Re: [PATCH 11/27] mm: vmscan: Do not reclaim from kswapd if there is any eligible zone

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On 06/09/2016 08:04 PM, Mel Gorman wrote:
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.

Looks like the code was even wrong before... if classzone_idx wasn't already set to the highmem zone in the first place, it wouldn't look at it.

Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

After fixing the bug below,

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

---
 mm/vmscan.c | 32 +++++++++++++++++++-------------
 1 file changed, 19 insertions(+), 13 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index e4f3e068b7a0..6663fc75c3bc 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3102,24 +3102,30 @@ 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++) {

                                                           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)) {
+			if (!zone_balanced(zone, sc.order, classzone_idx)) {
 				classzone_idx = i;
 				break;
 			}


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