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 12:55:56 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     mhocko@...e.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,
        ebiederm@...ssion.com, 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,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH 1/1] mm, oom_adj: don't loop through tasks in
 __set_oom_adj when not necessary

On 08/19, Suren Baghdasaryan wrote:
>
> Since the combination of CLONE_VM and !CLONE_SIGHAND is rarely
> used the additional mutex lock in that path of the clone() syscall should
> not affect its overall performance. Clearing the MMF_PROC_SHARED flag
> (when the last process sharing the mm exits) is left out of this patch to
> keep it simple and because it is believed that this threading model is
> rare.

vfork() ?

> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1403,6 +1403,15 @@ static int copy_mm(unsigned long clone_flags, struct task_struct *tsk)
>  	if (clone_flags & CLONE_VM) {
>  		mmget(oldmm);
>  		mm = oldmm;
> +		if (!(clone_flags & CLONE_SIGHAND)) {

I agree with Christian, you need CLONE_THREAD

> +			/* We need to synchronize with __set_oom_adj */
> +			mutex_lock(&oom_adj_lock);
> +			set_bit(MMF_PROC_SHARED, &mm->flags);
> +			/* Update the values in case they were changed after copy_signal */
> +			tsk->signal->oom_score_adj = current->signal->oom_score_adj;
> +			tsk->signal->oom_score_adj_min = current->signal->oom_score_adj_min;
> +			mutex_unlock(&oom_adj_lock);

I don't understand how this can close the race with __set_oom_adj...

What if __set_oom_adj() is called right after mutex_unlock() ? It will see
MMF_PROC_SHARED, but for_each_process() won't find the new child until
copy_process() does list_add_tail_rcu(&p->tasks, &init_task.tasks) ?

Oleg.