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

Michal Hocko
2012-06-11T07:34:50Z

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@google.com>

Acked-by: Michal Hocko <mhocko@suse.cz>

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