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 10:06:11 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Oleg Nesterov <oleg@...hat.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

Oleg Nesterov <oleg@...hat.com> writes:

> 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.

Yes.

Looking at include/uapi/linux/oom.h it appears that there are a lot of
oom_score_adj values that are reserved.  So it should be completely
possible to initialize vfork_oom_score_adj to -32768 aka SHRT_MIN, and
use that as a flag to see if it is active or not.

Likewise for vfork_oom_score_adj_min if we need to duplicate that one as
well.


That deals with that entire class of race.  We still have races during
exec about vfork_done being cleared before the new ->mm == new_mm.
While that is worth fixing is an independent issue.  

Eric