linux-kernel - Re: [PATCH 07/10] mm, oom: fortify task_will_free

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Message-Id: <201606172038.IIE43237.FtLMVSFOOHJFQO@I-love.SAKURA.ne.jp>
Date:	Fri, 17 Jun 2016 20:38:01 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	mhocko@...nel.org
Cc:	linux-mm@...ck.org, rientjes@...gle.com, oleg@...hat.com,
	vdavydov@...allels.com, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 07/10] mm, oom: fortify task_will_free_mem

Michal Hocko wrote:
> > > Anyway, would you be OK with the patch if I added the current->mm check
> > > and resolve its necessity in a separate patch?
> > 
> > Please correct task_will_free_mem() in oom_kill_process() as well.
> 
> We cannot hold task_lock over all task_will_free_mem I am even not sure
> we have to develop an elaborate way to make it raceless just for the nommu
> case. The current case is simple as we cannot race here. Is that
> sufficient for you?

We can use find_lock_task_mm() inside mark_oom_victim().
That is, call wake_oom_reaper() from mark_oom_victim() like

void mark_oom_victim(struct task_struct *tsk, bool can_use_oom_reaper)
{
	WARN_ON(oom_killer_disabled);
	/* OOM killer might race with memcg OOM */
	tsk = find_lock_task_mm(tsk);
	if (!tsk)
		return;
	if (test_and_set_tsk_thread_flag(tsk, TIF_MEMDIE)) {
		task_unlock(tsk);
		return;
	}
	task_unlock(tsk);
	atomic_inc(&tsk->signal->oom_victims);
	/*
	 * Make sure that the task is woken up from uninterruptible sleep
	 * if it is frozen because OOM killer wouldn't be able to free
	 * any memory and livelock. freezing_slow_path will tell the freezer
	 * that TIF_MEMDIE tasks should be ignored.
	 */
	__thaw_task(tsk);
	atomic_inc(&oom_victims);
	if (can_use_oom_reaper)
		wake_oom_reaper(tsk);
}

and move mark_oom_victim() by normal path to after task_unlock(victim).

 	do_send_sig_info(SIGKILL, SEND_SIG_FORCED, victim, true);
-	mark_oom_victim(victim);

-	if (can_oom_reap)
-		wake_oom_reaper(victim);
+	wake_oom_reaper(victim, can_oom_reap);

If you don't like possibility of showing different pid for

	pr_err("Killed process %d (%s)

and

	pr_info("oom_reaper: reaped process %d (%s)

messages, you can defer the former till mark_oom_victim() locks that task.