Re: [PATCH 8/8] memcg: get rid of mem_cgroup_from_task

Michal Hocko
2015-07-09T14:13:31Z

On Wed 08-07-15 20:43:31, Vladimir Davydov wrote:
> On Wed, Jul 08, 2015 at 02:27:52PM +0200, Michal Hocko wrote:
[...]
> > @@ -1091,12 +1079,14 @@ bool task_in_mem_cgroup(struct task_struct *task, struct mem_cgroup *memcg)
> > task_unlock(p);
> > } else {
> > /*
> > - * All threads may have already detached their mm's, but the oom
> > - * killer still needs to detect if they have already been oom
> > - * killed to prevent needlessly killing additional tasks.
> > + * All threads have already detached their mm's but we should
> > + * still be able to at least guess the original memcg from the
> > + * task_css. These two will match most of the time but there are
> > + * corner cases where task->mm and task_css refer to a different
> > + * cgroups.
> > */
> > rcu_read_lock();
> > - task_memcg = mem_cgroup_from_task(task);
> > + task_memcg = mem_cgroup_from_css(task_css(task, memory_cgrp_id));
> > css_get(&task_memcg->css);
>
> I wonder why it's safe to call css_get here.

What do you mean by safe? Memcg cannot go away because we are under rcu
lock.

>
> The patch itself looks good though,
>
> Reviewed-by: Vladimir Davydov <vdavydov@parallels.com>

Thanks!
--
Michal Hocko
SUSE Labs