bdev: implement freeze and thaw holder operations

The old method of implementing block device freeze and thaw operations
required us to rely on get_active_super() to walk the list of all
superblocks on the system to find any superblock that might use the
block device. This is wasteful and not very pleasant overall.

Now that we can finally go straight from block device to owning
superblock things become way simpler.

Link: https://lore.kernel.org/r/20231024-vfs-super-freeze-v2-5-599c19f4faac@kernel.orgReviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

block/bdev.c

@@ -222,31 +222,27 @@ EXPORT_SYMBOL(sync_blockdev_range);
  */
 int bdev_freeze(struct block_device *bdev)
 {
-	struct super_block *sb;
 	int error = 0;
 
 	mutex_lock(&bdev->bd_fsfreeze_mutex);
-	if (++bdev->bd_fsfreeze_count > 1)
-		goto done;
-
-	sb = get_active_super(bdev);
-	if (!sb)
-		goto sync;
-	if (sb->s_op->freeze_super)
-		error = sb->s_op->freeze_super(sb, FREEZE_HOLDER_USERSPACE);
-	else
-		error = freeze_super(sb, FREEZE_HOLDER_USERSPACE);
-	deactivate_super(sb);
 
-	if (error) {
-		bdev->bd_fsfreeze_count--;
-		goto done;
+	if (atomic_inc_return(&bdev->bd_fsfreeze_count) > 1) {
+		mutex_unlock(&bdev->bd_fsfreeze_mutex);
+		return 0;
+	}
+
+	mutex_lock(&bdev->bd_holder_lock);
+	if (bdev->bd_holder_ops && bdev->bd_holder_ops->freeze) {
+		error = bdev->bd_holder_ops->freeze(bdev);
+		lockdep_assert_not_held(&bdev->bd_holder_lock);
+	} else {
+		mutex_unlock(&bdev->bd_holder_lock);
+		error = sync_blockdev(bdev);
 	}
-	bdev->bd_fsfreeze_sb = sb;
 
-sync:
-	error = sync_blockdev(bdev);
-done:
+	if (error)
+		atomic_dec(&bdev->bd_fsfreeze_count);
+
 	mutex_unlock(&bdev->bd_fsfreeze_mutex);
 	return error;
 }
@@ -262,29 +258,32 @@ EXPORT_SYMBOL(bdev_freeze);
  */
 int bdev_thaw(struct block_device *bdev)
 {
-	struct super_block *sb;
-	int error = -EINVAL;
+	int error = -EINVAL, nr_freeze;
 
 	mutex_lock(&bdev->bd_fsfreeze_mutex);
-	if (!bdev->bd_fsfreeze_count)
+
+	/*
+	 * If this returns < 0 it means that @bd_fsfreeze_count was
+	 * already 0 and no decrement was performed.
+	 */
+	nr_freeze = atomic_dec_if_positive(&bdev->bd_fsfreeze_count);
+	if (nr_freeze < 0)
 		goto out;
 
 	error = 0;
-	if (--bdev->bd_fsfreeze_count > 0)
+	if (nr_freeze > 0)
 		goto out;
 
-	sb = bdev->bd_fsfreeze_sb;
-	if (!sb)
-		goto out;
+	mutex_lock(&bdev->bd_holder_lock);
+	if (bdev->bd_holder_ops && bdev->bd_holder_ops->thaw) {
+		error = bdev->bd_holder_ops->thaw(bdev);
+		lockdep_assert_not_held(&bdev->bd_holder_lock);
+	} else {
+		mutex_unlock(&bdev->bd_holder_lock);
+	}
 
-	if (sb->s_op->thaw_super)
-		error = sb->s_op->thaw_super(sb, FREEZE_HOLDER_USERSPACE);
-	else
-		error = thaw_super(sb, FREEZE_HOLDER_USERSPACE);
 	if (error)
-		bdev->bd_fsfreeze_count++;
-	else
-		bdev->bd_fsfreeze_sb = NULL;
+		atomic_inc(&bdev->bd_fsfreeze_count);
 out:
 	mutex_unlock(&bdev->bd_fsfreeze_mutex);
 	return error;

fs/super.c

@@ -1440,7 +1440,7 @@ 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;
@@ -1454,18 +1454,25 @@ 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);
 
-	locked = super_lock_shared(sb);
-	if (!locked || !sb->s_root || !(sb->s_flags & SB_ACTIVE)) {
-		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;
 }
 
@@ -1473,7 +1480,7 @@ 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;
 
@@ -1491,16 +1498,74 @@ 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;
+}
+
+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_HOLDER_USERSPACE);
+	else
+		error = freeze_super(sb, FREEZE_HOLDER_USERSPACE);
+	if (!error)
+		error = sync_blockdev(bdev);
+	deactivate_super(sb);
+	return error;
+}
+
+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_HOLDER_USERSPACE);
+	else
+		error = thaw_super(sb, 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);
 
@@ -1530,15 +1595,10 @@ 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 bdev_freeze() 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);
@@ -1551,7 +1611,6 @@ 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,

include/linux/blk_types.h

@@ -57,7 +57,7 @@ struct block_device {
 	const struct blk_holder_ops *bd_holder_ops;
 	struct mutex		bd_holder_lock;
 	/* The counter of freeze processes */
-	int			bd_fsfreeze_count;
+	atomic_t		bd_fsfreeze_count;
 	int			bd_holders;
 	struct kobject		*bd_holder_dir;