[5/7] btrfs: scrub: avoid create/commit empty transaction at finish_extent_writes_for_zoned()

Commit Message

Filipe Manana May 22, 2024, 2:36 p.m. UTC

From: Filipe Manana <[e-mail]>

At finish_extent_writes_for_zoned() we use btrfs_join_transaction() to
catch any running transaction and then commit it. This will however create
a new and empty transaction in case there's no running transaction anymore
(got committed by the transaction kthread or other task for example) or
there's a running transaction finishing its commit and with a state >=
TRANS_STATE_UNBLOCKED. In the former case we don't need to do anything
while in the second case we just need to wait for the transaction to
complete its commit.

So improve this by using btrfs_attach_transaction_barrier() instead, which
does not create a new transaction if there's none running, and if there's
a current transaction that is committing, it will wait for it to fully
commit and not create a new transaction. This helps avoiding creating and
committing empty transactions, saving IO, time and unnecessary rotation of
the backup roots in the super block.

Signed-off-by: Filipe Manana <[e-mail]>
---
 fs/btrfs/scrub.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

Patch

diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
index 376c5c2e9aed..6c7b5d52591e 100644
--- a/fs/btrfs/scrub.c
+++ b/fs/btrfs/scrub.c
@@ -2446,9 +2446,13 @@  static int finish_extent_writes_for_zoned(struct btrfs_root *root,
 	btrfs_wait_nocow_writers(cache);
 	btrfs_wait_ordered_roots(fs_info, U64_MAX, cache);
 
-	trans = btrfs_join_transaction(root);
-	if (IS_ERR(trans))
-		return PTR_ERR(trans);
+	trans = btrfs_attach_transaction_barrier(root);
+	if (IS_ERR(trans)) {
+		int ret = PTR_ERR(trans);
+
+		return (ret == -ENOENT) ? 0 : ret;
+	}
+
 	return btrfs_commit_transaction(trans);
 }