jbd2: use WRITE_SYNC_PLUG instead of WRITE_SYNC

When you are going to be submitting several sync writes, we want to
give the IO scheduler a chance to merge some of them. Instead of
using the implicitly unplugging WRITE_SYNC variant, use WRITE_SYNC_PLUG
and rely on sync_buffer() doing the unplug when someone does a
wait_on_buffer()/lock_buffer().
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/jbd2/commit.c

@@ -138,7 +138,7 @@ static int journal_submit_commit_record(journal_t *journal,
 		set_buffer_ordered(bh);
 		barrier_done = 1;
 	}
-	ret = submit_bh(WRITE_SYNC, bh);
+	ret = submit_bh(WRITE_SYNC_PLUG, bh);
 	if (barrier_done)
 		clear_buffer_ordered(bh);
 
@@ -159,7 +159,7 @@ static int journal_submit_commit_record(journal_t *journal,
 		lock_buffer(bh);
 		set_buffer_uptodate(bh);
 		clear_buffer_dirty(bh);
-		ret = submit_bh(WRITE_SYNC, bh);
+		ret = submit_bh(WRITE_SYNC_PLUG, bh);
 	}
 	*cbh = bh;
 	return ret;
@@ -190,7 +190,7 @@ static int journal_wait_on_commit_record(journal_t *journal,
 		set_buffer_uptodate(bh);
 		bh->b_end_io = journal_end_buffer_io_sync;
 
-		ret = submit_bh(WRITE_SYNC, bh);
+		ret = submit_bh(WRITE_SYNC_PLUG, bh);
 		if (ret) {
 			unlock_buffer(bh);
 			return ret;
@@ -402,8 +402,13 @@ void jbd2_journal_commit_transaction(journal_t *journal)
 	spin_lock(&journal->j_state_lock);
 	commit_transaction->t_state = T_LOCKED;
 
+	/*
+	 * Use plugged writes here, since we want to submit several before
+	 * we unplug the device. We don't do explicit unplugging in here,
+	 * instead we rely on sync_buffer() doing the unplug for us.
+	 */
 	if (commit_transaction->t_synchronous_commit)
-		write_op = WRITE_SYNC;
+		write_op = WRITE_SYNC_PLUG;
 	stats.u.run.rs_wait = commit_transaction->t_max_wait;
 	stats.u.run.rs_locked = jiffies;
 	stats.u.run.rs_running = jbd2_time_diff(commit_transaction->t_start,