wmb();

    /* Release s_umount to preserve sb_start_write -> s_umount ordering */
    up_write(&sb->s_umount);

    sb_wait_write(sb, SB_FREEZE_WRITE);

    /* Now we go and block page faults... */
    down_write(&sb->s_umount);
    sb->s_writers.frozen = SB_FREEZE_PAGEFAULT;
    smp_wmb();

    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;
    smp_wmb();
    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;
            smp_wmb();
            wake_up(&sb->s_writers.wait_unfrozen);
            deactivate_locked_super(sb);
            return ret;
        }
    }
    /*
     * This is just for debugging purposes so that fs can warn if it
     * sees write activity when frozen is set to SB_FREEZE_COMPLETE.
     */
    sb->s_writers.frozen = SB_FREEZE_COMPLETE;
    up_write(&sb->s_umount);
    return 0;
}
EXPORT_SYMBOL(freeze_super);

int thaw_super(struct super_block *sb)
{
    int error;

    down_write(&sb->s_umount);
    if (sb->s_writers.frozen == SB_UNFROZEN) {
        up_write(&sb->s_umount);
        return -EINVAL;
    }

    if (sb->s_flags & MS_RDONLY)
        goto out;

    if (sb->s_op->unfreeze_fs) {
        error = sb->s_op->unfreeze_fs(sb);
        if (error) {
            printk(KERN_ERR
                "VFS:Filesystem thaw failed\n");
            up_write(&sb->s_umount);
            return error;
        }
    }

out:
    sb->s_writers.frozen = SB_UNFROZEN;
    smp_wmb();
    wake_up(&sb->s_writers.wait_unfrozen);
    deactivate_locked_super(sb);

    return 0;
}
EXPORT_SYMBOL(thaw_super);