Re: [patch 2/3] mm: memcontrol: pull the NULL check from __mem_cgroup_same_or_subtree()

Vladimir Davydov
2014-10-24T14:49:03Z

On Fri, Oct 24, 2014 at 09:49:48AM -0400, Johannes Weiner wrote:
> The NULL in mm_match_cgroup() comes from a possibly exiting mm->owner.
> It makes a lot more sense to check where it's looked up, rather than
> check for it in __mem_cgroup_same_or_subtree() where it's unexpected.
>
> No other callsite passes NULL to __mem_cgroup_same_or_subtree().
>
> Signed-off-by: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>

Reviewed-by: Vladimir Davydov <vdavydov-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org>

> ---
> include/linux/memcontrol.h | 5 +++--
> mm/memcontrol.c | 2 +-
> 2 files changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index ea007615e8f9..e32ab948f589 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -83,11 +83,12 @@ static inline
> bool mm_match_cgroup(const struct mm_struct *mm, const struct mem_cgroup *memcg)
> {
> struct mem_cgroup *task_memcg;
> - bool match;
> + bool match = false;
>
> rcu_read_lock();
> task_memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
> - match = __mem_cgroup_same_or_subtree(memcg, task_memcg);
> + if (task_memcg)
> + match = __mem_cgroup_same_or_subtree(memcg, task_memcg);
> rcu_read_unlock();
> return match;
> }
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index bdf8520979cf..15b1c5110a8f 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1316,7 +1316,7 @@ bool __mem_cgroup_same_or_subtree(const struct mem_cgroup *root_memcg,
> {
> if (root_memcg == memcg)
> return true;
> - if (!root_memcg->use_hierarchy || !memcg)
> + if (!root_memcg->use_hierarchy)
> return false;
> return cgroup_is_descendant(memcg->css.cgroup, root_memcg->css.cgroup);
> }
> --
> 2.1.2
>