ext4: mark superblock writes synchronous for nobarrier mounts

Commit b685d3d65ac7 "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: b685d3d65ac791406e0dfd8779cc9b3707fea5a3
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;