writeback: disable periodic old data writeback for !dirty_writeback_centisecs

Prior to 2.6.32, setting /proc/sys/vm/dirty_writeback_centisecs disabled
periodic dirty writeback from kupdate. This got broken and now causes
excessive sys CPU usage if set to zero, as we'll keep beating on
schedule().

Cc: stable@kernel.org
Reported-by: Justin Maggard <jmaggard10@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

fs/fs-writeback.c

@@ -852,6 +852,12 @@ static long wb_check_old_data_flush(struct bdi_writeback *wb)
 	unsigned long expired;
 	long nr_pages;
 
+	/*
+	 * When set to zero, disable periodic writeback
+	 */
+	if (!dirty_writeback_interval)
+		return 0;
+
 	expired = wb->last_old_flush +
 			msecs_to_jiffies(dirty_writeback_interval * 10);
 	if (time_before(jiffies, expired))
@@ -947,8 +953,12 @@ int bdi_writeback_task(struct bdi_writeback *wb)
 				break;
 		}
 
-		wait_jiffies = msecs_to_jiffies(dirty_writeback_interval * 10);
-		schedule_timeout_interruptible(wait_jiffies);
+		if (dirty_writeback_interval) {
+			wait_jiffies = msecs_to_jiffies(dirty_writeback_interval * 10);
+			schedule_timeout_interruptible(wait_jiffies);
+		} else
+			schedule();
+
 		try_to_freeze();
 	}