Re: [PATCH] memcg: fix use_hierarchy css_is_ancestor oops regression

[Michal Hocko]

On Sun 10-06-12 11:54:47, Hugh Dickins wrote:
> If use_hierarchy is set, reclaim testing soon oopses in css_is_ancestor()
> called from __mem_cgroup_same_or_subtree() called from page_referenced():
> when processes are exiting, it's easy for mm_match_cgroup() to pass along
> a NULL memcg coming from a NULL mm->owner.
> 
> Check for that in __mem_cgroup_same_or_subtree().  Return true or false?
> False because we cannot know if it was in the hierarchy, but also false
> because it's better not to count a reference from an exiting process.
> 
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxx>

Thanks

> ---
> This a 3.5-rc issue: not needed for stable.
> 
>  mm/memcontrol.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- 3.5-rc2/mm/memcontrol.c	2012-05-30 08:17:19.400008280 -0700
> +++ linux/mm/memcontrol.c	2012-06-10 08:39:39.618182396 -0700
> @@ -1148,7 +1148,7 @@ bool __mem_cgroup_same_or_subtree(const
>  {
>  	if (root_memcg == memcg)
>  		return true;
> -	if (!root_memcg->use_hierarchy)
> +	if (!root_memcg->use_hierarchy || !memcg)
>  		return false;
>  	return css_is_ancestor(&memcg->css, &root_memcg->css);
>  }

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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