btrfs: fall back to global reservation when removing subvolumes

I recently did some ENOSPC testing that involved filling the disk
while create and removing snapshots in a loop. During the test cycle,
I ran into an ENOSPC when trying to remove a snapshot, leaving the fs
stuck in ENOSPC even after a umount/mount cycle.

This patch allow subvolume removal to fall back onto the global
block reservation in order to succeed when it would have failed
otherwise.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/ctree.h

@@ -3175,7 +3175,7 @@ void btrfs_orphan_release_metadata(struct inode *inode);
 int btrfs_subvolume_reserve_metadata(struct btrfs_root *root,
 				     struct btrfs_block_rsv *rsv,
 				     int nitems,
-				     u64 *qgroup_reserved);
+				     u64 *qgroup_reserved, bool use_global_rsv);
 void btrfs_subvolume_release_metadata(struct btrfs_root *root,
 				      struct btrfs_block_rsv *rsv,
 				      u64 qgroup_reserved);

fs/btrfs/extent-tree.c

@@ -4742,10 +4742,12 @@ void btrfs_orphan_release_metadata(struct inode *inode)
 int btrfs_subvolume_reserve_metadata(struct btrfs_root *root,
 				     struct btrfs_block_rsv *rsv,
 				     int items,
-				     u64 *qgroup_reserved)
+				     u64 *qgroup_reserved,
+				     bool use_global_rsv)
 {
 	u64 num_bytes;
 	int ret;
+	struct btrfs_block_rsv *global_rsv = &root->fs_info->global_block_rsv;
 
 	if (root->fs_info->quota_enabled) {
 		/* One for parent inode, two for dir entries */
@@ -4764,6 +4766,10 @@ int btrfs_subvolume_reserve_metadata(struct btrfs_root *root,
 					    BTRFS_BLOCK_GROUP_METADATA);
 	ret = btrfs_block_rsv_add(root, rsv, num_bytes,
 				  BTRFS_RESERVE_FLUSH_ALL);
+
+	if (ret == -ENOSPC && use_global_rsv)
+		ret = btrfs_block_rsv_migrate(global_rsv, rsv, num_bytes);
+
 	if (ret) {
 		if (*qgroup_reserved)
 			btrfs_qgroup_free(root, *qgroup_reserved);

fs/btrfs/ioctl.c

@@ -396,7 +396,7 @@ static noinline int create_subvol(struct inode *dir,
 	 * of create_snapshot().
 	 */
 	ret = btrfs_subvolume_reserve_metadata(root, &block_rsv,
-					       7, &qgroup_reserved);
+					       7, &qgroup_reserved, false);
 	if (ret)
 		return ret;
 
@@ -576,7 +576,8 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir,
 	 */
 	ret = btrfs_subvolume_reserve_metadata(BTRFS_I(dir)->root,
 					&pending_snapshot->block_rsv, 7,
-					&pending_snapshot->qgroup_reserved);
+					&pending_snapshot->qgroup_reserved,
+					false);
 	if (ret)
 		goto out;
 
@@ -2174,7 +2175,7 @@ static noinline int btrfs_ioctl_snap_destroy(struct file *file,
 	 * ref/backref.
 	 */
 	err = btrfs_subvolume_reserve_metadata(root, &block_rsv,
-					       5, &qgroup_reserved);
+					       5, &qgroup_reserved, true);
 	if (err)
 		goto out_up_write;