linux-kernel - Re: [PATCH] memcg: do not expose uninitialized mem_cgroup_per

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Message-ID: <20110601123913.GC4266@tiehlicka.suse.cz>
Date:	Wed, 1 Jun 2011 14:39:13 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Igor Mammedov <imammedo@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kamezawa.hiroyu@...fujitsu.com,
	balbir@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] memcg: do not expose uninitialized mem_cgroup_per_node
 to world

On Wed 01-06-11 12:44:04, Igor Mammedov wrote:
> Freshly allocated 'mem_cgroup_per_node' list entries must be
> initialized before the rest of the kernel can see them. Otherwise
> zero initialized list fields can lead to race condition at
> mem_cgroup_force_empty_list:
>   pc = list_entry(list->prev, struct page_cgroup, lru);
> where 'pc' will be something like 0xfffffffc if list->prev is 0
> and cause page fault later when 'pc' is dereferenced.

Have you ever seen such a race? I do not see how this could happen.
mem_cgroup_force_empty_list is called only from
mem_cgroup_force_empty_write (aka echo whatever > group/force_empty)
or mem_cgroup_pre_destroy when the group is destroyed. 

The initialization code is, however, called before a group is
given for use AFICS.

I am not saying tha the change is bad, I like it, but I do not think it
is a fix for potential race condition.

> 
> Signed-off-by: Igor Mammedov <imammedo@...hat.com>
> ---
>  mm/memcontrol.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index bd9052a..ee7cb4c 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4707,7 +4707,6 @@ static int alloc_mem_cgroup_per_zone_info(struct mem_cgroup *mem, int node)
>  	if (!pn)
>  		return 1;
>  
> -	mem->info.nodeinfo[node] = pn;
>  	for (zone = 0; zone < MAX_NR_ZONES; zone++) {
>  		mz = &pn->zoneinfo[zone];
>  		for_each_lru(l)
> @@ -4716,6 +4715,7 @@ static int alloc_mem_cgroup_per_zone_info(struct mem_cgroup *mem, int node)
>  		mz->on_tree = false;
>  		mz->mem = mem;
>  	}
> +	mem->info.nodeinfo[node] = pn;
>  	return 0;
>  }
>  
> -- 
> 1.7.1
> 
> --
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-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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