Re: [PATCH 1/1] mm, oom_adj: don't loop through tasks in __set_oom_adj when not necessary

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Aug 21, 2020 at 8:28 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Fri, Aug 21, 2020 at 4:16 AM Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > On 08/20, Eric W. Biederman wrote:
> > >
> > > That said if we are going for a small change why not:
> > >
> > >       /*
> > >        * Make sure we will check other processes sharing the mm if this is
> > >        * not vfrok which wants its own oom_score_adj.
> > >        * pin the mm so it doesn't go away and get reused after task_unlock
> > >        */
> > >       if (!task->vfork_done) {
> > >               struct task_struct *p = find_lock_task_mm(task);
> > >
> > >               if (p) {
> > > -                     if (atomic_read(&p->mm->mm_users) > 1) {
> > > +                     if (atomic_read(&p->mm->mm_users) > p->signal->nr_threads) {
> >
> > In theory this needs a barrier to avoid the race with do_exit(). And I'd
> > suggest to use signal->live, I think signal->nr_threads should die...
> > Something like
> >
> >         bool probably_has_other_mm_users(tsk)
> >         {
> >                 return  atomic_read_acquire(&tsk->mm->mm_users) >
> >                         atomic_read(&tsk->signal->live);
> >         }
> >
> > The barrier implied by _acquire ensures that if we race with the exiting
> > task and see the result of exit_mm()->mmput(mm), then we must also see
> > the result of atomic_dec_and_test(signal->live).
> >
> > Either way, if we want to fix the race with clone(CLONE_VM) we need other
> > changes.
>
> The way I understand this condition in __set_oom_adj() sync logic is
> that we would be ok with false positives (when we loop unnecessarily)
> but we can't tolerate false negatives (when oom_score_adj gets out of
> sync). With the clone(CLONE_VM) race not addressed we are allowing
> false negatives and IMHO that's not acceptable because it creates a
> possibility for userspace to get an inconsistent picture. When
> developing the patch I did think about using (p->mm->mm_users >
> p->signal->nr_threads) condition and had to reject it due to that
> reason.
>

Actually, reviewing again and considering where list_add_tail_rcu is
happening, maybe the race with clone(CLONE_VM) does not introduce
false negatives. However a false negative I think will happen when a
task shares mm with another task and also has an additional thread.
Shared mm will increment mm_users without adding to signal->live and
the additional thread will advance signal->live without adding to
mm_users. As a result these increments will balance themselves and
(mm->mm_users > signal->live) condition will yield false negative.

> >
> > Oleg.
> >