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

Oleg Nesterov
2020-08-21T18:00:27Z

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

On a second thought I agree that probably_has_other_mm_users() above can't
work ;) Compared to the test_bit(MMF_PROC_SHARED) check it is not _equally_
racy, it adds _another_ race with clone(CLONE_VM).

Suppose a single-threaded process P does

clone(CLONE_VM); // creates the child C

// mm_users == 2; P->signal->live == 1;

clone(CLONE_THREAD | CLONE_VM);

// mm_users == 3; P->signal->live == 2;

the problem is that in theory clone(CLONE_THREAD | CLONE_VM) can increment
_both_ counters between atomic_read_acquire(mm_users) and atomic_read(live)
in probably_has_other_mm_users() so it can observe mm_users == live == 2.

Oleg.