[PATCH] kmemcg: don't allocate extra memory for root memcg_cache_params (v2)

[Andrey Vagin <avagin@openvz.org>]

The memcg_cache_params structure contains the common part and the union,
which represents two different types of data: one for root cashes and
another for child caches.

The size of child data is fixed. The size of the memcg_caches array is
calculated in runtime.

Currently the size of memcg_cache_params for root caches is calculated
incorrectly, because it includes the size of parameters for child caches.

ssize_t size = memcg_caches_array_size(num_groups);
size *= sizeof(void *);

size += sizeof(struct memcg_cache_params);

v2: Fix a typo in calculations

Cc: Glauber Costa <glommer@openvz.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
---
 mm/memcontrol.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c5792a5..2e47fea 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3140,7 +3140,7 @@ int memcg_update_cache_size(struct kmem_cache *s, int num_groups)
 		ssize_t size = memcg_caches_array_size(num_groups);
 
 		size *= sizeof(void *);
-		size += sizeof(struct memcg_cache_params);
+		size += offsetof(struct memcg_cache_params, memcg_caches);
 
 		s->memcg_params = kzalloc(size, GFP_KERNEL);
 		if (!s->memcg_params) {
@@ -3183,13 +3183,16 @@ int memcg_update_cache_size(struct kmem_cache *s, int num_groups)
 int memcg_register_cache(struct mem_cgroup *memcg, struct kmem_cache *s,
 			 struct kmem_cache *root_cache)
 {
-	size_t size = sizeof(struct memcg_cache_params);
+	size_t size;
 
 	if (!memcg_kmem_enabled())
 		return 0;
 
-	if (!memcg)
+	if (!memcg) {
+		size = offsetof(struct memcg_cache_params, memcg_caches);
 		size += memcg_limited_groups_array_size * sizeof(void *);
+	} else
+		size = sizeof(struct memcg_cache_params);
 
 	s->memcg_params = kzalloc(size, GFP_KERNEL);
 	if (!s->memcg_params)
-- 
1.8.3.1

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