[PATCH v11 08/15] vmscan: take at least one pass with shrinkers

[Vladimir Davydov <vdavydov@parallels.com>]

From: Glauber Costa <glommer@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@openvz.org>
CC: Dave Chinner <dchinner@redhat.com>
CC: Carlos Maiolino <cmaiolino@redhat.com>
CC: "Theodore Ts'o" <tytso@mit.edu>
CC: Al Viro <viro@zeniv.linux.org.uk>
---
 mm/vmscan.c |   23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 36fc133..bfedcdc 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -311,20 +311,33 @@ 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) {
+	do {
 		unsigned long ret;
+		unsigned long nr_to_scan = min(batch_size, total_scan);
+		struct mem_cgroup *memcg = shrinkctl->target_mem_cgroup;
+
+		/*
+		 * Differentiate between "few objects" and "no objects"
+		 * as returned by the count step.
+		 */
+		if (!total_scan)
+			break;
+
+		if ((total_scan < batch_size) &&
+		   !(memcg && memcg_kmem_is_active(memcg)))
+			break;
 
-		shrinkctl->nr_to_scan = batch_size;
+		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();
-	}
+	} while (total_scan >= batch_size);
 
 	/*
 	 * move the unused scan count back into the shrinker in a
-- 
1.7.10.4

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