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

[Oleg Nesterov]

On 08/21, Suren Baghdasaryan wrote:
>
> On Fri, Aug 21, 2020 at 4:16 AM Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> >
> >         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).

Yes,

> 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.

Not sure I understand... I mean, the test_bit(MMF_PROC_SHARED) you propose
is equally racy and we need copy_oom_score() at the end of copy_process()
either way?

Oleg.