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Date: Wed, 18 Jul 2012 14:36:12 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Cc: linux-mm@kvack.org, Michal Hocko <mhocko@suse.cz>,
        Johannes Weiner <hannes@cmpxchg.org>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Gavin Shan <shangw@linux.vnet.ibm.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/memcg: wrap mem_cgroup_from_css function
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On Wed, 18 Jul 2012 11:05:30 +0800
Wanpeng Li <liwanp@linux.vnet.ibm.com> wrote:

> wrap mem_cgroup_from_css function to clarify get mem cgroup
> from cgroup_subsys_state.

This certainly adds clarity.

But it also adds a little more type-safety - these container_of() calls
can be invoked against *any* struct which has a field called "css". 
With your patch, we add a check that the code is indeed using a
cgroup_subsys_state*.  A small thing, but it's all good.


I changed the patch title to the more idiomatic "memcg: add
mem_cgroup_from_css() helper" and rewrote the changelog to

: Add a mem_cgroup_from_css() helper to replace open-coded invokations of
: container_of().  To clarify the code and to add a little more type safety.

> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -396,6 +396,12 @@ static void mem_cgroup_put(struct mem_cgroup *memcg);
>  #include <net/sock.h>
>  #include <net/ip.h>
>  
> +static inline
> +struct mem_cgroup *mem_cgroup_from_css(struct cgroup_subsys_state *s)
> +{
> +	return container_of(s, struct mem_cgroup, css);
> +}

And with great self-control, I avoided renaming this to
memcg_from_css().  Sigh.  I guess all that extra typing has cardio
benefits.