linux-kernel - Re: [PATCH RESEND v2 2/2] mm: delete unused MMF_OOM

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Date:   Mon, 22 Aug 2022 16:59:29 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Suren Baghdasaryan <surenb@...gle.com>,
        Michal Hocko <mhocko@...e.com>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>, Minchan Kim <minchan@...nel.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andrea Arcangeli <aarcange@...hat.com>, brauner@...nel.org,
        hch@...radead.org, oleg@...hat.com,
        David Hildenbrand <david@...hat.com>,
        Jann Horn <jannh@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Peter Xu <peterx@...hat.com>,
        John Hubbard <jhubbard@...dia.com>, shuah@...nel.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>, linux-kselftest@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH RESEND v2 2/2] mm: delete unused MMF_OOM_VICTIM flag

On Mon, Aug 22, 2022 at 4:48 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Mon, 22 Aug 2022 16:33:51 -0600 Yu Zhao <yuzhao@...gle.com> wrote:
>
> > > --- a/mm/vmscan.c~mm-delete-unused-mmf_oom_victim-flag-fix
> > > +++ a/mm/vmscan.c
> > > @@ -3429,9 +3429,6 @@ static bool should_skip_mm(struct mm_str
> > >         if (size < MIN_LRU_BATCH)
> > >                 return true;
> > >
> > > -       if (mm_is_oom_victim(mm))
> > > -               return true;
> > > -
> > >         return !mmget_not_zero(mm);
> > >  }
> > >
> > > @@ -4127,9 +4124,6 @@ restart:
> > >
> > >                 walk_pmd_range(&val, addr, next, args);
> > >
> > > -               if (mm_is_oom_victim(args->mm))
> > > -                       return 1;
> > > -
> > >                 /* a racy check to curtail the waiting time */
> > >                 if (wq_has_sleeper(&walk->lruvec->mm_state.wait))
> > >                         return 1;
> > > _
> > >
> > > Please confirm?
> >
> > LGTM.  The deleted checks are not about correctness.
>
> OK, for now.
>
> > I've queued
> >
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -3402,7 +3402,7 @@ static bool should_skip_mm(struct mm_struct *mm,
> > struct lru_gen_mm_walk *walk)
> >         if (size < MIN_LRU_BATCH)
> >                 return true;
> >
> > -       if (mm_is_oom_victim(mm))
> > +       if (test_bit(MMF_OOM_REAP_QUEUED, &mm->flags))
> >                 return true;
> >
> >         return !mmget_not_zero(mm);
> > @@ -4109,7 +4109,7 @@ static int walk_pud_range(p4d_t *p4d, unsigned
> > long start, unsigned long end,
> >
> >                 walk_pmd_range(&val, addr, next, args);
> >
> > -               if (mm_is_oom_victim(args->mm))
> > +               if (test_bit(MMF_OOM_REAP_QUEUED, &args->mm->flags))
> >                         return 1;
> >
> >                 /* a racy check to curtail the waiting time */
>
> Oh.  Why?  What does this change do?

The MMF_OOM_REAP_QUEUED flag is similar to the deleted MMF_OOM_VICTIM
flag, but it's set at a later stage during an OOM kill.

When either is set, the OOM reaper is probably already freeing the
memory of this mm_struct, or at least it's going to. So there is no
need to dwell on it in the reclaim path, hence not about correctness.