ext4: disable synchronous transaction batching if max_batch_time==0

commit 5dd21424 upstream.

The mount manpage says of the max_batch_time option,

	This optimization can be turned off entirely
	by setting max_batch_time to 0.

But the code doesn't do that.  So fix the code to do
that.
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
[bwh: Backported to 3.2: option parsing looks different]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/ext4/super.c

@@ -1663,8 +1663,6 @@ static int parse_options(char *options, struct super_block *sb,
 				return 0;
 			if (option < 0)
 				return 0;
-			if (option == 0)
-				option = EXT4_DEF_MAX_BATCH_TIME;
 			sbi->s_max_batch_time = option;
 			break;
 		case Opt_min_batch_time:

fs/jbd2/transaction.c

@@ -1388,9 +1388,12 @@ int jbd2_journal_stop(handle_t *handle)
 	 * to perform a synchronous write.  We do this to detect the
 	 * case where a single process is doing a stream of sync
 	 * writes.  No point in waiting for joiners in that case.
+	 *
+	 * Setting max_batch_time to 0 disables this completely.
 	 */
 	pid = current->pid;
-	if (handle->h_sync && journal->j_last_sync_writer != pid) {
+	if (handle->h_sync && journal->j_last_sync_writer != pid &&
+	    journal->j_max_batch_time) {
 		u64 commit_time, trans_time;
 
 		journal->j_last_sync_writer = pid;