On a filesystem with exhausted metadata, but still enough to start balance, it's possible to hit this error:

[324402.053842] BTRFS info (device loop0): 1 enospc errors during balance [324402.060769] BTRFS info (device loop0): balance: ended with status: -28 [324402.172295] BTRFS: error (device loop0) in reset_balance_state:3321: errno=-28 No space left

It fails inside reset_balance_state and turns the filesystem to read-only, which is unnecessary and should be fixed too, but the problem is caused by lack for space when the balance item is deleted. This is a one-time operation and from the same rank as unlink that is allowed to use the global block reserve. So do the same for the balance item.

Status of the filesystem (100GiB) just after the balance fails:

$ btrfs fi df mnt Data, single: total=80.01GiB, used=38.58GiB System, single: total=4.00MiB, used=16.00KiB Metadata, single: total=19.99GiB, used=19.48GiB GlobalReserve, single: total=512.00MiB, used=50.11MiB

CC: stable@vger.kernel.org # 4.4+ Signed-off-by: David Sterba dsterba@suse.com --- fs/btrfs/volumes.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c index f89625de1fff..aabc6c922e04 100644 --- a/fs/btrfs/volumes.c +++ b/fs/btrfs/volumes.c @@ -3231,7 +3231,7 @@ static int del_balance_item(struct btrfs_fs_info *fs_info) if (!path) return -ENOMEM;

- trans = btrfs_start_transaction(root, 0); + trans = btrfs_start_transaction_fallback_global_rsv(root, 0); if (IS_ERR(trans)) { btrfs_free_path(path); return PTR_ERR(trans);