[PATCH v14 18/18] memcg: flush memcg items upon memcg destruction

[Vladimir Davydov <vdavydov@parallels.com>]

From: Glauber Costa <glommer@openvz.org>

When a memcg is destroyed, it won't be imediately released until all
objects are gone. This means that if a memcg is restarted with the very
same workload - a very common case, the objects already cached won't be
billed to the new memcg. This is mostly undesirable since a container
can exploit this by restarting itself every time it reaches its limit,
and then coming up again with a fresh new limit.

Since now we have targeted reclaim, I sustain that we should assume that
a memcg that is destroyed should be flushed away. It makes perfect sense
if we assume that a memcg that goes away most likely indicates an
isolated workload that is terminated.

Signed-off-by: Glauber Costa <glommer@openvz.org>
Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
 mm/memcontrol.c |   28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 963285f..28d5472 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6162,12 +6162,40 @@ static void memcg_destroy_kmem(struct mem_cgroup *memcg)
 	memcg_destroy_all_lrus(memcg);
 }
 
+static void memcg_drop_slab(struct mem_cgroup *memcg)
+{
+	struct shrink_control shrink = {
+		.gfp_mask = GFP_KERNEL,
+		.target_mem_cgroup = memcg,
+	};
+	unsigned long nr_objects;
+
+	nodes_setall(shrink.nodes_to_scan);
+	do {
+		nr_objects = shrink_slab(&shrink, 1000, 1000);
+	} while (nr_objects > 0);
+}
+
 static void kmem_cgroup_css_offline(struct mem_cgroup *memcg)
 {
 	if (!memcg_kmem_is_active(memcg))
 		return;
 
 	/*
+	 * When a memcg is destroyed, it won't be imediately released until all
+	 * objects are gone. This means that if a memcg is restarted with the
+	 * very same workload - a very common case, the objects already cached
+	 * won't be billed to the new memcg. This is mostly undesirable since a
+	 * container can exploit this by restarting itself every time it
+	 * reaches its limit, and then coming up again with a fresh new limit.
+	 *
+	 * Therefore a memcg that is destroyed should be flushed away. It makes
+	 * perfect sense if we assume that a memcg that goes away indicates an
+	 * isolated workload that is terminated.
+	 */
+	memcg_drop_slab(memcg);
+
+	/*
 	 * kmem charges can outlive the cgroup. In the case of slab
 	 * pages, for instance, a page contain objects from various
 	 * processes. As we prevent from taking a reference for every
-- 
1.7.10.4

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