Merge tag 'vfs-6.8.super' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs

Pull vfs super updates from Christian Brauner:
 "This contains the super work for this cycle including the long-awaited
  series by Jan to make it possible to prevent writing to mounted block
  devices:

   - Writing to mounted devices is dangerous and can lead to filesystem
     corruption as well as crashes. Furthermore syzbot comes with more
     and more involved examples how to corrupt block device under a
     mounted filesystem leading to kernel crashes and reports we can do
     nothing about. Add tracking of writers to each block device and a
     kernel cmdline argument which controls whether other writeable
     opens to block devices open with BLK_OPEN_RESTRICT_WRITES flag are
     allowed.

     Note that this effectively only prevents modification of the
     particular block device's page cache by other writers. The actual
     device content can still be modified by other means - e.g. by
     issuing direct scsi commands, by doing writes through devices lower
     in the storage stack (e.g. in case loop devices, DM, or MD are
     involved) etc. But blocking direct modifications of the block
     device page cache is enough to give filesystems a chance to perform
     data validation when loading data from the underlying storage and
     thus prevent kernel crashes.

     Syzbot can use this cmdline argument option to avoid uninteresting
     crashes. Also users whose userspace setup does not need writing to
     mounted block devices can set this option for hardening. We expect
     that this will be interesting to quite a few workloads.

     Btrfs is currently opted out of this because they still haven't
     merged patches we require for this to work from three kernel
     releases ago.

   - Reimplement block device freezing and thawing as holder operations
     on the block device.

     This allows us to extend block device freezing to all devices
     associated with a superblock and not just the main device. It also
     allows us to remove get_active_super() and thus another function
     that scans the global list of superblocks.

     Freezing via additional block devices only works if the filesystem
     chooses to use @fs_holder_ops for these additional devices as well.
     That currently only includes ext4 and xfs.

     Earlier releases switched get_tree_bdev() and mount_bdev() to use
     @fs_holder_ops. The remaining nilfs2 open-coded version of
     mount_bdev() has been converted to rely on @fs_holder_ops as well.
     So block device freezing for the main block device will continue to
     work as before.

     There should be no regressions in functionality. The only special
     case is btrfs where block device freezing for the main block device
     never worked because sb->s_bdev isn't set. Block device freezing
     for btrfs can be fixed once they can switch to @fs_holder_ops but
     that can happen whenever they're ready"

* tag 'vfs-6.8.super' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs: (27 commits)
  block: Fix a memory leak in bdev_open_by_dev()
  super: don't bother with WARN_ON_ONCE()
  super: massage wait event mechanism
  ext4: Block writes to journal device
  xfs: Block writes to log device
  fs: Block writes to mounted block devices
  btrfs: Do not restrict writes to btrfs devices
  block: Add config option to not allow writing to mounted devices
  block: Remove blkdev_get_by_*() functions
  bcachefs: Convert to bdev_open_by_path()
  fs: handle freezing from multiple devices
  fs: remove dead check
  nilfs2: simplify device handling
  fs: streamline thaw_super_locked
  ext4: simplify device handling
  xfs: simplify device handling
  fs: simplify setup_bdev_super() calls
  blkdev: comment fs_holder_ops
  porting: document block device freeze and thaw changes
  fs: remove unused helper
  ...

1 files changed, 275 insertions, 223 deletions

diff --git a/fs/super.c b/fs/super.c
index 6fe482371633..e35936000408 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -81,16 +81,13 @@ static inline void super_unlock_shared(struct super_block *sb)
 	super_unlock(sb, false);
 }
 
-static inline bool wait_born(struct super_block *sb)
+static bool super_flags(const struct super_block *sb, unsigned int flags)
 {
-	unsigned int flags;
-
 	/*
 	 * Pairs with smp_store_release() in super_wake() and ensures
-	 * that we see SB_BORN or SB_DYING after we're woken.
+	 * that we see @flags after we're woken.
 	 */
-	flags = smp_load_acquire(&sb->s_flags);
-	return flags & (SB_BORN | SB_DYING);
+	return smp_load_acquire(&sb->s_flags) & flags;
 }
 
 /**
@@ -105,15 +102,21 @@ static inline bool wait_born(struct super_block *sb)
  *
  * The caller must have acquired a temporary reference on @sb->s_count.
  *
- * Return: This returns true if SB_BORN was set, false if SB_DYING was
- *         set. The function acquires s_umount and returns with it held.
+ * Return: The function returns true if SB_BORN was set and with
+ *         s_umount held. The function returns false if SB_DYING was
+ *         set and without s_umount held.
  */
 static __must_check bool super_lock(struct super_block *sb, bool excl)
 {
-
 	lockdep_assert_not_held(&sb->s_umount);
 
-relock:
+	/* wait until the superblock is ready or dying */
+	wait_var_event(&sb->s_flags, super_flags(sb, SB_BORN | SB_DYING));
+
+	/* Don't pointlessly acquire s_umount. */
+	if (super_flags(sb, SB_DYING))
+		return false;
+
 	__super_lock(sb, excl);
 
 	/*
@@ -121,32 +124,22 @@ relock:
 	 * @sb->s_root is NULL and @sb->s_active is 0. No one needs to
 	 * grab a reference to this. Tell them so.
 	 */
-	if (sb->s_flags & SB_DYING)
+	if (sb->s_flags & SB_DYING) {
+		super_unlock(sb, excl);
 		return false;
+	}
 
-	/* Has called ->get_tree() successfully. */
-	if (sb->s_flags & SB_BORN)
-		return true;
-
-	super_unlock(sb, excl);
-
-	/* wait until the superblock is ready or dying */
-	wait_var_event(&sb->s_flags, wait_born(sb));
-
-	/*
-	 * Neither SB_BORN nor SB_DYING are ever unset so we never loop.
-	 * Just reacquire @sb->s_umount for the caller.
-	 */
-	goto relock;
+	WARN_ON_ONCE(!(sb->s_flags & SB_BORN));
+	return true;
 }
 
-/* wait and acquire read-side of @sb->s_umount */
+/* wait and try to acquire read-side of @sb->s_umount */
 static inline bool super_lock_shared(struct super_block *sb)
 {
 	return super_lock(sb, false);
 }
 
-/* wait and acquire write-side of @sb->s_umount */
+/* wait and try to acquire write-side of @sb->s_umount */
 static inline bool super_lock_excl(struct super_block *sb)
 {
 	return super_lock(sb, true);
@@ -521,48 +514,7 @@ void deactivate_super(struct super_block *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)
-{
-	bool born;
-
-	s->s_count++;
-	spin_unlock(&sb_lock);
-	born = super_lock_excl(s);
-	if (born && atomic_inc_not_zero(&s->s_active)) {
-		put_super(s);
-		return 1;
-	}
-	super_unlock_excl(s);
-	put_super(s);
-	return 0;
-}
-
-static inline bool wait_dead(struct super_block *sb)
-{
-	unsigned int flags;
-
-	/*
-	 * Pairs with memory barrier in super_wake() and ensures
-	 * that we see SB_DEAD after we're woken.
-	 */
-	flags = smp_load_acquire(&sb->s_flags);
-	return flags & SB_DEAD;
-}
-
-/**
- * grab_super_dead - acquire an active reference to a superblock
+ * grab_super - acquire an active reference to a superblock
  * @sb: superblock to acquire
  *
  * Acquire a temporary reference on a superblock and try to trade it for
@@ -573,17 +525,21 @@ static inline bool wait_dead(struct super_block *sb)
  * Return: This returns true if an active reference could be acquired,
  *         false if not.
  */
-static bool grab_super_dead(struct super_block *sb)
+static bool grab_super(struct super_block *sb)
 {
+	bool locked;
 
 	sb->s_count++;
-	if (grab_super(sb)) {
-		put_super(sb);
-		lockdep_assert_held(&sb->s_umount);
-		return true;
+	spin_unlock(&sb_lock);
+	locked = super_lock_excl(sb);
+	if (locked) {
+		if (atomic_inc_not_zero(&sb->s_active)) {
+			put_super(sb);
+			return true;
+		}
+		super_unlock_excl(sb);
 	}
-	wait_var_event(&sb->s_flags, wait_dead(sb));
-	lockdep_assert_not_held(&sb->s_umount);
+	wait_var_event(&sb->s_flags, super_flags(sb, SB_DEAD));
 	put_super(sb);
 	return false;
 }
@@ -834,7 +790,7 @@ share_extant_sb:
 			warnfc(fc, "reusing existing filesystem in another namespace not allowed");
 		return ERR_PTR(-EBUSY);
 	}
-	if (!grab_super_dead(old))
+	if (!grab_super(old))
 		goto retry;
 	destroy_unused_super(s);
 	return old;
@@ -878,7 +834,7 @@ retry:
 				destroy_unused_super(s);
 				return ERR_PTR(-EBUSY);
 			}
-			if (!grab_super_dead(old))
+			if (!grab_super(old))
 				goto retry;
 			destroy_unused_super(s);
 			return old;
@@ -930,8 +886,7 @@ static void __iterate_supers(void (*f)(struct super_block *))
 
 	spin_lock(&sb_lock);
 	list_for_each_entry(sb, &super_blocks, s_list) {
-		/* Pairs with memory marrier in super_wake(). */
-		if (smp_load_acquire(&sb->s_flags) & SB_DYING)
+		if (super_flags(sb, SB_DYING))
 			continue;
 		sb->s_count++;
 		spin_unlock(&sb_lock);
@@ -961,15 +916,17 @@ void iterate_supers(void (*f)(struct super_block *, void *), void *arg)
 
 	spin_lock(&sb_lock);
 	list_for_each_entry(sb, &super_blocks, s_list) {
-		bool born;
+		bool locked;
 
 		sb->s_count++;
 		spin_unlock(&sb_lock);
 
-		born = super_lock_shared(sb);
-		if (born && sb->s_root)
-			f(sb, arg);
-		super_unlock_shared(sb);
+		locked = super_lock_shared(sb);
+		if (locked) {
+			if (sb->s_root)
+				f(sb, arg);
+			super_unlock_shared(sb);
+		}
 
 		spin_lock(&sb_lock);
 		if (p)
@@ -997,15 +954,17 @@ void iterate_supers_type(struct file_system_type *type,
 
 	spin_lock(&sb_lock);
 	hlist_for_each_entry(sb, &type->fs_supers, s_instances) {
-		bool born;
+		bool locked;
 
 		sb->s_count++;
 		spin_unlock(&sb_lock);
 
-		born = super_lock_shared(sb);
-		if (born && sb->s_root)
-			f(sb, arg);
-		super_unlock_shared(sb);
+		locked = super_lock_shared(sb);
+		if (locked) {
+			if (sb->s_root)
+				f(sb, arg);
+			super_unlock_shared(sb);
+		}
 
 		spin_lock(&sb_lock);
 		if (p)
@@ -1019,34 +978,6 @@ void iterate_supers_type(struct file_system_type *type,
 
 EXPORT_SYMBOL(iterate_supers_type);
 
-/**
- * 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;
-
-	spin_lock(&sb_lock);
-	list_for_each_entry(sb, &super_blocks, s_list) {
-		if (sb->s_bdev == bdev) {
-			if (!grab_super(sb))
-				return NULL;
-			super_unlock_excl(sb);
-			return sb;
-		}
-	}
-	spin_unlock(&sb_lock);
-	return NULL;
-}
-
 struct super_block *user_get_super(dev_t dev, bool excl)
 {
 	struct super_block *sb;
@@ -1054,15 +985,17 @@ struct super_block *user_get_super(dev_t dev, bool excl)
 	spin_lock(&sb_lock);
 	list_for_each_entry(sb, &super_blocks, s_list) {
 		if (sb->s_dev ==  dev) {
-			bool born;
+			bool locked;
 
 			sb->s_count++;
 			spin_unlock(&sb_lock);
 			/* still alive? */
-			born = super_lock(sb, excl);
-			if (born && sb->s_root)
-				return sb;
-			super_unlock(sb, excl);
+			locked = super_lock(sb, excl);
+			if (locked) {
+				if (sb->s_root)
+					return sb;
+				super_unlock(sb, excl);
+			}
 			/* nope, got unmounted */
 			spin_lock(&sb_lock);
 			__put_super(sb);
@@ -1173,9 +1106,9 @@ cancel_readonly:
 
 static void do_emergency_remount_callback(struct super_block *sb)
 {
-	bool born = super_lock_excl(sb);
+	bool locked = super_lock_excl(sb);
 
-	if (born && sb->s_root && sb->s_bdev && !sb_rdonly(sb)) {
+	if (locked && sb->s_root && sb->s_bdev && !sb_rdonly(sb)) {
 		struct fs_context *fc;
 
 		fc = fs_context_for_reconfigure(sb->s_root,
@@ -1186,7 +1119,8 @@ static void do_emergency_remount_callback(struct super_block *sb)
 			put_fs_context(fc);
 		}
 	}
-	super_unlock_excl(sb);
+	if (locked)
+		super_unlock_excl(sb);
 }
 
 static void do_emergency_remount(struct work_struct *work)
@@ -1209,16 +1143,17 @@ void emergency_remount(void)
 
 static void do_thaw_all_callback(struct super_block *sb)
 {
-	bool born = super_lock_excl(sb);
+	bool locked = super_lock_excl(sb);
 
-	if (born && sb->s_root) {
+	if (locked && sb->s_root) {
 		if (IS_ENABLED(CONFIG_BLOCK))
-			while (sb->s_bdev && !thaw_bdev(sb->s_bdev))
+			while (sb->s_bdev && !bdev_thaw(sb->s_bdev))
 				pr_warn("Emergency Thaw on %pg\n", sb->s_bdev);
 		thaw_super_locked(sb, FREEZE_HOLDER_USERSPACE);
-	} else {
-		super_unlock_excl(sb);
+		return;
 	}
+	if (locked)
+		super_unlock_excl(sb);
 }
 
 static void do_thaw_all(struct work_struct *work)
@@ -1428,11 +1363,11 @@ EXPORT_SYMBOL(sget_dev);
  *
  * The function must be called with bdev->bd_holder_lock and releases it.
  */
-static struct super_block *bdev_super_lock_shared(struct block_device *bdev)
+static struct super_block *bdev_super_lock(struct block_device *bdev, bool excl)
 	__releases(&bdev->bd_holder_lock)
 {
 	struct super_block *sb = bdev->bd_holder;
-	bool born;
+	bool locked;
 
 	lockdep_assert_held(&bdev->bd_holder_lock);
 	lockdep_assert_not_held(&sb->s_umount);
@@ -1442,19 +1377,25 @@ static struct super_block *bdev_super_lock_shared(struct block_device *bdev)
 	spin_lock(&sb_lock);
 	sb->s_count++;
 	spin_unlock(&sb_lock);
+
 	mutex_unlock(&bdev->bd_holder_lock);
 
-	born = super_lock_shared(sb);
-	if (!born || !sb->s_root || !(sb->s_flags & SB_ACTIVE)) {
-		super_unlock_shared(sb);
-		put_super(sb);
-		return NULL;
-	}
+	locked = super_lock(sb, excl);
+
 	/*
-	 * The superblock is active and we hold s_umount, we can drop our
-	 * temporary reference now.
-	 */
+	 * If the superblock wasn't already SB_DYING then we hold
+	 * s_umount and can safely drop our temporary reference.
+         */
 	put_super(sb);
+
+	if (!locked)
+		return NULL;
+
+	if (!sb->s_root || !(sb->s_flags & SB_ACTIVE)) {
+		super_unlock(sb, excl);
+		return NULL;
+	}
+
 	return sb;
 }
 
@@ -1462,7 +1403,7 @@ static void fs_bdev_mark_dead(struct block_device *bdev, bool surprise)
 {
 	struct super_block *sb;
 
-	sb = bdev_super_lock_shared(bdev);
+	sb = bdev_super_lock(bdev, false);
 	if (!sb)
 		return;
 
@@ -1480,16 +1421,110 @@ static void fs_bdev_sync(struct block_device *bdev)
 {
 	struct super_block *sb;
 
-	sb = bdev_super_lock_shared(bdev);
+	sb = bdev_super_lock(bdev, false);
 	if (!sb)
 		return;
+
 	sync_filesystem(sb);
 	super_unlock_shared(sb);
 }
 
+static struct super_block *get_bdev_super(struct block_device *bdev)
+{
+	bool active = false;
+	struct super_block *sb;
+
+	sb = bdev_super_lock(bdev, true);
+	if (sb) {
+		active = atomic_inc_not_zero(&sb->s_active);
+		super_unlock_excl(sb);
+	}
+	if (!active)
+		return NULL;
+	return sb;
+}
+
+/**
+ * fs_bdev_freeze - freeze owning filesystem of block device
+ * @bdev: block device
+ *
+ * Freeze the filesystem that owns this block device if it is still
+ * active.
+ *
+ * A filesystem that owns multiple block devices may be frozen from each
+ * block device and won't be unfrozen until all block devices are
+ * unfrozen. Each block device can only freeze the filesystem once as we
+ * nest freezes for block devices in the block layer.
+ *
+ * Return: If the freeze was successful zero is returned. If the freeze
+ *         failed a negative error code is returned.
+ */
+static int fs_bdev_freeze(struct block_device *bdev)
+{
+	struct super_block *sb;
+	int error = 0;
+
+	lockdep_assert_held(&bdev->bd_fsfreeze_mutex);
+
+	sb = get_bdev_super(bdev);
+	if (!sb)
+		return -EINVAL;
+
+	if (sb->s_op->freeze_super)
+		error = sb->s_op->freeze_super(sb,
+				FREEZE_MAY_NEST | FREEZE_HOLDER_USERSPACE);
+	else
+		error = freeze_super(sb,
+				FREEZE_MAY_NEST | FREEZE_HOLDER_USERSPACE);
+	if (!error)
+		error = sync_blockdev(bdev);
+	deactivate_super(sb);
+	return error;
+}
+
+/**
+ * fs_bdev_thaw - thaw owning filesystem of block device
+ * @bdev: block device
+ *
+ * Thaw the filesystem that owns this block device.
+ *
+ * A filesystem that owns multiple block devices may be frozen from each
+ * block device and won't be unfrozen until all block devices are
+ * unfrozen. Each block device can only freeze the filesystem once as we
+ * nest freezes for block devices in the block layer.
+ *
+ * Return: If the thaw was successful zero is returned. If the thaw
+ *         failed a negative error code is returned. If this function
+ *         returns zero it doesn't mean that the filesystem is unfrozen
+ *         as it may have been frozen multiple times (kernel may hold a
+ *         freeze or might be frozen from other block devices).
+ */
+static int fs_bdev_thaw(struct block_device *bdev)
+{
+	struct super_block *sb;
+	int error;
+
+	lockdep_assert_held(&bdev->bd_fsfreeze_mutex);
+
+	sb = get_bdev_super(bdev);
+	if (WARN_ON_ONCE(!sb))
+		return -EINVAL;
+
+	if (sb->s_op->thaw_super)
+		error = sb->s_op->thaw_super(sb,
+				FREEZE_MAY_NEST | FREEZE_HOLDER_USERSPACE);
+	else
+		error = thaw_super(sb,
+				FREEZE_MAY_NEST | FREEZE_HOLDER_USERSPACE);
+	deactivate_super(sb);
+	return error;
+}
+
 const struct blk_holder_ops fs_holder_ops = {
 	.mark_dead		= fs_bdev_mark_dead,
 	.sync			= fs_bdev_sync,
+	.freeze			= fs_bdev_freeze,
+	.thaw			= fs_bdev_thaw,
 };
 EXPORT_SYMBOL_GPL(fs_holder_ops);
 
@@ -1519,15 +1554,10 @@ int setup_bdev_super(struct super_block *sb, int sb_flags,
 	}
 
 	/*
-	 * Until SB_BORN flag is set, there can be no active superblock
-	 * references and thus no filesystem freezing. get_active_super() will
-	 * just loop waiting for SB_BORN so even freeze_bdev() cannot proceed.
-	 *
-	 * It is enough to check bdev was not frozen before we set s_bdev.
+	 * It is enough to check bdev was not frozen before we set
+	 * s_bdev as freezing will wait until SB_BORN is set.
 	 */
-	mutex_lock(&bdev->bd_fsfreeze_mutex);
-	if (bdev->bd_fsfreeze_count > 0) {
-		mutex_unlock(&bdev->bd_fsfreeze_mutex);
+	if (atomic_read(&bdev->bd_fsfreeze_count) > 0) {
 		if (fc)
 			warnf(fc, "%pg: Can't mount, blockdev is frozen", bdev);
 		bdev_release(bdev_handle);
@@ -1540,7 +1570,6 @@ int setup_bdev_super(struct super_block *sb, int sb_flags,
 	if (bdev_stable_writes(bdev))
 		sb->s_iflags |= SB_I_STABLE_WRITES;
 	spin_unlock(&sb_lock);
-	mutex_unlock(&bdev->bd_fsfreeze_mutex);
 
 	snprintf(sb->s_id, sizeof(sb->s_id), "%pg", bdev);
 	shrinker_debugfs_rename(sb->s_shrink, "sb-%s:%s", sb->s_type->name,
@@ -1585,15 +1614,7 @@ int get_tree_bdev(struct fs_context *fc,
 			return -EBUSY;
 		}
 	} else {
-		/*
-		 * We drop s_umount here because we need to open the bdev and
-		 * bdev->open_mutex ranks above s_umount (blkdev_put() ->
-		 * bdev_mark_dead()). It is safe because we have active sb
-		 * reference and SB_BORN is not set yet.
-		 */
-		super_unlock_excl(s);
 		error = setup_bdev_super(s, fc->sb_flags, fc);
-		__super_lock_excl(s);
 		if (!error)
 			error = fill_super(s, fc);
 		if (error) {
@@ -1637,15 +1658,7 @@ struct dentry *mount_bdev(struct file_system_type *fs_type,
 			return ERR_PTR(-EBUSY);
 		}
 	} else {
-		/*
-		 * We drop s_umount here because we need to open the bdev and
-		 * bdev->open_mutex ranks above s_umount (blkdev_put() ->
-		 * bdev_mark_dead()). It is safe because we have active sb
-		 * reference and SB_BORN is not set yet.
-		 */
-		super_unlock_excl(s);
 		error = setup_bdev_super(s, flags, NULL);
-		__super_lock_excl(s);
 		if (!error)
 			error = fill_super(s, data, flags & SB_SILENT ? 1 : 0);
 		if (error) {
@@ -1914,6 +1927,47 @@ static int wait_for_partially_frozen(struct super_block *sb)
 	return ret;
 }
 
+#define FREEZE_HOLDERS (FREEZE_HOLDER_KERNEL | FREEZE_HOLDER_USERSPACE)
+#define FREEZE_FLAGS (FREEZE_HOLDERS | FREEZE_MAY_NEST)
+
+static inline int freeze_inc(struct super_block *sb, enum freeze_holder who)
+{
+	WARN_ON_ONCE((who & ~FREEZE_FLAGS));
+	WARN_ON_ONCE(hweight32(who & FREEZE_HOLDERS) > 1);
+
+	if (who & FREEZE_HOLDER_KERNEL)
+		++sb->s_writers.freeze_kcount;
+	if (who & FREEZE_HOLDER_USERSPACE)
+		++sb->s_writers.freeze_ucount;
+	return sb->s_writers.freeze_kcount + sb->s_writers.freeze_ucount;
+}
+
+static inline int freeze_dec(struct super_block *sb, enum freeze_holder who)
+{
+	WARN_ON_ONCE((who & ~FREEZE_FLAGS));
+	WARN_ON_ONCE(hweight32(who & FREEZE_HOLDERS) > 1);
+
+	if ((who & FREEZE_HOLDER_KERNEL) && sb->s_writers.freeze_kcount)
+		--sb->s_writers.freeze_kcount;
+	if ((who & FREEZE_HOLDER_USERSPACE) && sb->s_writers.freeze_ucount)
+		--sb->s_writers.freeze_ucount;
+	return sb->s_writers.freeze_kcount + sb->s_writers.freeze_ucount;
+}
+
+static inline bool may_freeze(struct super_block *sb, enum freeze_holder who)
+{
+	WARN_ON_ONCE((who & ~FREEZE_FLAGS));
+	WARN_ON_ONCE(hweight32(who & FREEZE_HOLDERS) > 1);
+
+	if (who & FREEZE_HOLDER_KERNEL)
+		return (who & FREEZE_MAY_NEST) ||
+		       sb->s_writers.freeze_kcount == 0;
+	if (who & FREEZE_HOLDER_USERSPACE)
+		return (who & FREEZE_MAY_NEST) ||
+		       sb->s_writers.freeze_ucount == 0;
+	return false;
+}
+
 /**
  * freeze_super - lock the filesystem and force it into a consistent state
  * @sb: the super to lock
@@ -1926,6 +1980,7 @@ static int wait_for_partially_frozen(struct super_block *sb)
  * @who should be:
  * * %FREEZE_HOLDER_USERSPACE if userspace wants to freeze the fs;
  * * %FREEZE_HOLDER_KERNEL if the kernel wants to freeze the fs.
+ * * %FREEZE_MAY_NEST whether nesting freeze and thaw requests is allowed.
  *
  * The @who argument distinguishes between the kernel and userspace trying to
  * freeze the filesystem.  Although there cannot be multiple kernel freezes or
@@ -1933,6 +1988,13 @@ static int wait_for_partially_frozen(struct super_block *sb)
  * userspace can both hold a filesystem frozen.  The filesystem remains frozen
  * until there are no kernel or userspace freezes in effect.
  *
+ * A filesystem may hold multiple devices and thus a filesystems may be
+ * frozen through the block layer via multiple block devices. In this
+ * case the request is marked as being allowed to nest by passing
+ * FREEZE_MAY_NEST. The filesystem remains frozen until all block
+ * devices are unfrozen. If multiple freezes are attempted without
+ * FREEZE_MAY_NEST -EBUSY will be returned.
+ *
  * During this function, sb->s_writers.frozen goes through these values:
  *
  * SB_UNFROZEN: File system is normal, all writes progress as usual.
@@ -1957,31 +2019,29 @@ static int wait_for_partially_frozen(struct super_block *sb)
  * mostly auxiliary for filesystems to verify they do not modify frozen fs.
  *
  * sb->s_writers.frozen is protected by sb->s_umount.
+ *
+ * Return: If the freeze was successful zero is returned. If the freeze
+ *         failed a negative error code is returned.
  */
 int freeze_super(struct super_block *sb, enum freeze_holder who)
 {
 	int ret;
 
+	if (!super_lock_excl(sb)) {
+		WARN_ON_ONCE("Dying superblock while freezing!");
+		return -EINVAL;
+	}
 	atomic_inc(&sb->s_active);
-	if (!super_lock_excl(sb))
-		WARN(1, "Dying superblock while freezing!");
 
 retry:
 	if (sb->s_writers.frozen == SB_FREEZE_COMPLETE) {
-		if (sb->s_writers.freeze_holders & who) {
-			deactivate_locked_super(sb);
-			return -EBUSY;
-		}
-
-		WARN_ON(sb->s_writers.freeze_holders == 0);
-
-		/*
-		 * Someone else already holds this type of freeze; share the
-		 * freeze and assign the active ref to the freeze.
-		 */
-		sb->s_writers.freeze_holders |= who;
-		super_unlock_excl(sb);
-		return 0;
+		if (may_freeze(sb, who))
+			ret = !!WARN_ON_ONCE(freeze_inc(sb, who) == 1);
+		else
+			ret = -EBUSY;
+		/* All freezers share a single active reference. */
+		deactivate_locked_super(sb);
+		return ret;
 	}
 
 	if (sb->s_writers.frozen != SB_UNFROZEN) {
@@ -1994,14 +2054,9 @@ retry:
 		goto retry;
 	}
 
-	if (!(sb->s_flags & SB_BORN)) {
-		super_unlock_excl(sb);
-		return 0;	/* sic - it's "nothing to do" */
-	}
-
 	if (sb_rdonly(sb)) {
 		/* Nothing to do really... */
-		sb->s_writers.freeze_holders |= who;
+		WARN_ON_ONCE(freeze_inc(sb, who) > 1);
 		sb->s_writers.frozen = SB_FREEZE_COMPLETE;
 		wake_up_var(&sb->s_writers.frozen);
 		super_unlock_excl(sb);
@@ -2012,8 +2067,7 @@ retry:
 	/* Release s_umount to preserve sb_start_write -> s_umount ordering */
 	super_unlock_excl(sb);
 	sb_wait_write(sb, SB_FREEZE_WRITE);
-	if (!super_lock_excl(sb))
-		WARN(1, "Dying superblock while freezing!");
+	__super_lock_excl(sb);
 
 	/* Now we go and block page faults... */
 	sb->s_writers.frozen = SB_FREEZE_PAGEFAULT;
@@ -2049,7 +2103,7 @@ retry:
 	 * 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.freeze_holders |= who;
+	WARN_ON_ONCE(freeze_inc(sb, who) > 1);
 	sb->s_writers.frozen = SB_FREEZE_COMPLETE;
 	wake_up_var(&sb->s_writers.frozen);
 	lockdep_sb_freeze_release(sb);
@@ -2066,34 +2120,22 @@ EXPORT_SYMBOL(freeze_super);
  */
 static int thaw_super_locked(struct super_block *sb, enum freeze_holder who)
 {
-	int error;
+	int error = -EINVAL;
 
-	if (sb->s_writers.frozen == SB_FREEZE_COMPLETE) {
-		if (!(sb->s_writers.freeze_holders & who)) {
-			super_unlock_excl(sb);
-			return -EINVAL;
-		}
+	if (sb->s_writers.frozen != SB_FREEZE_COMPLETE)
+		goto out_unlock;
 
-		/*
-		 * Freeze is shared with someone else.  Release our hold and
-		 * drop the active ref that freeze_super assigned to the
-		 * freezer.
-		 */
-		if (sb->s_writers.freeze_holders & ~who) {
-			sb->s_writers.freeze_holders &= ~who;
-			deactivate_locked_super(sb);
-			return 0;
-		}
-	} else {
-		super_unlock_excl(sb);
-		return -EINVAL;
-	}
+	/*
+	 * All freezers share a single active reference.
+	 * So just unlock in case there are any left.
+	 */
+	if (freeze_dec(sb, who))
+		goto out_unlock;
 
 	if (sb_rdonly(sb)) {
-		sb->s_writers.freeze_holders &= ~who;
 		sb->s_writers.frozen = SB_UNFROZEN;
 		wake_up_var(&sb->s_writers.frozen);
-		goto out;
+		goto out_deactivate;
 	}
 
 	lockdep_sb_freeze_acquire(sb);
@@ -2101,20 +2143,23 @@ static int thaw_super_locked(struct super_block *sb, enum freeze_holder who)
 	if (sb->s_op->unfreeze_fs) {
 		error = sb->s_op->unfreeze_fs(sb);
 		if (error) {
-			printk(KERN_ERR "VFS:Filesystem thaw failed\n");
+			pr_err("VFS: Filesystem thaw failed\n");
+			freeze_inc(sb, who);
 			lockdep_sb_freeze_release(sb);
-			super_unlock_excl(sb);
-			return error;
+			goto out_unlock;
 		}
 	}
 
-	sb->s_writers.freeze_holders &= ~who;
 	sb->s_writers.frozen = SB_UNFROZEN;
 	wake_up_var(&sb->s_writers.frozen);
 	sb_freeze_unlock(sb, SB_FREEZE_FS);
-out:
+out_deactivate:
 	deactivate_locked_super(sb);
 	return 0;
+
+out_unlock:
+	super_unlock_excl(sb);
+	return error;
 }
 
 /**
@@ -2128,11 +2173,18 @@ out:
  * @who should be:
  * * %FREEZE_HOLDER_USERSPACE if userspace wants to thaw the fs;
  * * %FREEZE_HOLDER_KERNEL if the kernel wants to thaw the fs.
+ * * %FREEZE_MAY_NEST whether nesting freeze and thaw requests is allowed
+ *
+ * A filesystem may hold multiple devices and thus a filesystems may
+ * have been frozen through the block layer via multiple block devices.
+ * The filesystem remains frozen until all block devices are unfrozen.
  */
 int thaw_super(struct super_block *sb, enum freeze_holder who)
 {
-	if (!super_lock_excl(sb))
-		WARN(1, "Dying superblock while thawing!");
+	if (!super_lock_excl(sb)) {
+		WARN_ON_ONCE("Dying superblock while thawing!");
+		return -EINVAL;
+	}
 	return thaw_super_locked(sb, who);
 }
 EXPORT_SYMBOL(thaw_super);