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

    Oleg.

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