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 11:53 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Fri, Aug 21, 2020 at 11:00 AM Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > 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.
>
> I see. So even though live is incremented after mm_users, the observer
> from __set_oom_adj still can see them becoming equal because it reads
> mm_users first.
>
> Do you see any such races if I incorporate the changes proposed by
> Michal in http://lkml.kernel.org/r/20200820124109.GI5033@dhcp22.suse.cz
> ? I have the new patch and I'm testing it right now. So far it behaves
> well but maybe I'm missing some rare race here that won't show up in
> my testing?
>

FYI: v2 of this patch with proposed changes is posted at:
https://lore.kernel.org/patchwork/patch/1294520/

>
> >
> > Oleg.
> >
> > --
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> >