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

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Message-ID: <CAJuCfpGcmSmMc2zEBR-0FgbeZVre+Tbqmu04y8deEwySasvD3Q@mail.gmail.com>
Date:   Fri, 21 Aug 2020 10:22:34 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Michal Hocko <mhocko@...e.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Tim Murray <timmurray@...gle.com>, mingo@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, esyr@...hat.com,
        christian@...lner.me, areber@...hat.com,
        Shakeel Butt <shakeelb@...gle.com>, cyphar@...har.com,
        adobriyan@...il.com, Andrew Morton <akpm@...ux-foundation.org>,
        gladkov.alexey@...il.com, Michel Lespinasse <walken@...gle.com>,
        daniel.m.jordan@...cle.com, avagin@...il.com,
        bernd.edlinger@...mail.de,
        John Johansen <john.johansen@...onical.com>,
        laoar.shao@...il.com, Minchan Kim <minchan@...nel.org>,
        kernel-team <kernel-team@...roid.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, linux-mm <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 Fri, Aug 21, 2020 at 9:37 AM Oleg Nesterov <oleg@...hat.com> wrote:
>
> again, don't really understand...
>
> On 08/21, Suren Baghdasaryan wrote:
> >
> > Actually, reviewing again and considering where list_add_tail_rcu is
> > happening, maybe the race with clone(CLONE_VM) does not introduce
> > false negatives.
>
> I think it does... Whatever we check, mm_users or MMF_PROC_SHARED,
> the task can do clone(CLONE_VM) right after the check.

Ah, yes of course. I missed this same just like in the original patch.

>
> > However a false negative I think will happen when a
> > task shares mm with another task and also has an additional thread.
> > Shared mm will increment mm_users without adding to signal->live
>
> Yes,
>
> > and
> > the additional thread will advance signal->live without adding to
> > mm_users.
>
> No, please note that CLONE_THREAD requires CLONE_VM.

My fault. Forgot that CLONE_VM means "share VM" and not "dup VM". Need
some coffee.
Thanks Oleg!

>
> Oleg.
>