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Subject[PATCH -mm v15 01/13] memcg: make cache index determination more robust
From: Glauber Costa <>

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 <>
Signed-off-by: Vladimir Davydov <>
Cc: Michal Hocko <>
Cc: Johannes Weiner <>
Cc: Balbir Singh <>
Cc: KAMEZAWA Hiroyuki <>
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)

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