ext4: mark superblock writes synchronous for nobarrier mounts

Commit 8eecb540590c "block: treat REQ_FUA and REQ_PREFLUSH as
synchronous" removed REQ_SYNC flag from WRITE_FUA implementation.
generic_make_request_checks() however strips REQ_FUA flag from a bio
when the storage doesn't report volatile write cache and thus write
effectively becomes asynchronous which can lead to performance
regressions. This affects superblock writes for ext4. Fix the problem
by marking superblock writes always as synchronous.

Fixes: 8eecb540590c70d97c0366ea02a6a436c87fa8ef
CC: linux-ext4@vger.kernel.org
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 9a40e0c16dcc770ca2a96493ff49e95914d8e3be..931053cf20d6ae616fa6066bc53ae3883307f703 100644 (file)
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -4637,7 +4637,7 @@ static int ext4_commit_super(struct super_block *sb, int sync)
        if (sync) {
                unlock_buffer(sbh);
                error = __sync_dirty_buffer(sbh,
-                       test_opt(sb, BARRIER) ? REQ_FUA : REQ_SYNC);
+                       REQ_SYNC | (test_opt(sb, BARRIER) ? REQ_FUA : 0));
                if (error)
                        return error;