[PATCH] Btrfs: fix unexpected balance crash due to BUG_ON

[Liu Bo <bo.li.liu@oracle.com>]

Mounting a btrfs can resume previous balance operations asynchronously.
An user got a crash when one drive has some corrupt sectors.

Since balance can cancel itself in case of any error, we can gracefully
return errors to upper layers and let balance do the cancel job.

Reported-by: sash <master.b.at.raven@chefmail.de>
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
---
 fs/btrfs/volumes.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index bd0f45f..5aed2e2 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -3418,13 +3418,25 @@ static int __btrfs_balance(struct btrfs_fs_info *fs_info)
 		ret = btrfs_shrink_device(device, old_size - size_to_free);
 		if (ret == -ENOSPC)
 			break;
-		BUG_ON(ret);
+		if (ret) {
+			/* btrfs_shrink_device never returns ret > 0 */
+			WARN_ON_ONCE(ret > 0);
+			goto error;
+		}
 
 		trans = btrfs_start_transaction(dev_root, 0);
-		BUG_ON(IS_ERR(trans));
+		if (IS_ERR(trans)) {
+			ret = PTR_ERR(trans);
+			goto error;
+		}
 
 		ret = btrfs_grow_device(trans, device, old_size);
-		BUG_ON(ret);
+		if (ret) {
+			btrfs_end_transaction(trans, dev_root);
+			/* btrfs_grow_device never returns ret > 0 */
+			WARN_ON_ONCE(ret > 0);
+			goto error;
+		}
 
 		btrfs_end_transaction(trans, dev_root);
 	}
-- 
2.5.5