linux-kernel - Re: [PATCH 1/2] mm, oom: marks all killed tasks as oom victims

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Date:   Tue, 8 Jan 2019 05:58:41 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 1/2] mm, oom: marks all killed tasks as oom victims

On 2019/01/07 23:38, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
> 
> Historically we have called mark_oom_victim only to the main task
> selected as the oom victim because oom victims have access to memory
> reserves and granting the access to all killed tasks could deplete
> memory reserves very quickly and cause even larger problems.
> 
> Since only a partial access to memory reserves is allowed there is no
> longer this risk and so all tasks killed along with the oom victim
> can be considered as well.
> 
> The primary motivation for that is that process groups which do not
> shared signals would behave more like standard thread groups wrt oom
> handling (aka tsk_is_oom_victim will work the same way for them).
> 
> - Use find_lock_task_mm to stabilize mm as suggested by Tetsuo
> 
> Signed-off-by: Michal Hocko <mhocko@...e.com>
> ---
>  mm/oom_kill.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index f0e8cd9edb1a..0246c7a4e44e 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -892,6 +892,7 @@ static void __oom_kill_process(struct task_struct *victim)
>  	 */
>  	rcu_read_lock();
>  	for_each_process(p) {
> +		struct task_struct *t;
>  		if (!process_shares_mm(p, mm))
>  			continue;
>  		if (same_thread_group(p, victim))
> @@ -911,6 +912,11 @@ static void __oom_kill_process(struct task_struct *victim)
>  		if (unlikely(p->flags & PF_KTHREAD))
>  			continue;
>  		do_send_sig_info(SIGKILL, SEND_SIG_PRIV, p, PIDTYPE_TGID);
> +		t = find_lock_task_mm(p);
> +		if (!t)
> +			continue;
> +		mark_oom_victim(t);
> +		task_unlock(t);

Thank you for updating this patch. This patch is correct from the point of
view of avoiding TIF_MEMDIE race. But if I recall correctly, the reason we
did not do this is to avoid depleting memory reserves. And we still grant
full access to memory reserves for CONFIG_MMU=n case. Shouldn't the changelog
mention CONFIG_MMU=n case?

>  	}
>  	rcu_read_unlock();
>  
>