Re: [PATCH RESEND v2 2/2] mm: delete unused MMF_OOM_VICTIM flag

[Yu Zhao <yuzhao@google.com>]

On Tue, Aug 23, 2022 at 2:36 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Mon 22-08-22 17:20:17, Yu Zhao wrote:
> > On Mon, Aug 22, 2022 at 5:16 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> > >
> > > On Mon, 22 Aug 2022 16:59:29 -0600 Yu Zhao <yuzhao@google.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.

I understand where you're coming from, however, I don't share this
POV. I see it as cooperation -- the page reclaim and the oom/reaper
can't (or at least shouldn't) operate in isolation.

> You cannot really make any
> assumptions about oom_reaper and how quickly it is going to free the
> memory.

Agreed. But here we are talking about heuristics, not dependencies on
certain behaviors. Assume we are playing a guessing game: there are
multiple mm_structs available for reclaim, would the oom-killed ones
be more profitable on average? I'd say no, because I assume it's more
likely than unlikely that the oom reaper is doing/to do its work. Note
that the assumption is about likelihood, hence arguably valid.

> 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.

I definitely can artificially create a test case that runs oom a few
times per second, to prove this two-liner is beneficial to that
scenario. Then there is the question how much it would benefit the
real-world scenarios.

I'd recommend keeping this two-liner if we still had
mm_is_oom_victim(), because it's simple, clear and intuitive. With
MMF_OOM_REAP_QUEUED, I don't have a strong opinion. Since you do, I'll
just delete it.