[PATCH] mm, vmscan: consider eligible zones in get_scan_count

[Michal Hocko]

get_scan_count considers the whole node LRU size when
- doing SCAN_FILE due to many page cache inactive pages
- calculating the number of pages to scan

in both cases this might lead to unexpected behavior especially on 32b
systems where we can expect lowmem memory pressure very often.

A large highmem zone can easily distort SCAN_FILE heuristic because
there might be only few file pages from the eligible zones on the node
lru and we would still enforce file lru scanning which can lead to
trashing while we could still scan anonymous pages.

The later use of lruvec_lru_size can be problematic as well. Especially
when there are not many pages from the eligible zones. We would have to
skip over many pages to find anything to reclaim but shrink_node_memcg
would only reduce the remaining number to scan by SWAP_CLUSTER_MAX
at maximum. Therefore we can end up going over a large LRU many times
without actually having chance to reclaim much if anything at all. The
closer we are out of memory on lowmem zone the worse the problem will
be.

Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
 mm/vmscan.c | 30 ++++++++++++++++++++++++++++--
 1 file changed, 28 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index c98b1a585992..785b4d7fb8a0 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -252,6 +252,32 @@ unsigned long lruvec_zone_lru_size(struct lruvec *lruvec, enum lru_list lru, int
 }
 
 /*
+ * Return the number of pages on the given lru which are eligibne for the
+ * given zone_idx
+ */
+static unsigned long lruvec_lru_size_zone_idx(struct lruvec *lruvec,
+		enum lru_list lru, int zone_idx)
+{
+	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
+	unsigned long lru_size;
+	int zid;
+
+	lru_size = lruvec_lru_size(lruvec, lru);
+	for (zid = zone_idx + 1; zid < MAX_NR_ZONES; zid++) {
+		struct zone *zone = &pgdat->node_zones[zid];
+		unsigned long size;
+
+		if (!managed_zone(zone))
+			continue;
+
+		size = lruvec_zone_lru_size(lruvec, lru, zid);
+		lru_size -= min(size, lru_size);
+	}
+
+	return lru_size;
+}
+
+/*
  * Add a shrinker callback to be called from the vm.
  */
 int register_shrinker(struct shrinker *shrinker)
@@ -2207,7 +2233,7 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
 	 * system is under heavy pressure.
 	 */
 	if (!inactive_list_is_low(lruvec, true, sc) &&
-	    lruvec_lru_size(lruvec, LRU_INACTIVE_FILE) >> sc->priority) {
+	    lruvec_lru_size_zone_idx(lruvec, LRU_INACTIVE_FILE, sc->reclaim_idx) >> sc->priority) {
 		scan_balance = SCAN_FILE;
 		goto out;
 	}
@@ -2274,7 +2300,7 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
 			unsigned long size;
 			unsigned long scan;
 
-			size = lruvec_lru_size(lruvec, lru);
+			size = lruvec_lru_size_zone_idx(lruvec, lru, sc->reclaim_idx);
 			scan = size >> sc->priority;
 
 			if (!scan && pass && force_scan)
-- 
2.10.2

-- 
Michal Hocko
SUSE Labs