[PATCH -mm v15 01/13] memcg: make cache index determination more robust

[Vladimir Davydov <vdavydov@parallels.com>]

From: Glauber Costa <glommer@openvz.org>

I caught myself doing something like the following outside memcg core:

	memcg_id = -1;
	if (memcg && memcg_kmem_is_active(memcg))
		memcg_id = memcg_cache_id(memcg);

to be able to handle all possible memcgs in a sane manner. In particular, the
root cache will have kmemcg_id = -1 (just because we don't call memcg_kmem_init
to the root cache since it is not limitable). We have always coped with that by
making sure we sanitize which cache is passed to memcg_cache_id. Although this
example is given for root, what we really need to know is whether or not a
cache is kmem active.

But outside the memcg core testing for root, for instance, is not trivial since
we don't export mem_cgroup_is_root. I ended up realizing that this tests really
belong inside memcg_cache_id. This patch moves a similar but stronger test
inside memcg_cache_id and make sure it always return a meaningful value.

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 |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 53385cd4e6f0..75758fc5c50c 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3110,7 +3110,9 @@ static void memcg_uncharge_kmem(struct mem_cgroup *memcg, u64 size)
  */
 int memcg_cache_id(struct mem_cgroup *memcg)
 {
-	return memcg ? memcg->kmemcg_id : -1;
+	if (!memcg || !memcg_can_account_kmem(memcg))
+		return -1;
+	return memcg->kmemcg_id;
 }
 
 static size_t memcg_caches_array_size(int num_groups)
-- 
1.7.10.4

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