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

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

On 05/25, Michal Hocko wrote:
>
> On Fri 22-05-15 20:30:42, Oleg Nesterov wrote:
> > On 05/22, Michal Hocko wrote:
> > >
> > > On Fri 22-05-15 18:29:00, Oleg Nesterov wrote:
> > > >
> > > > In the likely case (if CLONE_VM without CLONE_THREAD was not used) the
> > > > last for_each_process() in mm_update_next_owner() will find another thread
> > > > from the same group.
> > >
> > > My understanding was that for_each_process will iterate only over
> > > processes (represented by the thread group leaders).
> >
> > Yes. But note the inner for_each_thread() loop. And note that we
> > we need this loop exactly because the leader can be zombie.
>
> I was too vague, sorry about that.

Looks like, we confused each other somehow ;) Not sure I understand your
concerns...

But,

> What I meant was that
> for_each_process would pick up a group leader

Yes. In the case above it will find the caller (current) too,

> and the inner
> for_each_thread will return it as the first element in the list.

Yes, and this will be "current" task. But current->mm == NULL, so
for_each_thread() will continue and find another thread which becomes
the new mm->owner.

Just in case, note the BUG_ON(c == p). I think that BUG_ON(p->mm) at
the start will look much better. This is what mm_update_next_owner()
actually assumes.

Oleg.