[Devel] [PATCH v14 13/18] vmscan: take at least one pass with shrinkers

[Vladimir Davydov]

From: Glauber Costa <glommer at openvz.org>

In very low free kernel memory situations, it may be the case that we
have less objects to free than our initial batch size. If this is the
case, it is better to shrink those, and open space for the new workload
then to keep them and fail the new allocations.

In particular, we are concerned with the direct reclaim case for memcg.
Although this same technique can be applied to other situations just as
well, we will start conservative and apply it for that case, which is
the one that matters the most.

Signed-off-by: Glauber Costa <glommer at openvz.org>
Signed-off-by: Vladimir Davydov <vdavydov at parallels.com>
Cc: Dave Chinner <dchinner at redhat.com>
Cc: Michal Hocko <mhocko at suse.cz>
Cc: Johannes Weiner <hannes at cmpxchg.org>
Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: Mel Gorman <mgorman at suse.de>
Cc: Rik van Riel <riel at redhat.com>
---
 mm/vmscan.c |   13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 67c1950..95fd2c3 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -281,17 +281,22 @@ shrink_slab_node(struct shrink_control *shrinkctl, struct shrinker *shrinker,
 				nr_pages_scanned, lru_pages,
 				max_pass, delta, total_scan);
 
-	while (total_scan >= batch_size) {
+	while (total_scan > 0) {
 		unsigned long ret;
+		unsigned long nr_to_scan = min(batch_size, total_scan);
 
-		shrinkctl->nr_to_scan = batch_size;
+		if (!shrinkctl->target_mem_cgroup &&
+		    total_scan < batch_size)
+			break;
+
+		shrinkctl->nr_to_scan = nr_to_scan;
 		ret = shrinker->scan_objects(shrinker, shrinkctl);
 		if (ret == SHRINK_STOP)
 			break;
 		freed += ret;
 
-		count_vm_events(SLABS_SCANNED, batch_size);
-		total_scan -= batch_size;
+		count_vm_events(SLABS_SCANNED, nr_to_scan);
+		total_scan -= nr_to_scan;
 
 		cond_resched();
 	}
-- 
1.7.10.4