linux-kernel - Re: [PATCH] memcg: don't initialize kmem-cache destroying work for root caches

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Message-Id: <20130805130530.fd38ec4866ba7f1d9a400218@linux-foundation.org>
Date:	Mon, 5 Aug 2013 13:05:30 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Andrey Vagin <avagin@...nvz.org>
Cc:	linux-mm@...ck.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org, Glauber Costa <glommer@...nvz.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Balbir Singh <bsingharora@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	<stable@...r.kernel.org>
Subject: Re: [PATCH] memcg: don't initialize kmem-cache destroying work for
 root caches

On Mon,  5 Aug 2013 20:09:40 +0400 Andrey Vagin <avagin@...nvz.org> 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@...r.kernel.org>    [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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