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 // 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);
 
 static LIST_HEAD(super_blocks);
 static DEFINE_SPINLOCK(sb_lock);
 
 static char *sb_writers_name[SB_FREEZE_LEVELS] = {
     "sb_writers",
     "sb_pagefaults",
     "sb_internal",
 };
 
 /*
  * One thing we have to be careful of with a per-sb shrinker is that we don't
  * drop the last active reference to the superblock from within the shrinker.
  * If that happens we could trigger unregistering the shrinker from within the
  * shrinker path and that leads to deadlock on the shrinker_rwsem. Hence we
  * take a passive reference to the superblock to avoid this from occurring.
  */
 static unsigned long super_cache_scan(struct shrinker *shrink,
                       struct shrink_control *sc)
 {
     struct super_block *sb;
     long    fs_objects = 0;
     long    total_objects;
     long    freed = 0;
     long    dentries;
     long    inodes;
 
     sb = container_of(shrink, struct super_block, s_shrink);
 
     /*
      * Deadlock avoidance.  We may hold various FS locks, and we don't want
      * to recurse into the FS that called us in clear_inode() and friends..
      */
     if (!(sc->gfp_mask & __GFP_FS))
         return SHRINK_STOP;
 
     if (!trylock_super(sb))
         return SHRINK_STOP;
 
     if (sb->s_op->nr_cached_objects)
         fs_objects = sb->s_op->nr_cached_objects(sb, sc);
 
     inodes = list_lru_shrink_count(&sb->s_inode_lru, sc);
     dentries = list_lru_shrink_count(&sb->s_dentry_lru, sc);
     total_objects = dentries + inodes + fs_objects + 1;
     if (!total_objects)
         total_objects = 1;
 
     /* proportion the scan between the caches */
     dentries = mult_frac(sc->nr_to_scan, dentries, total_objects);
     inodes = mult_frac(sc->nr_to_scan, inodes, total_objects);
     fs_objects = mult_frac(sc->nr_to_scan, fs_objects, total_objects);
 
     /*
      * prune the dcache first as the icache is pinned by it, then
      * prune the icache, followed by the filesystem specific caches
      *
      * Ensure that we always scan at least one object - memcg kmem
      * accounting uses this to fully empty the caches.
      */
     sc->nr_to_scan = dentries + 1;
     freed = prune_dcache_sb(sb, sc);
     sc->nr_to_scan = inodes + 1;
     freed += prune_icache_sb(sb, sc);
 
     if (fs_objects) {
         sc->nr_to_scan = fs_objects + 1;
         freed += sb->s_op->free_cached_objects(sb, sc);
     }
 
     up_read(&sb->s_umount);
     return freed;
 }
 
 static unsigned long super_cache_count(struct shrinker *shrink,
                        struct shrink_control *sc)
 {
     struct super_block *sb;
     long    total_objects = 0;
 
     sb = container_of(shrink, struct super_block, s_shrink);
 
     /*
      * We don't call trylock_super() here as it is a scalability bottleneck,
      * so we're exposed to partial setup state. The shrinker rwsem does not
      * protect filesystem operations backing list_lru_shrink_count() or
      * s_op->nr_cached_objects(). Counts can change between
      * super_cache_count and super_cache_scan, so we really don't need locks
      * here.
      *
      * However, if we are currently mounting the superblock, the underlying
      * filesystem might be in a state of partial construction and hence it
      * is dangerous to access it.  trylock_super() uses a SB_BORN check to
      * avoid this situation, so do the same here. The memory barrier is
      * matched with the one in mount_fs() as we don't hold locks here.
      */
     if (!(sb->s_flags & SB_BORN))
         return 0;
     smp_rmb();
 
     if (sb->s_op && sb->s_op->nr_cached_objects)
         total_objects = sb->s_op->nr_cached_objects(sb, sc);
 
     total_objects += list_lru_shrink_count(&sb->s_dentry_lru, sc);
     total_objects += list_lru_shrink_count(&sb->s_inode_lru, sc);
 
     if (!total_objects)
         return SHRINK_EMPTY;
 
     total_objects = vfs_pressure_ratio(total_objects);
     return total_objects;
 }
 
 static void destroy_super_work(struct work_struct *work)
 {
     struct super_block *s = container_of(work, struct super_block,
                             destroy_work);
     int i;
 
     for (i = 0; i < SB_FREEZE_LEVELS; i++)
         percpu_free_rwsem(&s->s_writers.rw_sem[i]);
     kfree(s);
 }
 
 static void destroy_super_rcu(struct rcu_head *head)
 {
     struct super_block *s = container_of(head, struct super_block, rcu);
     INIT_WORK(&s->destroy_work, destroy_super_work);
     schedule_work(&s->destroy_work);
 }
 
 /* Free a superblock that has never been seen by anyone */
 static void destroy_unused_super(struct super_block *s)
 {
     if (!s)
         return;
     up_write(&s->s_umount);
     list_lru_destroy(&s->s_dentry_lru);
     list_lru_destroy(&s->s_inode_lru);
     security_sb_free(s);
     put_user_ns(s->s_user_ns);
     kfree(s->s_subtype);
     free_prealloced_shrinker(&s->s_shrink);
     /* no delays needed */
     destroy_super_work(&s->destroy_work);
 }
 
 /**
  *  alloc_super -   create new superblock
  *  @type:  filesystem type superblock should belong to
  *  @flags: the mount flags
  *  @user_ns: User namespace for the super_block
  *
  *  Allocates and initializes a new &struct super_block.  alloc_super()
  *  returns a pointer new superblock or %NULL if allocation had failed.
  */
 static struct super_block *alloc_super(struct file_system_type *type, int flags,
                        struct user_namespace *user_ns)
 {
     struct super_block *s = kzalloc(sizeof(struct super_block),  GFP_USER);
     static const struct super_operations default_op;
     int i;
 
     if (!s)
         return NULL;
 
     INIT_LIST_HEAD(&s->s_mounts);
     s->s_user_ns = get_user_ns(user_ns);
     init_rwsem(&s->s_umount);
     lockdep_set_class(&s->s_umount, &type->s_umount_key);
     /*
      * sget() can have s_umount recursion.
      *
      * When it cannot find a suitable sb, it allocates a new
      * one (this one), and tries again to find a suitable old
      * one.
      *
      * In case that succeeds, it will acquire the s_umount
      * lock of the old one. Since these are clearly distrinct
      * locks, and this object isn't exposed yet, there's no
      * risk of deadlocks.
      *
      * Annotate this by putting this lock in a different
      * subclass.
      */
     down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING);
 
     if (security_sb_alloc(s))
         goto fail;
 
     for (i = 0; i < SB_FREEZE_LEVELS; i++) {
         if (__percpu_init_rwsem(&s->s_writers.rw_sem[i],
                     sb_writers_name[i],
                     &type->s_writers_key[i]))
             goto fail;
     }
     init_waitqueue_head(&s->s_writers.wait_unfrozen);
     s->s_bdi = &noop_backing_dev_info;
     s->s_flags = flags;
     if (s->s_user_ns != &init_user_ns)
         s->s_iflags |= SB_I_NODEV;
     INIT_HLIST_NODE(&s->s_instances);
     INIT_HLIST_BL_HEAD(&s->s_roots);
     mutex_init(&s->s_sync_lock);
     INIT_LIST_HEAD(&s->s_inodes);
     spin_lock_init(&s->s_inode_list_lock);
     INIT_LIST_HEAD(&s->s_inodes_wb);
     spin_lock_init(&s->s_inode_wblist_lock);
 
     s->s_count = 1;
     atomic_set(&s->s_active, 1);
     mutex_init(&s->s_vfs_rename_mutex);
     lockdep_set_class(&s->s_vfs_rename_mutex, &type->s_vfs_rename_key);
     init_rwsem(&s->s_dquot.dqio_sem);
     s->s_maxbytes = MAX_NON_LFS;
     s->s_op = &default_op;
     s->s_time_gran = 1000000000;
     s->s_time_min = TIME64_MIN;
     s->s_time_max = TIME64_MAX;
 
     s->s_shrink.seeks = DEFAULT_SEEKS;
     s->s_shrink.scan_objects = super_cache_scan;
     s->s_shrink.count_objects = super_cache_count;
     s->s_shrink.batch = 1024;
     s->s_shrink.flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE;
     if (prealloc_shrinker(&s->s_shrink, "sb-%s", type->name))
         goto fail;
     if (list_lru_init_memcg(&s->s_dentry_lru, &s->s_shrink))
         goto fail;
     if (list_lru_init_memcg(&s->s_inode_lru, &s->s_shrink))
         goto fail;
     return s;
 
 fail:
     destroy_unused_super(s);
     return NULL;
 }
 
 /* Superblock refcounting  */
 
 /*
  * Drop a superblock's refcount.  The caller must hold sb_lock.
  */
 static void __put_super(struct super_block *s)
 {
     if (!--s->s_count) {
         list_del_init(&s->s_list);
         WARN_ON(s->s_dentry_lru.node);
         WARN_ON(s->s_inode_lru.node);
         WARN_ON(!list_empty(&s->s_mounts));
         security_sb_free(s);
         fscrypt_sb_free(s);
         put_user_ns(s->s_user_ns);
         kfree(s->s_subtype);
         call_rcu(&s->rcu, destroy_super_rcu);
     }
 }
 
 /**
  *  put_super   -   drop a temporary reference to superblock
  *  @sb: superblock in question
  *
  *  Drops a temporary reference, frees superblock if there's no
  *  references left.
  */
 void put_super(struct super_block *sb)
 {
     spin_lock(&sb_lock);
     __put_super(sb);
     spin_unlock(&sb_lock);
 }
 
 
 /**
  *  deactivate_locked_super -   drop an active reference to superblock
  *  @s: superblock to deactivate
  *
  *  Drops an active reference to superblock, converting it into a temporary
  *  one if there is no other active references left.  In that case we
  *  tell fs driver to shut it down and drop the temporary reference we
  *  had just acquired.
  *
  *  Caller holds exclusive lock on superblock; that lock is released.
  */
 void deactivate_locked_super(struct super_block *s)
 {
     struct file_system_type *fs = s->s_type;
     if (atomic_dec_and_test(&s->s_active)) {
         unregister_shrinker(&s->s_shrink);
         fs->kill_sb(s);
 
         /*
          * Since list_lru_destroy() may sleep, we cannot call it from
          * put_super(), where we hold the sb_lock. Therefore we destroy
          * the lru lists right now.
          */
         list_lru_destroy(&s->s_dentry_lru);
         list_lru_destroy(&s->s_inode_lru);
 
         put_filesystem(fs);
         put_super(s);
     } else {
         up_write(&s->s_umount);
     }
 }
 
 EXPORT_SYMBOL(deactivate_locked_super);
 
 /**
  *  deactivate_super    -   drop an active reference to superblock
  *  @s: superblock to deactivate
  *
  *  Variant of deactivate_locked_super(), except that superblock is *not*
  *  locked by caller.  If we are going to drop the final active reference,
  *  lock will be acquired prior to that.
  */
 void deactivate_super(struct super_block *s)
 {
     if (!atomic_add_unless(&s->s_active, -1, 1)) {
         down_write(&s->s_umount);
         deactivate_locked_super(s);
     }
 }
 
 EXPORT_SYMBOL(deactivate_super);
 
 /**
  *  grab_super - acquire an active reference
  *  @s: reference we are trying to make active
  *
  *  Tries to acquire an active reference.  grab_super() is used when we
  *  had just found a superblock in super_blocks or fs_type->fs_supers
  *  and want to turn it into a full-blown active reference.  grab_super()
  *  is called with sb_lock held and drops it.  Returns 1 in case of
  *  success, 0 if we had failed (superblock contents was already dead or
  *  dying when grab_super() had been called).  Note that this is only
  *  called for superblocks not in rundown mode (== ones still on ->fs_supers
  *  of their type), so increment of ->s_count is OK here.
  */
 static int grab_super(struct super_block *s) __releases(sb_lock)
 {
     s->s_count++;
     spin_unlock(&sb_lock);
     down_write(&s->s_umount);
     if ((s->s_flags & SB_BORN) && atomic_inc_not_zero(&s->s_active)) {
         put_super(s);
         return 1;
     }
     up_write(&s->s_umount);
     put_super(s);
     return 0;
 }
 
 /*
  *  trylock_super - try to grab ->s_umount shared
  *  @sb: reference we are trying to grab
  *
  *  Try to prevent fs shutdown.  This is used in places where we
  *  cannot take an active reference but we need to ensure that the
  *  filesystem is not shut down while we are working on it. It returns
  *  false if we cannot acquire s_umount or if we lose the race and
  *  filesystem already got into shutdown, and returns true with the s_umount
  *  lock held in read mode in case of success. On successful return,
  *  the caller must drop the s_umount lock when done.
  *
  *  Note that unlike get_super() et.al. this one does *not* bump ->s_count.
  *  The reason why it's safe is that we are OK with doing trylock instead
  *  of down_read().  There's a couple of places that are OK with that, but
  *  it's very much not a general-purpose interface.
  */
 bool trylock_super(struct super_block *sb)
 {
     if (down_read_trylock(&sb->s_umount)) {
         if (!hlist_unhashed(&sb->s_instances) &&
             sb->s_root && (sb->s_flags & SB_BORN))
             return true;
         up_read(&sb->s_umount);
     }
 
     return false;
 }
 
 /**
  *  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);
     down_write(&sb->s_umount);
     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;
     up_write(&sb->s_umount);
 }
 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();
 
         /* evict all inodes with zero refcount */
         evict_inodes(sb);
         /* only nonzero refcount inodes can have marks */
         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);
 
         if (!list_empty(&sb->s_inodes)) {
             printk("VFS: Busy inodes after unmount of %s. "
                "Self-destruct in 5 seconds.  Have a nice day...\n",
                sb->s_id);
         }
     }
     spin_lock(&sb_lock);
     /* should be initialized for __put_super_and_need_restart() */
     hlist_del_init(&sb->s_instances);
     spin_unlock(&sb_lock);
     up_write(&sb->s_umount);
     if (sb->s_bdi != &noop_backing_dev_info) {
         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
  *
  * Find or create a superblock using the parameters stored in the filesystem
  * context and the two callback functions.
  *
  * If an extant superblock is matched, then that will be returned with an
  * elevated reference count that the caller must transfer or discard.
  *
  * If no match is made, a new superblock will be allocated and basic
  * initialisation will be performed (s_type, s_fs_info and s_id will be set and
  * the set() callback will be invoked), the superblock will be published and it
  * will be returned in a partially constructed state with SB_BORN and SB_ACTIVE
  * as yet unset.
  */
 struct super_block *sget_fc(struct fs_context *fc,
                 int (*test)(struct super_block *, struct fs_context *),
                 int (*set)(struct super_block *, struct fs_context *))
 {
     struct super_block *s = NULL;
     struct super_block *old;
     struct user_namespace *user_ns = fc->global ? &init_user_ns : fc->user_ns;
     int err;
 
 retry:
     spin_lock(&sb_lock);
     if (test) {
         hlist_for_each_entry(old, &fc->fs_type->fs_supers, s_instances) {
             if (test(old, fc))
                 goto share_extant_sb;
         }
     }
     if (!s) {
         spin_unlock(&sb_lock);
         s = alloc_super(fc->fs_type, fc->sb_flags, user_ns);
         if (!s)
             return ERR_PTR(-ENOMEM);
         goto retry;
     }
 
     s->s_fs_info = fc->s_fs_info;
     err = set(s, fc);
     if (err) {
         s->s_fs_info = NULL;
         spin_unlock(&sb_lock);
         destroy_unused_super(s);
         return ERR_PTR(err);
     }
     fc->s_fs_info = NULL;
     s->s_type = fc->fs_type;
     s->s_iflags |= fc->s_iflags;
     strlcpy(s->s_id, s->s_type->name, sizeof(s->s_id));
     list_add_tail(&s->s_list, &super_blocks);
     hlist_add_head(&s->s_instances, &s->s_type->fs_supers);
     spin_unlock(&sb_lock);
     get_filesystem(s->s_type);
     register_shrinker_prepared(&s->s_shrink);
     return s;
 
 share_extant_sb:
     if (user_ns != old->s_user_ns) {
         spin_unlock(&sb_lock);
         destroy_unused_super(s);
         return ERR_PTR(-EBUSY);
     }
     if (!grab_super(old))
         goto retry;
     destroy_unused_super(s);
     return old;
 }
 EXPORT_SYMBOL(sget_fc);
 
 /**
  *  sget    -   find or create a superblock
  *  @type:    filesystem type superblock should belong to
  *  @test:    comparison callback
  *  @set:     setup callback
  *  @flags:   mount flags
  *  @data:    argument to each of them
  */
 struct super_block *sget(struct file_system_type *type,
             int (*test)(struct super_block *,void *),
             int (*set)(struct super_block *,void *),
             int flags,
             void *data)
 {
     struct user_namespace *user_ns = current_user_ns();
     struct super_block *s = NULL;
     struct super_block *old;
     int err;
 
     /* We don't yet pass the user namespace of the parent
      * mount through to here so always use &init_user_ns
      * until that changes.
      */
     if (flags & SB_SUBMOUNT)
         user_ns = &init_user_ns;
 
 retry:
     spin_lock(&sb_lock);
     if (test) {
         hlist_for_each_entry(old, &type->fs_supers, s_instances) {
             if (!test(old, data))
                 continue;
             if (user_ns != old->s_user_ns) {
                 spin_unlock(&sb_lock);
                 destroy_unused_super(s);
                 return ERR_PTR(-EBUSY);
             }
             if (!grab_super(old))
                 goto retry;
             destroy_unused_super(s);
             return old;
         }
     }
     if (!s) {
         spin_unlock(&sb_lock);
         s = alloc_super(type, (flags & ~SB_SUBMOUNT), user_ns);
         if (!s)
             return ERR_PTR(-ENOMEM);
         goto retry;
     }
 
     err = set(s, data);
     if (err) {
         spin_unlock(&sb_lock);
         destroy_unused_super(s);
         return ERR_PTR(err);
     }
     s->s_type = type;
     strlcpy(s->s_id, type->name, sizeof(s->s_id));
     list_add_tail(&s->s_list, &super_blocks);
     hlist_add_head(&s->s_instances, &type->fs_supers);
     spin_unlock(&sb_lock);
     get_filesystem(type);
     register_shrinker_prepared(&s->s_shrink);
     return s;
 }
 EXPORT_SYMBOL(sget);
 
 void drop_super(struct super_block *sb)
 {
     up_read(&sb->s_umount);
     put_super(sb);
 }
 
 EXPORT_SYMBOL(drop_super);
 
 void drop_super_exclusive(struct super_block *sb)
 {
     up_write(&sb->s_umount);
     put_super(sb);
 }
 EXPORT_SYMBOL(drop_super_exclusive);
 
 static void __iterate_supers(void (*f)(struct super_block *))
 {
     struct super_block *sb, *p = NULL;
 
     spin_lock(&sb_lock);
     list_for_each_entry(sb, &super_blocks, s_list) {
         if (hlist_unhashed(&sb->s_instances))
             continue;
         sb->s_count++;
         spin_unlock(&sb_lock);
 
         f(sb);
 
         spin_lock(&sb_lock);
         if (p)
             __put_super(p);
         p = sb;
     }
     if (p)
         __put_super(p);
     spin_unlock(&sb_lock);
 }
 /**
  *  iterate_supers - call function for all active superblocks
  *  @f: function to call
  *  @arg: argument to pass to it
  *
  *  Scans the superblock list and calls given function, passing it
  *  locked superblock and given argument.
  */
 void iterate_supers(void (*f)(struct super_block *, void *), void *arg)
 {
     struct super_block *sb, *p = NULL;
 
     spin_lock(&sb_lock);
     list_for_each_entry(sb, &super_blocks, s_list) {
         if (hlist_unhashed(&sb->s_instances))
             continue;
         sb->s_count++;
         spin_unlock(&sb_lock);
 
         down_read(&sb->s_umount);
         if (sb->s_root && (sb->s_flags & SB_BORN))
             f(sb, arg);
         up_read(&sb->s_umount);
 
         spin_lock(&sb_lock);
         if (p)
             __put_super(p);
         p = sb;
     }
     if (p)
         __put_super(p);
     spin_unlock(&sb_lock);
 }
 
 /**
  *  iterate_supers_type - call function for superblocks of given type
  *  @type: fs type
  *  @f: function to call
  *  @arg: argument to pass to it
  *
  *  Scans the superblock list and calls given function, passing it
  *  locked superblock and given argument.
  */
 void iterate_supers_type(struct file_system_type *type,
     void (*f)(struct super_block *, void *), void *arg)
 {
     struct super_block *sb, *p = NULL;
 
     spin_lock(&sb_lock);
     hlist_for_each_entry(sb, &type->fs_supers, s_instances) {
         sb->s_count++;
         spin_unlock(&sb_lock);
 
         down_read(&sb->s_umount);
         if (sb->s_root && (sb->s_flags & SB_BORN))
             f(sb, arg);
         up_read(&sb->s_umount);
 
         spin_lock(&sb_lock);
         if (p)
             __put_super(p);
         p = sb;
     }
     if (p)
         __put_super(p);
     spin_unlock(&sb_lock);
 }
 
 EXPORT_SYMBOL(iterate_supers_type);
 
 /**
  * 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 & SB_BORN))
                 return sb;
             up_read(&sb->s_umount);
             /* nope, got unmounted */
             spin_lock(&sb_lock);
             __put_super(sb);
             goto rescan;
         }
     }
     spin_unlock(&sb_lock);
     return NULL;
 }
 
 /**
  * get_active_super - get an active reference to the superblock of a device
  * @bdev: device to get the superblock for
  *
  * Scans the superblock list and finds the superblock of the file system
  * mounted on the device given.  Returns the superblock with an active
  * reference or %NULL if none was found.
  */
 struct super_block *get_active_super(struct block_device *bdev)
 {
     struct super_block *sb;
 
     if (!bdev)
         return NULL;
 
 restart:
     spin_lock(&sb_lock);
     list_for_each_entry(sb, &super_blocks, s_list) {
         if (hlist_unhashed(&sb->s_instances))
             continue;
         if (sb->s_bdev == bdev) {
             if (!grab_super(sb))
                 goto restart;
             up_write(&sb->s_umount);
             return sb;
         }
     }
     spin_unlock(&sb_lock);
     return NULL;
 }
 
 struct super_block *user_get_super(dev_t dev, bool excl)
 {
     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);
             if (excl)
                 down_write(&sb->s_umount);
             else
                 down_read(&sb->s_umount);
             /* still alive? */
             if (sb->s_root && (sb->s_flags & SB_BORN))
                 return sb;
             if (excl)
                 up_write(&sb->s_umount);
             else
                 up_read(&sb->s_umount);
             /* nope, got unmounted */
             spin_lock(&sb_lock);
             __put_super(sb);
             goto rescan;
         }
     }
     spin_unlock(&sb_lock);
     return NULL;
 }
 
 /**
  * reconfigure_super - asks filesystem to change superblock parameters
  * @fc: The superblock and configuration
  *
  * Alters the configuration parameters of a live superblock.
  */
 int reconfigure_super(struct fs_context *fc)
 {
     struct super_block *sb = fc->root->d_sb;
     int retval;
     bool remount_ro = false;
     bool force = fc->sb_flags & SB_FORCE;
 
     if (fc->sb_flags_mask & ~MS_RMT_MASK)
         return -EINVAL;
     if (sb->s_writers.frozen != SB_UNFROZEN)
         return -EBUSY;
 
     retval = security_sb_remount(sb, fc->security);
     if (retval)
         return retval;
 
     if (fc->sb_flags_mask & SB_RDONLY) {
 #ifdef CONFIG_BLOCK
         if (!(fc->sb_flags & SB_RDONLY) && sb->s_bdev &&
             bdev_read_only(sb->s_bdev))
             return -EACCES;
 #endif
 
         remount_ro = (fc->sb_flags & SB_RDONLY) && !sb_rdonly(sb);
     }
 
     if (remount_ro) {
         if (!hlist_empty(&sb->s_pins)) {
             up_write(&sb->s_umount);
             group_pin_kill(&sb->s_pins);
             down_write(&sb->s_umount);
             if (!sb->s_root)
                 return 0;
             if (sb->s_writers.frozen != SB_UNFROZEN)
                 return -EBUSY;
             remount_ro = !sb_rdonly(sb);
         }
     }
     shrink_dcache_sb(sb);
 
     /* If we are reconfiguring to RDONLY and current sb is read/write,
      * make sure there are no files open for writing.
      */
     if (remount_ro) {
         if (force) {
             sb->s_readonly_remount = 1;
             smp_wmb();
         } else {
             retval = sb_prepare_remount_readonly(sb);
             if (retval)
                 return retval;
         }
     }
 
     if (fc->ops->reconfigure) {
         retval = fc->ops->reconfigure(fc);
         if (retval) {
             if (!force)
                 goto cancel_readonly;
             /* If forced remount, go ahead despite any errors */
             WARN(1, "forced remount of a %s fs returned %i\n",
                  sb->s_type->name, retval);
         }
     }
 
     WRITE_ONCE(sb->s_flags, ((sb->s_flags & ~fc->sb_flags_mask) |
                  (fc->sb_flags & fc->sb_flags_mask)));
     /* Needs to be ordered wrt mnt_is_readonly() */
     smp_wmb();
     sb->s_readonly_remount = 0;
 
     /*
      * Some filesystems modify their metadata via some other path than the
      * bdev buffer cache (eg. use a private mapping, or directories in
      * pagecache, etc). Also file data modifications go via their own
      * mappings. So If we try to mount readonly then copy the filesystem
      * from bdev, we could get stale data, so invalidate it to give a best
      * effort at coherency.
      */
     if (remount_ro && sb->s_bdev)
         invalidate_bdev(sb->s_bdev);
     return 0;
 
 cancel_readonly:
     sb->s_readonly_remount = 0;
     return retval;
 }
 
 static void do_emergency_remount_callback(struct super_block *sb)
 {
     down_write(&sb->s_umount);
     if (sb->s_root && sb->s_bdev && (sb->s_flags & SB_BORN) &&
         !sb_rdonly(sb)) {
         struct fs_context *fc;
 
         fc = fs_context_for_reconfigure(sb->s_root,
                     SB_RDONLY | SB_FORCE, SB_RDONLY);
         if (!IS_ERR(fc)) {
             if (parse_monolithic_mount_data(fc, NULL) == 0)
                 (void)reconfigure_super(fc);
             put_fs_context(fc);
         }
     }
     up_write(&sb->s_umount);
 }
 
 static void do_emergency_remount(struct work_struct *work)
 {
     __iterate_supers(do_emergency_remount_callback);
     kfree(work);
     printk("Emergency Remount complete\n");
 }
 
 void emergency_remount(void)
 {
     struct work_struct *work;
 
     work = kmalloc(sizeof(*work), GFP_ATOMIC);
     if (work) {
         INIT_WORK(work, do_emergency_remount);
         schedule_work(work);
     }
 }
 
 static void do_thaw_all_callback(struct super_block *sb)
 {
     down_write(&sb->s_umount);
     if (sb->s_root && sb->s_flags & SB_BORN) {
         emergency_thaw_bdev(sb);
         thaw_super_locked(sb);
     } else {
         up_write(&sb->s_umount);
     }
 }
 
 static void do_thaw_all(struct work_struct *work)
 {
     __iterate_supers(do_thaw_all_callback);
     kfree(work);
     printk(KERN_WARNING "Emergency Thaw complete\n");
 }
 
 /**
  * emergency_thaw_all -- forcibly thaw every frozen filesystem
  *
  * Used for emergency unfreeze of all filesystems via SysRq
  */
 void emergency_thaw_all(void)
 {
     struct work_struct *work;
 
     work = kmalloc(sizeof(*work), GFP_ATOMIC);
     if (work) {
         INIT_WORK(work, do_thaw_all);
         schedule_work(work);
     }
 }
 
 static DEFINE_IDA(unnamed_dev_ida);
 
 /**
  * get_anon_bdev - Allocate a block device for filesystems which don't have one.
  * @p: Pointer to a dev_t.
  *
  * Filesystems which don't use real block devices can call this function
  * to allocate a virtual block device.
  *
  * Context: Any context.  Frequently called while holding sb_lock.
  * Return: 0 on success, -EMFILE if there are no anonymous bdevs left
  * or -ENOMEM if memory allocation failed.
  */
 int get_anon_bdev(dev_t *p)
 {
     int dev;
 
     /*
      * Many userspace utilities consider an FSID of 0 invalid.
      * Always return at least 1 from get_anon_bdev.
      */
     dev = ida_alloc_range(&unnamed_dev_ida, 1, (1 << MINORBITS) - 1,
             GFP_ATOMIC);
     if (dev == -ENOSPC)
         dev = -EMFILE;
     if (dev < 0)
         return dev;
 
     *p = MKDEV(0, dev);
     return 0;
 }
 EXPORT_SYMBOL(get_anon_bdev);
 
 void free_anon_bdev(dev_t dev)
 {
     ida_free(&unnamed_dev_ida, MINOR(dev));
 }
 EXPORT_SYMBOL(free_anon_bdev);
 
 int set_anon_super(struct super_block *s, void *data)
 {
     return get_anon_bdev(&s->s_dev);
 }
 EXPORT_SYMBOL(set_anon_super);
 
 void kill_anon_super(struct super_block *sb)
 {
     dev_t dev = sb->s_dev;
     generic_shutdown_super(sb);
     free_anon_bdev(dev);
 }
 EXPORT_SYMBOL(kill_anon_super);
 
 void kill_litter_super(struct super_block *sb)
 {
     if (sb->s_root)
         d_genocide(sb->s_root);
     kill_anon_super(sb);
 }
 EXPORT_SYMBOL(kill_litter_super);
 
 int set_anon_super_fc(struct super_block *sb, struct fs_context *fc)
 {
     return set_anon_super(sb, NULL);
 }
 EXPORT_SYMBOL(set_anon_super_fc);
 
 static int test_keyed_super(struct super_block *sb, struct fs_context *fc)
 {
     return sb->s_fs_info == fc->s_fs_info;
 }
 
 static int test_single_super(struct super_block *s, struct fs_context *fc)
 {
     return 1;
 }
 
 /**
  * vfs_get_super - Get a superblock with a search key set in s_fs_info.
  * @fc: The filesystem context holding the parameters
  * @keying: How to distinguish superblocks
  * @fill_super: Helper to initialise a new superblock
  *
  * Search for a superblock and create a new one if not found.  The search
  * criterion is controlled by @keying.  If the search fails, a new superblock
  * is created and @fill_super() is called to initialise it.
  *
  * @keying can take one of a number of values:
  *
  * (1) vfs_get_single_super - Only one superblock of this type may exist on the
  *     system.  This is typically used for special system filesystems.
  *
  * (2) vfs_get_keyed_super - Multiple superblocks may exist, but they must have
  *     distinct keys (where the key is in s_fs_info).  Searching for the same
  *     key again will turn up the superblock for that key.
  *
  * (3) vfs_get_independent_super - Multiple superblocks may exist and are
  *     unkeyed.  Each call will get a new superblock.
  *
  * A permissions check is made by sget_fc() unless we're getting a superblock
  * for a kernel-internal mount or a submount.
  */
 int vfs_get_super(struct fs_context *fc,
           enum vfs_get_super_keying keying,
           int (*fill_super)(struct super_block *sb,
                     struct fs_context *fc))
 {
     int (*test)(struct super_block *, struct fs_context *);
     struct super_block *sb;
     int err;
 
     switch (keying) {
     case vfs_get_single_super:
     case vfs_get_single_reconf_super:
         test = test_single_super;
         break;
     case vfs_get_keyed_super:
         test = test_keyed_super;
         break;
     case vfs_get_independent_super:
         test = NULL;
         break;
     default:
         BUG();
     }
 
     sb = sget_fc(fc, test, set_anon_super_fc);
     if (IS_ERR(sb))
         return PTR_ERR(sb);
 
     if (!sb->s_root) {
         err = fill_super(sb, fc);
         if (err)
             goto error;
 
         sb->s_flags |= SB_ACTIVE;
         fc->root = dget(sb->s_root);
     } else {
         fc->root = dget(sb->s_root);
         if (keying == vfs_get_single_reconf_super) {
             err = reconfigure_super(fc);
             if (err < 0) {
                 dput(fc->root);
                 fc->root = NULL;
                 goto error;
             }
         }
     }
 
     return 0;
 
 error:
     deactivate_locked_super(sb);
     return err;
 }
 EXPORT_SYMBOL(vfs_get_super);
 
 int get_tree_nodev(struct fs_context *fc,
           int (*fill_super)(struct super_block *sb,
                     struct fs_context *fc))
 {
     return vfs_get_super(fc, vfs_get_independent_super, fill_super);
 }
 EXPORT_SYMBOL(get_tree_nodev);
 
 int get_tree_single(struct fs_context *fc,
           int (*fill_super)(struct super_block *sb,
                     struct fs_context *fc))
 {
     return vfs_get_super(fc, vfs_get_single_super, fill_super);
 }
 EXPORT_SYMBOL(get_tree_single);
 
 int get_tree_single_reconf(struct fs_context *fc,
           int (*fill_super)(struct super_block *sb,
                     struct fs_context *fc))
 {
     return vfs_get_super(fc, vfs_get_single_reconf_super, fill_super);
 }
 EXPORT_SYMBOL(get_tree_single_reconf);
 
 int get_tree_keyed(struct fs_context *fc,
           int (*fill_super)(struct super_block *sb,
                     struct fs_context *fc),
         void *key)
 {
     fc->s_fs_info = key;
     return vfs_get_super(fc, vfs_get_keyed_super, fill_super);
 }
 EXPORT_SYMBOL(get_tree_keyed);
 
 #ifdef CONFIG_BLOCK
 
 static int set_bdev_super(struct super_block *s, void *data)
 {
     s->s_bdev = data;
     s->s_dev = s->s_bdev->bd_dev;
     s->s_bdi = bdi_get(s->s_bdev->bd_disk->bdi);
 
     if (bdev_stable_writes(s->s_bdev))
         s->s_iflags |= SB_I_STABLE_WRITES;
     return 0;
 }
 
 static int set_bdev_super_fc(struct super_block *s, struct fs_context *fc)
 {
     return set_bdev_super(s, fc->sget_key);
 }
 
 static int test_bdev_super_fc(struct super_block *s, struct fs_context *fc)
 {
     return !(s->s_iflags & SB_I_RETIRED) && s->s_bdev == fc->sget_key;
 }
 
 /**
  * get_tree_bdev - Get a superblock based on a single block device
  * @fc: The filesystem context holding the parameters
  * @fill_super: Helper to initialise a new superblock
  */
 int get_tree_bdev(struct fs_context *fc,
         int (*fill_super)(struct super_block *,
                   struct fs_context *))
 {
     struct block_device *bdev;
     struct super_block *s;
     fmode_t mode = FMODE_READ | FMODE_EXCL;
     int error = 0;
 
     if (!(fc->sb_flags & SB_RDONLY))
         mode |= FMODE_WRITE;
 
     if (!fc->source)
         return invalf(fc, "No source specified");
 
     bdev = blkdev_get_by_path(fc->source, mode, fc->fs_type);
     if (IS_ERR(bdev)) {
         errorf(fc, "%s: Can't open blockdev", fc->source);
         return PTR_ERR(bdev);
     }
 
     /* Once the superblock is inserted into the list by sget_fc(), s_umount
      * will protect the lockfs code from trying to start a snapshot while
      * we are mounting
      */
     mutex_lock(&bdev->bd_fsfreeze_mutex);
     if (bdev->bd_fsfreeze_count > 0) {
         mutex_unlock(&bdev->bd_fsfreeze_mutex);
         warnf(fc, "%pg: Can't mount, blockdev is frozen", bdev);
         blkdev_put(bdev, mode);
         return -EBUSY;
     }
 
     fc->sb_flags |= SB_NOSEC;
     fc->sget_key = bdev;
     s = sget_fc(fc, test_bdev_super_fc, set_bdev_super_fc);
     mutex_unlock(&bdev->bd_fsfreeze_mutex);
     if (IS_ERR(s)) {
         blkdev_put(bdev, mode);
         return PTR_ERR(s);
     }
 
     if (s->s_root) {
         /* Don't summarily change the RO/RW state. */
         if ((fc->sb_flags ^ s->s_flags) & SB_RDONLY) {
             warnf(fc, "%pg: Can't mount, would change RO state", bdev);
             deactivate_locked_super(s);
             blkdev_put(bdev, mode);
             return -EBUSY;
         }
 
         /*
          * s_umount nests inside open_mutex during
          * __invalidate_device().  blkdev_put() acquires
          * open_mutex and can't be called under s_umount.  Drop
          * s_umount temporarily.  This is safe as we're
          * holding an active reference.
          */
         up_write(&s->s_umount);
         blkdev_put(bdev, mode);
         down_write(&s->s_umount);
     } else {
         s->s_mode = mode;
         snprintf(s->s_id, sizeof(s->s_id), "%pg", bdev);
         shrinker_debugfs_rename(&s->s_shrink, "sb-%s:%s",
                     fc->fs_type->name, s->s_id);
         sb_set_blocksize(s, block_size(bdev));
         error = fill_super(s, fc);
         if (error) {
             deactivate_locked_super(s);
             return error;
         }
 
         s->s_flags |= SB_ACTIVE;
         bdev->bd_super = s;
     }
 
     BUG_ON(fc->root);
     fc->root = dget(s->s_root);
     return 0;
 }
 EXPORT_SYMBOL(get_tree_bdev);
 
 static int test_bdev_super(struct super_block *s, void *data)
 {
     return !(s->s_iflags & SB_I_RETIRED) && (void *)s->s_bdev == data;
 }
 
 struct dentry *mount_bdev(struct file_system_type *fs_type,
     int flags, const char *dev_name, void *data,
     int (*fill_super)(struct super_block *, void *, int))
 {
     struct block_device *bdev;
     struct super_block *s;
     fmode_t mode = FMODE_READ | FMODE_EXCL;
     int error = 0;
 
     if (!(flags & SB_RDONLY))
         mode |= FMODE_WRITE;
 
     bdev = blkdev_get_by_path(dev_name, mode, fs_type);
     if (IS_ERR(bdev))
         return ERR_CAST(bdev);
 
     /*
      * once the super is inserted into the list by sget, s_umount
      * will protect the lockfs code from trying to start a snapshot
      * while we are mounting
      */
     mutex_lock(&bdev->bd_fsfreeze_mutex);
     if (bdev->bd_fsfreeze_count > 0) {
         mutex_unlock(&bdev->bd_fsfreeze_mutex);
         error = -EBUSY;
         goto error_bdev;
     }
     s = sget(fs_type, test_bdev_super, set_bdev_super, flags | SB_NOSEC,
          bdev);
     mutex_unlock(&bdev->bd_fsfreeze_mutex);
     if (IS_ERR(s))
         goto error_s;
 
     if (s->s_root) {
         if ((flags ^ s->s_flags) & SB_RDONLY) {
             deactivate_locked_super(s);
             error = -EBUSY;
             goto error_bdev;
         }
 
         /*
          * s_umount nests inside open_mutex during
          * __invalidate_device().  blkdev_put() acquires
          * open_mutex and can't be called under s_umount.  Drop
          * s_umount temporarily.  This is safe as we're
          * holding an active reference.
          */
         up_write(&s->s_umount);
         blkdev_put(bdev, mode);
         down_write(&s->s_umount);
     } else {
         s->s_mode = mode;
         snprintf(s->s_id, sizeof(s->s_id), "%pg", bdev);
         shrinker_debugfs_rename(&s->s_shrink, "sb-%s:%s",
                     fs_type->name, s->s_id);
         sb_set_blocksize(s, block_size(bdev));
         error = fill_super(s, data, flags & SB_SILENT ? 1 : 0);
         if (error) {
             deactivate_locked_super(s);
             goto error;
         }
 
         s->s_flags |= SB_ACTIVE;
         bdev->bd_super = s;
     }
 
     return dget(s->s_root);
 
 error_s:
     error = PTR_ERR(s);
 error_bdev:
     blkdev_put(bdev, mode);
 error:
     return ERR_PTR(error);
 }
 EXPORT_SYMBOL(mount_bdev);
 
 void kill_block_super(struct super_block *sb)
 {
     struct block_device *bdev = sb->s_bdev;
     fmode_t mode = sb->s_mode;
 
     bdev->bd_super = NULL;
     generic_shutdown_super(sb);
     sync_blockdev(bdev);
     WARN_ON_ONCE(!(mode & FMODE_EXCL));
     blkdev_put(bdev, mode | FMODE_EXCL);
 }
 
 EXPORT_SYMBOL(kill_block_super);
 #endif
 
 struct dentry *mount_nodev(struct file_system_type *fs_type,
     int flags, void *data,
     int (*fill_super)(struct super_block *, void *, int))
 {
     int error;
     struct super_block *s = sget(fs_type, NULL, set_anon_super, flags, NULL);
 
     if (IS_ERR(s))
         return ERR_CAST(s);
 
     error = fill_super(s, data, flags & SB_SILENT ? 1 : 0);
     if (error) {
         deactivate_locked_super(s);
         return ERR_PTR(error);
     }
     s->s_flags |= SB_ACTIVE;
     return dget(s->s_root);
 }
 EXPORT_SYMBOL(mount_nodev);
 
 int reconfigure_single(struct super_block *s,
                int flags, void *data)
 {
     struct fs_context *fc;
     int ret;
 
     /* The caller really need to be passing fc down into mount_single(),
      * then a chunk of this can be removed.  [Bollocks -- AV]
      * Better yet, reconfiguration shouldn't happen, but rather the second
      * mount should be rejected if the parameters are not compatible.
      */
     fc = fs_context_for_reconfigure(s->s_root, flags, MS_RMT_MASK);
     if (IS_ERR(fc))
         return PTR_ERR(fc);
 
     ret = parse_monolithic_mount_data(fc, data);
     if (ret < 0)
         goto out;
 
     ret = reconfigure_super(fc);
 out:
     put_fs_context(fc);
     return ret;
 }
 
 static int compare_single(struct super_block *s, void *p)
 {
     return 1;
 }
 
 struct dentry *mount_single(struct file_system_type *fs_type,
     int flags, void *data,
     int (*fill_super)(struct super_block *, void *, int))
 {
     struct super_block *s;
     int error;
 
     s = sget(fs_type, compare_single, set_anon_super, flags, NULL);
     if (IS_ERR(s))
         return ERR_CAST(s);
     if (!s->s_root) {
         error = fill_super(s, data, flags & SB_SILENT ? 1 : 0);
         if (!error)
             s->s_flags |= SB_ACTIVE;
     } else {
         error = reconfigure_single(s, flags, data);
     }
     if (unlikely(error)) {
         deactivate_locked_super(s);
         return ERR_PTR(error);
     }
     return dget(s->s_root);
 }
 EXPORT_SYMBOL(mount_single);
 
 /**
  * vfs_get_tree - Get the mountable root
  * @fc: The superblock configuration context.
  *
  * The filesystem is invoked to get or create a superblock which can then later
  * be used for mounting.  The filesystem places a pointer to the root to be
  * used for mounting in @fc->root.
  */
 int vfs_get_tree(struct fs_context *fc)
 {
     struct super_block *sb;
     int error;
 
     if (fc->root)
         return -EBUSY;
 
     /* Get the mountable root in fc->root, with a ref on the root and a ref
      * on the superblock.
      */
     error = fc->ops->get_tree(fc);
     if (error < 0)
         return error;
 
     if (!fc->root) {
         pr_err("Filesystem %s get_tree() didn't set fc->root\n",
                fc->fs_type->name);
         /* We don't know what the locking state of the superblock is -
          * if there is a superblock.
          */
         BUG();
     }
 
     sb = fc->root->d_sb;
     WARN_ON(!sb->s_bdi);
 
     /*
      * Write barrier is for super_cache_count(). We place it before setting
      * SB_BORN as the data dependency between the two functions is the
      * superblock structure contents that we just set up, not the SB_BORN
      * flag.
      */
     smp_wmb();
     sb->s_flags |= SB_BORN;
 
     error = security_sb_set_mnt_opts(sb, fc->security, 0, NULL);
     if (unlikely(error)) {
         fc_drop_locked(fc);
         return error;
     }
 
     /*
      * filesystems should never set s_maxbytes larger than MAX_LFS_FILESIZE
      * but s_maxbytes was an unsigned long long for many releases. Throw
      * this warning for a little while to try and catch filesystems that
      * violate this rule.
      */
     WARN((sb->s_maxbytes < 0), "%s set sb->s_maxbytes to "
         "negative value (%lld)\n", fc->fs_type->name, sb->s_maxbytes);
 
     return 0;
 }
 EXPORT_SYMBOL(vfs_get_tree);
 
 /*
  * Setup private BDI for given superblock. It gets automatically cleaned up
  * in generic_shutdown_super().
  */
 int super_setup_bdi_name(struct super_block *sb, char *fmt, ...)
 {
     struct backing_dev_info *bdi;
     int err;
     va_list args;
 
     bdi = bdi_alloc(NUMA_NO_NODE);
     if (!bdi)
         return -ENOMEM;
 
     va_start(args, fmt);
     err = bdi_register_va(bdi, fmt, args);
     va_end(args);
     if (err) {
         bdi_put(bdi);
         return err;
     }
     WARN_ON(sb->s_bdi != &noop_backing_dev_info);
     sb->s_bdi = bdi;
     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, 0, _THIS_IP_);
 }
 
 /*
  * Tell lockdep we are holding these locks before we call ->unfreeze_fs(sb).
  */
 static void lockdep_sb_freeze_acquire(struct super_block *sb)
 {
     int level;
 
     for (level = 0; level < SB_FREEZE_LEVELS; ++level)
         percpu_rwsem_acquire(sb->s_writers.rw_sem + level, 0, _THIS_IP_);
 }
 
 static void sb_freeze_unlock(struct super_block *sb, int level)
 {
     for (level--; level >= 0; level--)
         percpu_up_write(sb->s_writers.rw_sem + level);
 }
 
 /**
  * freeze_super - lock the filesystem and force it into a consistent state
  * @sb: the super to lock
  *
  * Syncs the super to make sure the filesystem is consistent and calls the fs's
  * freeze_fs.  Subsequent calls to this without first thawing the fs will return
  * -EBUSY.
  *
  * During this function, sb->s_writers.frozen goes through these values:
  *
  * SB_UNFROZEN: File system is normal, all writes progress as usual.
  *
  * SB_FREEZE_WRITE: The file system is in the process of being frozen.  New
  * writes should be blocked, though page faults are still allowed. We wait for
  * all writes to complete and then proceed to the next stage.
  *
  * SB_FREEZE_PAGEFAULT: Freezing continues. Now also page faults are blocked
  * but internal fs threads can still modify the filesystem (although they
  * should not dirty new pages or inodes), writeback can run etc. After waiting
  * for all running page faults we sync the filesystem which will clean all
  * dirty pages and inodes (no new dirty pages or inodes can be created when
  * sync is running).
  *
  * SB_FREEZE_FS: The file system is frozen. Now all internal sources of fs
  * modification are blocked (e.g. XFS preallocation truncation on inode
  * reclaim). This is usually implemented by blocking new transactions for
  * filesystems that have them and need this additional guard. After all
  * internal writers are finished we call ->freeze_fs() to finish filesystem
  * freezing. Then we transition to SB_FREEZE_COMPLETE state. This state is
  * mostly auxiliary for filesystems to verify they do not modify frozen fs.
  *
  * sb->s_writers.frozen is protected by sb->s_umount.
  */
 int freeze_super(struct super_block *sb)
 {
     int ret;
 
     atomic_inc(&sb->s_active);
     down_write(&sb->s_umount);
     if (sb->s_writers.frozen != SB_UNFROZEN) {
         deactivate_locked_super(sb);
         return -EBUSY;
     }
 
     if (!(sb->s_flags & SB_BORN)) {
         up_write(&sb->s_umount);
         return 0;   /* sic - it's "nothing to do" */
     }
 
     if (sb_rdonly(sb)) {
         /* Nothing to do really... */
         sb->s_writers.frozen = SB_FREEZE_COMPLETE;
         up_write(&sb->s_umount);
         return 0;
     }
 
     sb->s_writers.frozen = SB_FREEZE_WRITE;
     /* Release s_umount to preserve sb_start_write -> s_umount ordering */
     up_write(&sb->s_umount);
     sb_wait_write(sb, SB_FREEZE_WRITE);
     down_write(&sb->s_umount);
 
     /* Now we go and block page faults... */
     sb->s_writers.frozen = SB_FREEZE_PAGEFAULT;
     sb_wait_write(sb, SB_FREEZE_PAGEFAULT);
 
     /* All writers are done so after syncing there won't be dirty data */
     ret = sync_filesystem(sb);
     if (ret) {
         sb->s_writers.frozen = SB_UNFROZEN;
         sb_freeze_unlock(sb, SB_FREEZE_PAGEFAULT);
         wake_up(&sb->s_writers.wait_unfrozen);
         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(&sb->s_writers.wait_unfrozen);
             deactivate_locked_super(sb);
             return ret;
         }
     }
     /*
      * For debugging purposes so that fs can warn if it sees write activity
      * when frozen is set to SB_FREEZE_COMPLETE, and for thaw_super().
      */
     sb->s_writers.frozen = SB_FREEZE_COMPLETE;
     lockdep_sb_freeze_release(sb);
     up_write(&sb->s_umount);
     return 0;
 }
 EXPORT_SYMBOL(freeze_super);
 
 static int thaw_super_locked(struct super_block *sb)
 {
     int error;
 
     if (sb->s_writers.frozen != SB_FREEZE_COMPLETE) {
         up_write(&sb->s_umount);
         return -EINVAL;
     }
 
     if (sb_rdonly(sb)) {
         sb->s_writers.frozen = SB_UNFROZEN;
         goto out;
     }
 
     lockdep_sb_freeze_acquire(sb);
 
     if (sb->s_op->unfreeze_fs) {
         error = sb->s_op->unfreeze_fs(sb);
         if (error) {
             printk(KERN_ERR
                 "VFS:Filesystem thaw failed\n");
             lockdep_sb_freeze_release(sb);
             up_write(&sb->s_umount);
             return error;
         }
     }
 
     sb->s_writers.frozen = SB_UNFROZEN;
     sb_freeze_unlock(sb, SB_FREEZE_FS);
 out:
     wake_up(&sb->s_writers.wait_unfrozen);
     deactivate_locked_super(sb);
     return 0;
 }
 
 /**
  * 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)
 {
     down_write(&sb->s_umount);
     return thaw_super_locked(sb);
 }
 EXPORT_SYMBOL(thaw_super);

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