[PATCH] don't shrink slab for highmem allocations

From: William Lee Irwin III <wli@holomorphy.com>

If one's goal is to free highmem pages, shrink_slab() is an ineffective
method of recovering them, as slab pages are all ZONE_NORMAL or ZONE_DMA.
Hence, this "FIXME: do not do for zone highmem".  Presumably this is a
question of policy, as highmem allocations may be satisfied by reaping slab
pages and handing them back; but the FIXME says what we should do.

mm/vmscan.c

@@ -135,8 +135,6 @@ void remove_shrinker(struct shrinker *shrinker)
  * If the vm encounted mapped pages on the LRU it increase the pressure on
  * slab to avoid swapping.
  *
- * FIXME: do not do for zone highmem
- *
  * We do weird things to avoid (scanned*seeks*entries) overflowing 32 bits.
  */
 static int shrink_slab(long scanned, unsigned int gfp_mask)
@@ -835,6 +833,7 @@ int try_to_free_pages(struct zone *classzone,
 
 		/* Take a nap, wait for some writeback to complete */
 		blk_congestion_wait(WRITE, HZ/10);
+		if (classzone - classzone->zone_pgdat->node_zones < ZONE_HIGHMEM)
 			shrink_slab(total_scanned, gfp_mask);
 	}
 	if ((gfp_mask & __GFP_FS) && !(gfp_mask & __GFP_NORETRY))
@@ -895,6 +894,7 @@ static int balance_pgdat(pg_data_t *pgdat, int nr_pages, struct page_state *ps)
 				max_scan = SWAP_CLUSTER_MAX;
 			to_free -= shrink_zone(zone, max_scan, GFP_KERNEL,
 					to_reclaim, &nr_mapped, ps, priority);
+			if (i < ZONE_HIGHMEM)
 				shrink_slab(max_scan + nr_mapped, GFP_KERNEL);
 			if (zone->all_unreclaimable)
 				continue;