[linux-2.6-microblaze.git] / fs / super.c

/*
 *  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/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 !sc->nr_to_scan ? 0 : -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]);
}

/**
 *      alloc_super     -       create new superblock
 *      @type:  filesystem type superblock should belong to
 *      @flags: the mount flags
 *
 *      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 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);
                mutex_init(&s->s_lock);
                lockdep_set_class(&s->s_umount, &type->s_umount_key);
                /*
                 * The locking rules for s_lock are up to the
                 * filesystem. For example ext3fs has different
                 * lock ordering than usbfs:
                 */
                lockdep_set_class(&s->s_lock, &type->s_lock_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);
                init_waitqueue_head(&s->s_wait_unfrozen);
                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;
}

/**
 *      destroy_super   -       frees a superblock
 *      @s: superblock to free
 *
 *      Frees a superblock.
 */
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);
        }
}

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


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

                /* caches are now gone, we can safely kill the shrinker now */
                unregister_shrinker(&s->s_shrink);

                /*
                 * We need to call rcu_barrier so all the delayed rcu free
                 * inodes are flushed before we release the fs module.
                 */
                rcu_barrier();
                put_filesystem(fs);
                put_super(s);
        } else {
                up_write(&s->s_umount);
        }
}

EXPORT_SYMBOL(deactivate_locked_super);

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

EXPORT_SYMBOL(deactivate_super);

/**
 *      grab_super - acquire an active reference
 *      @s: reference we are trying to make active
 *
 *      Tries to acquire an active reference.  grab_super() is used when we
 *      had just found a superblock in super_blocks or fs_type->fs_supers
 *      and want to turn it into a full-blown active reference.  grab_super()
 *      is called with sb_lock held and drops it.  Returns 1 in case of
 *      success, 0 if we had failed (superblock contents was already dead or
 *      dying when grab_super() had been called).
 */
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
 *      @s: 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;
}

/*
 * Superblock locking.  We really ought to get rid of these two.
 */
void lock_super(struct super_block * sb)
{
        mutex_lock(&sb->s_lock);
}

void unlock_super(struct super_block * sb)
{
        mutex_unlock(&sb->s_lock);
}

EXPORT_SYMBOL(lock_super);
EXPORT_SYMBOL(unlock_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 &= ~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);

/**
 *      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 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);

/**
 * sync_supers - helper for periodic superblock writeback
 *
 * Call the write_super method if present on all dirty superblocks in
 * the system.  This is for the periodic writeback used by most older
 * filesystems.  For data integrity superblock writeback use
 * sync_filesystems() instead.
 *
 * Note: check the dirty flag before waiting, so we don't
 * hold up the sync while mounting a device. (The newly
 * mounted device won't need syncing.)
 */
void sync_supers(void)
{
        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;
                if (sb->s_op->write_super && sb->s_dirt) {
                        sb->s_count++;
                        spin_unlock(&sb_lock);

                        down_read(&sb->s_umount);
                        if (sb->s_root && sb->s_dirt && (sb->s_flags & MS_BORN))
                                sb->s_op->write_super(sb);
                        up_read(&sb->s_umount);

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

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

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

                down_read(&sb->s_umount);
                if (sb->s_root && (sb->s_flags & 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);
}

/**
 *      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;
        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);

/**
 *      get_super - get the superblock of a device
 *      @bdev: device to get the superblock for
 *      
 *      Scans the superblock list and finds the superblock of the file system
 *      mounted on the device given. %NULL is returned if no match is found.
 */

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

/**
 *      get_super_thawed - get thawed superblock of a device
 *      @bdev: device to get the superblock for
 *
 *      Scans the superblock list and finds the superblock of the file system
 *      mounted on the device. The superblock is returned once it is thawed
 *      (or immediately if it was not frozen). %NULL is returned if no match
 *      is found.
 */
struct super_block *get_super_thawed(struct block_device *bdev)
{
        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);

/**
 * get_active_super - get an active reference to the superblock of a device
 * @bdev: device to get the superblock for
 *
 * Scans the superblock list and finds the superblock of the file system
 * mounted on the device given.  Returns the superblock with an active
 * reference or %NULL if none was found.
 */
struct super_block *get_active_super(struct block_device *bdev)
{
        struct super_block *sb;

        if (!bdev)
                return NULL;

restart:
        spin_lock(&sb_lock);
        list_for_each_entry(sb, &super_blocks, s_list) {
                if (hlist_unhashed(&sb->s_instances))
                        continue;
                if (sb->s_bdev == bdev) {
                        if (grab_super(sb)) /* 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;
}

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

/**
 * 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. Caller of this function should make sure there can be no new writers
 * of type @level before calling this function. Otherwise this function can
 * livelock.
 */
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);
}

/**
 * freeze_super - lock the filesystem and force it into a consistent state
 * @sb: the super to lock
 *
 * Syncs the super to make sure the filesystem is consistent and calls the fs's
 * freeze_fs.  Subsequent calls to this without first thawing the fs will return
 * -EBUSY.
 *
 * During this function, sb->s_writers.frozen goes through these values:
 *
 * SB_UNFROZEN: File system is normal, all writes progress as usual.
 *
 * SB_FREEZE_WRITE: The file system is in the process of being frozen.  New
 * writes should be blocked, though page faults are still allowed. We wait for
 * all writes to complete and then proceed to the next stage.
 *
 * SB_FREEZE_PAGEFAULT: Freezing continues. Now also page faults are blocked
 * but internal fs threads can still modify the filesystem (although they
 * should not dirty new pages or inodes), writeback can run etc. After waiting
 * for all running page faults we sync the filesystem which will clean all
 * dirty pages and inodes (no new dirty pages or inodes can be created when
 * sync is running).
 *
 * SB_FREEZE_FS: The file system is frozen. Now all internal sources of fs
 * modification are blocked (e.g. XFS preallocation truncation on inode
 * reclaim). This is usually implemented by blocking new transactions for
 * filesystems that have them and need this additional guard. After all
 * internal writers are finished we call ->freeze_fs() to finish filesystem
 * freezing. Then we transition to SB_FREEZE_COMPLETE state. This state is
 * mostly auxiliary for filesystems to verify they do not modify frozen fs.
 *
 * sb->s_writers.frozen is protected by sb->s_umount.
 */
int freeze_super(struct super_block *sb)
{
        int ret;

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

        if (!(sb->s_flags & 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);

/**
 * thaw_super -- unlock filesystem
 * @sb: the super to thaw
 *
 * Unlocks the filesystem and marks it writeable again after 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);