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 23:39:25 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Michal Hocko <mhocko@...e.com>
Cc:     Suren Baghdasaryan <surenb@...gle.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,
        Oleg Nesterov <oleg@...hat.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

Michal Hocko <mhocko@...e.com> writes:

> On Thu 20-08-20 08:56:53, Suren Baghdasaryan wrote:
> [...]
>> Catching up on the discussion which was going on while I was asleep...
>> So it sounds like there is a consensus that oom_adj should be moved to
>> mm_struct rather than trying to synchronize it among tasks sharing mm.
>> That sounds reasonable to me too. Michal answered all the earlier
>> questions about this patch, so I won't be reiterating them, thanks
>> Michal. If any questions are still lingering about the original patch
>> I'll be glad to answer them.
>
> I think it still makes some sense to go with a simpler (aka less tricky)
> solution which would be your original patch with an incremental fix for
> vfork and the proper ordering (http://lkml.kernel.org/r/20200820124109.GI5033@dhcp22.suse.cz)
> and then make a more complex shift to mm struct on top of that. The
> former will be less tricky to backport to stable IMHO.

So I am confused.

I don't know how a subtle dependency on something in clone
is better than something flat footed in exec.


That said if we are going for a small change why not:

	/*
	 * Make sure we will check other processes sharing the mm if this is
	 * not vfrok which wants its own oom_score_adj.
	 * pin the mm so it doesn't go away and get reused after task_unlock
	 */
	if (!task->vfork_done) {
		struct task_struct *p = find_lock_task_mm(task);

		if (p) {
-			if (atomic_read(&p->mm->mm_users) > 1) {
+			if (atomic_read(&p->mm->mm_users) > p->signal->nr_threads) {
				mm = p->mm;
				mmgrab(mm);
			}
			task_unlock(p);
		}
	}

That would seem to be the minimal change to make this happen.  That has
the advantage that if a processes does vfork it won't always have to
take the slow path.

Moving to the mm_struct is much less racy but this is simple.

Eric