UBUNTU: SAUCE: (no-up) If zone is so small that watermarks are the same, stop zone balance.

BugLink: http://bugs.launchpad.net/bugs/1518457

On an AWS t2.micro instance (Xeon E5-2670, 991MiB of memory).
Occasionally (about once a day), kswapd0 falls into a busy loop and
spins on 100% CPU usage indefinitely. Reject to do the zone balance
when the memory is too small.
Signed-off-by: Dan Streetman <dan.streetman@canonical.com>
Signed-off-by: Gavin Guo <gavin.guo@canonical.com>
Tested-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Acked-by: Leann Ogasawara <leann.ogasawara@canonical.com>
Acked-by: Brad Figg <brad.figg@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>

mm/vmscan.c

@@ -2959,6 +2959,13 @@ static void age_active_anon(struct zone *zone, struct scan_control *sc)
 static bool zone_balanced(struct zone *zone, int order,
 			  unsigned long balance_gap, int classzone_idx)
 {
+	/*
+	 * if zone is so small that watermarks are the same, don't bother trying
+	 * to balance; kswapd would just spin continuously trying to balance it.
+	 */
+	if (low_wmark_pages(zone) == high_wmark_pages(zone))
+		return true;
+
 	if (!zone_watermark_ok_safe(zone, order, high_wmark_pages(zone) +
 				    balance_gap, classzone_idx))
 		return false;