btrfs: allow use of global block reserve for balance item deletion

BugLink: https://bugs.launchpad.net/bugs/1892899
commit 3502a8c0dc1bd4b4970b59b06e348f22a1c05581 upstream.

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+
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 1e6e3c1d97dfa67f771453fe102d8a3b9f8bf08d..a9c84595fb2453534b6bdbd7e2d734cfe9ce5adf 100644 (file)
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -3283,7 +3283,7 @@ static int insert_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);