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

[Michal Hocko]

On Mon 22-08-22 17:20:17, Yu Zhao wrote:
> On Mon, Aug 22, 2022 at 5:16 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Mon, 22 Aug 2022 16:59:29 -0600 Yu Zhao <yuzhao@xxxxxxxxxx> 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