Re: [PATCH 3/7] memcg: immigrate charges only when a threadgroup leader is moved

[Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On 05/20, Michal Hocko wrote:
>
> On Wed 20-05-15 19:53:02, Oleg Nesterov wrote:
> >
> > Yes, yes, the group leader can't go away until the whole thread-group dies.
>
> OK, then we should have a guarantee that mm->owner is always thread
> group leader, right?

No, please note that the exiting leader does exit_mm()->mm_update_next_owner()
and this changes mm->owner.

Btw, this connects to other potential cleanups... task_struct->mm looks
a bit strange, we probably want to move it into signal_struct->mm and
make exit_mm/etc per-process. But this is not trivial, and off-topic.

Offtopic, because the exiting leader has to call mm_update_next_owner()
in any case. Simply because it does cgroup_exit() after that, so
get_mem_cgroup_from_mm() can't work if mm->owner is zombie.

This also means that get_mem_cgroup_from_mm() can race with the exiting
mm->owner unless I missed something.

> > But can't we kill mm->owner somehow?
>
> I would be happy about that. But it is not that simple.

Yes, yes, I understand.

Oleg.