btrfs: Make flush bios explicitely sync

Commit 8eecb540590c "block: treat REQ_FUA and REQ_PREFLUSH as
synchronous" removed REQ_SYNC flag from WRITE_{FUA|PREFLUSH|...}
definitions.  generic_make_request_checks() however strips REQ_FUA and
REQ_PREFLUSH flags from a bio when the storage doesn't report volatile
write cache and thus write effectively becomes asynchronous which can
lead to performance regressions

Fix the problem by making sure all bios which are synchronous are
properly marked with REQ_SYNC.

CC: David Sterba <dsterba@suse.com>
CC: linux-btrfs@vger.kernel.org
Fixes: 8eecb540590c70d97c0366ea02a6a436c87fa8ef
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 683194242deb169cc5832ff682aca3799a846e8e..b3579d11fb26d15c20372182218263462b8dcb01 100644 (file)
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3488,10 +3488,12 @@ static int write_dev_supers(struct btrfs_device *device,
                 * we fua the first super.  The others we allow
                 * to go down lazy.
                 */
-               if (i == 0)
-                       ret = btrfsic_submit_bh(REQ_OP_WRITE, REQ_FUA, bh);
-               else
+               if (i == 0) {
+                       ret = btrfsic_submit_bh(REQ_OP_WRITE,
+                                               REQ_SYNC | REQ_FUA, bh);
+               } else {
                        ret = btrfsic_submit_bh(REQ_OP_WRITE, REQ_SYNC, bh);
+               }
                if (ret)
                        errors++;
        }
@@ -3556,7 +3558,7 @@ static int write_dev_flush(struct btrfs_device *device, int wait)
 
        bio->bi_end_io = btrfs_end_empty_barrier;
        bio->bi_bdev = device->bdev;
-       bio->bi_opf = REQ_OP_WRITE | REQ_PREFLUSH;
+       bio->bi_opf = REQ_OP_WRITE | REQ_SYNC | REQ_PREFLUSH;
        init_completion(&device->flush_wait);
        bio->bi_private = &device->flush_wait;
        device->flush_bio = bio;