btrfs: avoid start and commit empty transaction when flushing qgroups

When flushing qgroups, we try to join a running transaction, with
btrfs_join_transaction(), and then commit the transaction. However using
btrfs_join_transaction() will result in creating a new transaction in case
there isn't any running or if there's an existing one already committing.
This is pointless as we only need to attach to an existing one that is
not committing and in case there's an existing one committing, wait for
its commit to complete. Creating and committing an empty transaction is
wasteful, pointless IO and unnecessary rotation of the backup roots.

So use btrfs_attach_transaction_barrier() instead, to avoid creating and
committing empty transactions.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index 1ef60ea8f0bc..b99230db3c82 100644
--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -3758,9 +3758,11 @@ static int try_flush_qgroup(struct btrfs_root *root)
 		goto out;
 	btrfs_wait_ordered_extents(root, U64_MAX, 0, (u64)-1);
 
-	trans = btrfs_join_transaction(root);
+	trans = btrfs_attach_transaction_barrier(root);
 	if (IS_ERR(trans)) {
 		ret = PTR_ERR(trans);
+		if (ret == -ENOENT)
+			ret = 0;
 		goto out;
 	}