linux-kernel - Re: [PATCH] kmemcg: don't allocate extra memory for root memcg_cache

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Message-Id: <20130814134710.ff123b0ea802efa7261d7e26@linux-foundation.org>
Date:	Wed, 14 Aug 2013 13:47:10 -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>
Subject: Re: [PATCH] kmemcg: don't allocate extra memory for root
 memcg_cache_params

On Wed, 14 Aug 2013 14:31:21 +0400 Andrey Vagin <avagin@...nvz.org> wrote:

> 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);
> 
> ...
>
> --- 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 += sizeof(offsetof(struct memcg_cache_params, memcg_caches));

This looks wrong. offsetof() returns size_t, so this is equivalent to

		size += sizeof(size_t);

>  		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)

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