mm: vmscan: do not reclaim from kswapd if there is any eligible zone

kswapd scans from highest to lowest for a zone that requires balancing.
This was necessary when reclaim was per-zone to fairly age pages on
lower zones.  Now that we are reclaiming on a per-node basis, any
eligible zone can be used and pages will still be aged fairly.  This
patch avoids reclaiming excessively unless buffer_heads are over the
limit and it's necessary to reclaim from a higher zone than requested by
the waker of kswapd to relieve low memory pressure.

[hillf.zj@alibaba-inc.com: Force kswapd reclaim no more than needed]
Link: http://lkml.kernel.org/r/1466518566-30034-12-git-send-email-mgorman@techsingularity.net
Link: http://lkml.kernel.org/r/1467970510-21195-13-git-send-email-mgorman@techsingularity.net
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 8b39b903bd14dde14b25849d26cd560d7e7a943b..b7a276f4b1b02ef61ced58c24a0a5e1387dfa16c 100644 (file)
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3144,31 +3144,39 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
 
                sc.nr_reclaimed = 0;
 
-               /* Scan from the highest requested zone to dma */
-               for (i = classzone_idx; i >= 0; i--) {
-                       zone = pgdat->node_zones + i;
-                       if (!populated_zone(zone))
-                               continue;
-
-                       /*
-                        * If the number of buffer_heads in the machine
-                        * exceeds the maximum allowed level and this node
-                        * has a highmem zone, force kswapd to reclaim from
-                        * it to relieve lowmem pressure.
-                        */
-                       if (buffer_heads_over_limit && is_highmem_idx(i)) {
-                               classzone_idx = i;
-                               break;
-                       }
+               /*
+                * If the number of buffer_heads in the machine exceeds the
+                * maximum allowed level then reclaim from all zones. This is
+                * not specific to highmem as highmem may not exist but it is
+                * it is expected that buffer_heads are stripped in writeback.
+                */
+               if (buffer_heads_over_limit) {
+                       for (i = MAX_NR_ZONES - 1; i >= 0; i--) {
+                               zone = pgdat->node_zones + i;
+                               if (!populated_zone(zone))
+                                       continue;
 
-                       if (!zone_balanced(zone, order, 0)) {
                                classzone_idx = i;
                                break;
                        }
                }
 
-               if (i < 0)
-                       goto out;
+               /*
+                * Only reclaim if there are no eligible zones. Check from
+                * high to low zone as allocations prefer higher zones.
+                * Scanning from low to high zone would allow congestion to be
+                * cleared during a very small window when a small low
+                * zone was balanced even under extreme pressure when the
+                * overall node may be congested.
+                */
+               for (i = classzone_idx; i >= 0; i--) {
+                       zone = pgdat->node_zones + i;
+                       if (!populated_zone(zone))
+                               continue;
+
+                       if (zone_balanced(zone, sc.order, classzone_idx))
+                               goto out;
+               }
 
                /*
                 * Do some background aging of the anon list, to give
@@ -3213,19 +3221,6 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
                if (try_to_freeze() || kthread_should_stop())
                        break;
 
-               /*
-                * Stop reclaiming if any eligible zone is balanced and clear
-                * node writeback or congested.
-                */
-               for (i = 0; i <= classzone_idx; i++) {
-                       zone = pgdat->node_zones + i;
-                       if (!populated_zone(zone))
-                               continue;
-
-                       if (zone_balanced(zone, sc.order, classzone_idx))
-                               goto out;
-               }
-
                /*
                 * Raise priority if scanning rate is too low or there was no
                 * progress in reclaiming pages