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[PATCH 12/27] mm: vmscan: Do not reclaim from kswapd if there is any eligible zone

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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.

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
 mm/vmscan.c | 32 +++++++++++++++++++-------------
 1 file changed, 19 insertions(+), 13 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 1f5d3829d35e..39ad2ab76a2b 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3123,24 +3123,30 @@ 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 */
+		/*
+		 * 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) {
+			for (i = MAX_NR_ZONES - 1; i >= 0; i++) {
+				zone = pgdat->node_zones + i;
+				if (!populated_zone(zone))
+					continue;
+
+				if (is_highmem_idx(i))
+					classzone_idx = i;
+				break;
+			}
+		}
+
+		/* Only reclaim if there are no eligible zones */
 		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 (!zone_balanced(zone, order, 0, 0)) {
+			if (!zone_balanced(zone, order, 0, classzone_idx)) {
 				classzone_idx = i;
 				break;
 			}
-- 
2.6.4

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[RFC PATCH 00/27] Move LRU page reclaim from zones to nodes v2 Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 14:50 +0100
  [PATCH 12/27] mm: vmscan: Do not reclaim from kswapd if there is any eligible zone Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 14:50 +0100
  [PATCH 11/27] mm, vmscan: Clear congestion, dirty and need for compaction on a per-node basis Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 14:50 +0100
  [PATCH 02/27] mm, vmscan: Check if cpusets are enabled during direct reclaim Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 14:50 +0100
  [PATCH 08/27] mm, vmscan: Make kswapd reclaim in terms of nodes Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 14:50 +0100
  [PATCH 13/27] mm, vmscan: Make shrink_node decisions more node-centric Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 14:50 +0100
  [PATCH 09/27] mm, vmscan: Simplify the logic deciding whether kswapd sleeps Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 14:50 +0100
  [PATCH 10/27] mm, vmscan: By default have direct reclaim only shrink once per node Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 14:50 +0100
  [PATCH 04/27] mm, vmscan: Move lru_lock to the node Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 14:50 +0100
  [PATCH 14/27] mm, memcg: Move memcg limit enforcement from zones to nodes Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 15:00 +0100
  [PATCH 15/27] mm, workingset: Make working set detection node-aware Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 15:00 +0100

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