[PATCH v12 14/18] vmscan: take at least one pass with shrinkers

[Vladimir Davydov <vdavydov@parallels.com>]

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

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 10e6b2f..749e492 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -287,17 +287,22 @@ shrink_slab_node(struct shrink_control *shrinkctl, struct shrinker *shrinker,
 				fraction, denominator,
 				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

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