linux-kernel - Re: [PATCH 1/1] mm, oom_adj: don't loop through tasks in __set_oom

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Message-ID: <20200821175943.GD19445@redhat.com>
Date:   Fri, 21 Aug 2020 19:59:43 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Michal Hocko <mhocko@...e.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Tim Murray <timmurray@...gle.com>, mingo@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, esyr@...hat.com,
        christian@...lner.me, areber@...hat.com,
        Shakeel Butt <shakeelb@...gle.com>, cyphar@...har.com,
        adobriyan@...il.com, Andrew Morton <akpm@...ux-foundation.org>,
        gladkov.alexey@...il.com, Michel Lespinasse <walken@...gle.com>,
        daniel.m.jordan@...cle.com, avagin@...il.com,
        bernd.edlinger@...mail.de,
        John Johansen <john.johansen@...onical.com>,
        laoar.shao@...il.com, Minchan Kim <minchan@...nel.org>,
        kernel-team <kernel-team@...roid.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH 1/1] mm, oom_adj: don't loop through tasks in
 __set_oom_adj when not necessary

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