[PATCH] btrfs: always write superblocks synchronously

[Davidlohr Bueso]

Commit b685d3d65ac7 "block: treat REQ_FUA and REQ_PREFLUSH as
synchronous" removed REQ_SYNC flag from WRITE_FUA implementation.
Since REQ_FUA and REQ_FLUSH flags are stripped from submitted IO
when the disk doesn't have volatile write cache and thus effectively
make the write async. This was seen to cause performance hits up
to 90% regression in disk IO related benchmarks such as reaim and
dbench[1].

Fix the problem by making sure the first superblock write is also
treated as synchronous since they can block progress of the
journalling (commit, log syncs) machinery and thus the whole filesystem.

[1] https://www.spinics.net/lists/linux-ext4/msg56238.html

Fixes: b685d3d65ac (block: treat REQ_FUA and REQ_PREFLUSH as synchronous)
Cc: stable <stable@xxxxxxxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
---
 fs/btrfs/disk-io.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 061c1d1f774f..51b2fd8ceccb 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3468,7 +3468,7 @@ static int write_dev_supers(struct btrfs_device *device,
 		 * to go down lazy.
 		 */
 		if (i == 0)
-			ret = btrfsic_submit_bh(REQ_OP_WRITE, REQ_FUA, bh);
+			ret = btrfsic_submit_bh(REQ_OP_WRITE, REQ_FUA | REQ_SYNC, bh);
 		else
 			ret = btrfsic_submit_bh(REQ_OP_WRITE, REQ_SYNC, bh);
 		if (ret)
-- 
2.12.0