[PATCH] vmscan: handle empty zones

There's a lockup condition where kswapd ensounters an empty zone which has all
its metadata set to zero.

Fix that by adding explicit checks for empty zones and just skip over them.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/vmscan.c

@@ -851,6 +851,9 @@ shrink_caches(struct zone **zones, struct scan_control *sc)
 	for (i = 0; zones[i] != NULL; i++) {
 		struct zone *zone = zones[i];
 
+		if (zone->present_pages == 0)
+			continue;
+
 		zone->temp_priority = sc->priority;
 		if (zone->prev_priority > sc->priority)
 			zone->prev_priority = sc->priority;
@@ -1004,6 +1007,9 @@ static int balance_pgdat(pg_data_t *pgdat, int nr_pages)
 			for (i = pgdat->nr_zones - 1; i >= 0; i--) {
 				struct zone *zone = pgdat->node_zones + i;
 
+				if (zone->present_pages == 0)
+					continue;
+
 				if (zone->all_unreclaimable &&
 						priority != DEF_PRIORITY)
 					continue;
@@ -1036,6 +1042,9 @@ static int balance_pgdat(pg_data_t *pgdat, int nr_pages)
 		for (i = 0; i <= end_zone; i++) {
 			struct zone *zone = pgdat->node_zones + i;
 
+			if (zone->present_pages == 0)
+				continue;
+
 			if (zone->all_unreclaimable && priority != DEF_PRIORITY)
 				continue;
 
@@ -1159,6 +1168,8 @@ static int kswapd(void *p)
  */
 void wakeup_kswapd(struct zone *zone)
 {
+	if (zone->present_pages == 0)
+		return;
 	if (zone->free_pages > zone->pages_low)
 		return;
 	if (!waitqueue_active(&zone->zone_pgdat->kswapd_wait))