Re: [PATCH] memcg: don't initialize kmem-cache destroying work forroot caches

From: Andrew Morton
Date: Mon Aug 05 2013 - 16:05:37 EST

On Mon, 5 Aug 2013 20:09:40 +0400 Andrey Vagin <avagin@xxxxxxxxxx> wrote:

> struct memcg_cache_params has a union. Different parts of this union
> are used for root and non-root caches. A part with destroying work is
> used only for non-root caches.
> I fixed the same problem in another place v3.9-rc1-16204-gf101a94, but
> didn't notice this one.
> Cc: <stable@xxxxxxxxxxxxxxx> [3.9.x]

hm, why the cc:stable?

> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3195,11 +3195,11 @@ int memcg_register_cache(struct mem_cgroup *memcg, struct kmem_cache *s,
> if (!s->memcg_params)
> return -ENOMEM;
> - INIT_WORK(&s->memcg_params->destroy,
> - kmem_cache_destroy_work_func);
> if (memcg) {
> s->memcg_params->memcg = memcg;
> s->memcg_params->root_cache = root_cache;
> + INIT_WORK(&s->memcg_params->destroy,
> + kmem_cache_destroy_work_func);
> } else
> s->memcg_params->is_root_cache = true;

So the bug here is that we'll scribble on some entries in
memcg_caches[]. Those scribbles may or may not be within the part of
that array which is actually used. If there's code which expects
memcg_caches[] entries to be zeroed at initialisation then yes, we have
a problem.

But I rather doubt whether this bug was causing runtime problems?

Presently memcg_register_cache() allocates too much memory for the
memcg_caches[] array. If that was fixed then this INIT_WORK() might
scribble into unknown memory, which is of course serious.
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