[PATCH] kswapd: avoid unnecessary reclaiming from higher zones

Currently kswapd walks across all zones in dma->normal->highmem order,
performing proportional scanning until all zones are OK.  This means that
pressure against ZONE_NORMAL causes unnecessary reclaim of ZONE_HIGHMEM.

To fix that up we change kswapd so that it walks the zones in the
high->normal->dma direction, skipping zones which are OK.  Once it encounters
a zone which needs some reclaim kswapd will perform proportional scanning
against that zone as well as all the succeeding lower zones.

We scan the lower zones even if they have sufficient free pages.  This is
because

a) the lower zone may be above pages_high, but because of the incremental
   min, the lower zone may still not be eligible for allocations.  That's bad
   because cache in that lower zone will then not be scanned at the correct
   rate.

b) pages in this lower zone are usable for allocations against the higher
   zone.  So we do want to san all the relevant zones at an equal rate.

mm/vmscan.c

@@ -898,6 +898,13 @@ int try_to_free_pages(struct zone **zones,
  * scanned twice and there has been zero successful reclaim.  Mark the zone as
  * dead and from now on, only perform a short scan.  Basically we're polling
  * the zone for when the problem goes away.
+ *
+ * kswapd scans the zones in the highmem->normal->dma direction.  It skips
+ * zones which have free_pages > pages_high, but once a zone is found to have
+ * free_pages <= pages_high, we scan that zone and the lower zones regardless
+ * of the number of free pages in the lower zones.  This interoperates with
+ * the page allocator fallback scheme to ensure that aging of pages is balanced
+ * across the zones.
  */
 static int balance_pgdat(pg_data_t *pgdat, int nr_pages, struct page_state *ps)
 {
@@ -918,7 +925,7 @@ static int balance_pgdat(pg_data_t *pgdat, int nr_pages, struct page_state *ps)
 		int all_zones_ok = 1;
 		int pages_scanned = 0;
 
-		for (i = 0; i < pgdat->nr_zones; i++) {
+		for (i = pgdat->nr_zones - 1; i >= 0; i--) {
 			struct zone *zone = pgdat->node_zones + i;
 			int total_scanned = 0;
 			int max_scan;
@@ -930,6 +937,8 @@ static int balance_pgdat(pg_data_t *pgdat, int nr_pages, struct page_state *ps)
 			if (nr_pages == 0) {	/* Not software suspend */
 				if (zone->free_pages <= zone->pages_high)
 					all_zones_ok = 0;
+				if (all_zones_ok)
+					continue;
 			}
 			zone->temp_priority = priority;
 			max_scan = zone->nr_inactive >> priority;