linux-kernel - Re: [PATCH] memcg, oom: lock mem_cgroup_print_oom

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Date:	Wed, 11 Dec 2013 14:23:18 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Michal Hocko <mhocko@...e.cz>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] memcg, oom: lock mem_cgroup_print_oom_info

On Wed, 11 Dec 2013, Michal Hocko wrote:

> mem_cgroup_print_oom_info uses a static buffer (memcg_name) to store the
> name of the cgroup. This is not safe as pointed out by David Rientjes
> because memcg oom is locked only for its hierarchy and nothing prevents
> another parallel hierarchy to trigger oom as well and overwrite the
> already in-use buffer.
> 
> This patch introduces oom_info_lock hidden inside mem_cgroup_print_oom_info
> which is held throughout the function. It make access to memcg_name safe
> and as a bonus it also prevents parallel memcg ooms to interleave their
> statistics which would make the printed data hard to analyze otherwise.
> 
> Signed-off-by: Michal Hocko <mhocko@...e.cz>

Acked-by: David Rientjes <rientjes@...gle.com>

> ---
>  mm/memcontrol.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 28c9221b74ea..c72b03bf9679 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1647,13 +1647,13 @@ static void move_unlock_mem_cgroup(struct mem_cgroup *memcg,
>   */
>  void mem_cgroup_print_oom_info(struct mem_cgroup *memcg, struct task_struct *p)
>  {
> -	struct cgroup *task_cgrp;
> -	struct cgroup *mem_cgrp;
>  	/*
> -	 * Need a buffer in BSS, can't rely on allocations. The code relies
> -	 * on the assumption that OOM is serialized for memory controller.
> -	 * If this assumption is broken, revisit this code.
> +	 * protects memcg_name and makes sure that parallel ooms do not
> +	 * interleave

Parallel memcg oom kills can happen in disjoint memcg hierarchies, this 
just prevents the printing of the statistics from interleaving.  I'm not 
sure if that's clear from this comment.
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