linux-kernel - Re: [PATCH 1/1] mm, oom_adj: don't loop through tasks in __set_oom

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Date:   Thu, 20 Aug 2020 16:36:27 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Michal Hocko <mhocko@...e.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        christian.brauner@...ntu.com, mingo@...nel.org,
        peterz@...radead.org, tglx@...utronix.de, esyr@...hat.com,
        christian@...lner.me, areber@...hat.com, shakeelb@...gle.com,
        cyphar@...har.com, adobriyan@...il.com, akpm@...ux-foundation.org,
        gladkov.alexey@...il.com, walken@...gle.com,
        daniel.m.jordan@...cle.com, avagin@...il.com,
        bernd.edlinger@...mail.de, john.johansen@...onical.com,
        laoar.shao@...il.com, timmurray@...gle.com, minchan@...nel.org,
        kernel-team@...roid.com, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/1] mm, oom_adj: don't loop through tasks in
 __set_oom_adj when not necessary

On 08/20, Oleg Nesterov wrote:
>
> On 08/20, Eric W. Biederman wrote:
> >
> > --- a/fs/exec.c
> > +++ b/fs/exec.c
> > @@ -1139,6 +1139,10 @@ static int exec_mmap(struct mm_struct *mm)
> >  	vmacache_flush(tsk);
> >  	task_unlock(tsk);
> >  	if (old_mm) {
> > +		mm->oom_score_adj = old_mm->oom_score_adj;
> > +		mm->oom_score_adj_min = old_mm->oom_score_adj_min;
> > +		if (tsk->vfork_done)
> > +			mm->oom_score_adj = tsk->vfork_oom_score_adj;
>
> too late, ->vfork_done is NULL after mm_release().
>
> And this can race with __set_oom_adj(). Yes, the current code is racy too,
> but this change adds another race, __set_oom_adj() could already observe
> ->mm != NULL and update mm->oom_score_adj.
  ^^^^^^^^^^^^

I meant ->mm == new_mm.

And another problem. Suppose we have

	if (!vfork()) {
		change_oom_score();
		exec();
	}

the parent can be killed before the child execs, in this case vfork_oom_score_adj
will be lost.

Oleg.