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

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Date:   Tue, 23 Aug 2022 10:36:15 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Suren Baghdasaryan <surenb@...gle.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,
        Christoph Hellwig <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 22-08-22 17:20:17, Yu Zhao wrote:
> On Mon, Aug 22, 2022 at 5:16 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
> >
> > On Mon, 22 Aug 2022 16:59:29 -0600 Yu Zhao <yuzhao@...gle.com> wrote:
> >
> > > > > @@ -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.
> >
> > Thanks.  That sounds worthy of some code comments?
> 
> Will do. Thanks.

I would rather not see this abuse. You cannot really make any
assumptions about oom_reaper and how quickly it is going to free the
memory. If this is really worth it (and I have to say I doubt it) then
it should be a separate patch with numbers justifying it.

-- 
Michal Hocko
SUSE Labs