[PATCH] Fix bug in try_to_free_pages and balance_pgdat when they fail to reclaim pages

[Martin Bligh <mbligh@google.com>]

The same bug is contained in both try_to_free_pages and balance_pgdat.
On reclaiming the requisite number of pages we correctly set
prev_priority back to DEF_PRIORITY. However, we ALSO do this even
if we loop over all priorities and fail to reclaim.

Setting prev_priority artificially high causes reclaimers to set
distress artificially low, and fail to reclaim mapped pages, when
they are, in fact, under severe memory pressure (their priority
may be as low as 0). This causes the OOM killer to fire incorrectly.

This patch changes that to set prev_priority to 0 instead, if we
fail to reclaim.

Signed-off-by: Martin J. Bligh <mbligh@google.com>

diff -aurpN -X /home/mbligh/.diff.exclude linux-2.6.18/mm/vmscan.c 2.6.18-prev_reset/mm/vmscan.c
--- linux-2.6.18/mm/vmscan.c	2006-09-20 12:24:42.000000000 -0700
+++ 2.6.18-prev_reset/mm/vmscan.c	2006-10-16 17:23:48.000000000 -0700
@@ -962,7 +962,6 @@ static unsigned long shrink_zones(int pr
 unsigned long try_to_free_pages(struct zone **zones, gfp_t gfp_mask)
 {
 	int priority;
-	int ret = 0;
 	unsigned long total_scanned = 0;
 	unsigned long nr_reclaimed = 0;
 	struct reclaim_state *reclaim_state = current->reclaim_state;
@@ -1000,8 +999,15 @@ unsigned long try_to_free_pages(struct z
 		}
 		total_scanned += sc.nr_scanned;
 		if (nr_reclaimed >= sc.swap_cluster_max) {
-			ret = 1;
-			goto out;
+			for (i = 0; zones[i] != 0; i++) {
+				struct zone *zone = zones[i];
+
+				if (!cpuset_zone_allowed(zone, __GFP_HARDWALL))
+					continue;
+
+				zone->prev_priority = zone->temp_priority;
+			}
+			return 1;
 		}
 
 		/*
@@ -1021,16 +1027,15 @@ unsigned long try_to_free_pages(struct z
 		if (sc.nr_scanned && priority < DEF_PRIORITY - 2)
 			blk_congestion_wait(WRITE, HZ/10);
 	}
-out:
 	for (i = 0; zones[i] != 0; i++) {
 		struct zone *zone = zones[i];
 
 		if (!cpuset_zone_allowed(zone, __GFP_HARDWALL))
 			continue;
 
-		zone->prev_priority = zone->temp_priority;
+		zone->prev_priority = 0;
 	}
-	return ret;
+	return 0;
 }
 
 /*
@@ -1186,7 +1191,10 @@ out:
 	for (i = 0; i < pgdat->nr_zones; i++) {
 		struct zone *zone = pgdat->node_zones + i;
 
-		zone->prev_priority = zone->temp_priority;
+		if (priority < 0)		/* we failed to reclaim */
+			zone->prev_priority = 0;
+		else
+			zone->prev_priority = zone->temp_priority;
 	}
 	if (!all_zones_ok) {
 		cond_resched();