[PATCH] vmscan: balancing fix

Revert a patch which went into 2.6.8-rc1.  The changelog for that patch was:

  The shrink_zone() logic can, under some circumstances, cause far too many
  pages to be reclaimed.  Say, we're scanning at high priority and suddenly
  hit a large number of reclaimable pages on the LRU.

  Change things so we bale out when SWAP_CLUSTER_MAX pages have been
  reclaimed.

Problem is, this change caused significant imbalance in inter-zone scan
balancing by truncating scans of larger zones.

Suppose, for example, ZONE_HIGHMEM is 10x the size of ZONE_NORMAL.  The zone
balancing algorithm would require that if we're scanning 100 pages of
ZONE_HIGHMEM, we should scan 10 pages of ZONE_NORMAL.  But this logic will
cause the scanning of ZONE_HIGHMEM to bale out after only 32 pages are
reclaimed.  Thus effectively causing smaller zones to be scanned relatively
harder than large ones.

Now I need to remember what the workload was which caused me to write this
patch originally, then fix it up in a different way...
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/vmscan.c

@@ -63,9 +63,6 @@ struct scan_control {
 
 	unsigned long nr_mapped;	/* From page_state */
 
-	/* How many pages shrink_cache() should reclaim */
-	int nr_to_reclaim;
-
 	/* Ask shrink_caches, or shrink_zone to scan at this priority */
 	unsigned int priority;
 
@@ -656,7 +653,6 @@ static void shrink_cache(struct zone *zone, struct scan_control *sc)
 		if (current_is_kswapd())
 			mod_page_state(kswapd_steal, nr_freed);
 		mod_page_state_zone(zone, pgsteal, nr_freed);
-		sc->nr_to_reclaim -= nr_freed;
 
 		spin_lock_irq(&zone->lru_lock);
 		/*
@@ -856,8 +852,6 @@ shrink_zone(struct zone *zone, struct scan_control *sc)
 	else
 		nr_inactive = 0;
 
-	sc->nr_to_reclaim = sc->swap_cluster_max;
-
 	while (nr_active || nr_inactive) {
 		if (nr_active) {
 			sc->nr_to_scan = min(nr_active,
@@ -871,8 +865,6 @@ shrink_zone(struct zone *zone, struct scan_control *sc)
 					(unsigned long)sc->swap_cluster_max);
 			nr_inactive -= sc->nr_to_scan;
 			shrink_cache(zone, sc);
-			if (sc->nr_to_reclaim <= 0)
-				break;
 		}
 	}