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>

1 files changed, 27 insertions, 32 deletions

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 8b39b903bd14..b7a276f4b1b0 100644
--- 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
@@ -3214,19 +3222,6 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
                         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
                  */