linux-kernel - Re: can't oom-kill zap the victim's memory?

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Message-ID: <20150922144551.GA31154@redhat.com>
Date:	Tue, 22 Sep 2015 16:45:51 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	mhocko@...nel.org, torvalds@...ux-foundation.org,
	kwalker@...hat.com, cl@...ux.com, akpm@...ux-foundation.org,
	rientjes@...gle.com, hannes@...xchg.org, vdavydov@...allels.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	skozina@...hat.com
Subject: Re: can't oom-kill zap the victim's memory?

On 09/22, Tetsuo Handa wrote:
>
> Oleg Nesterov wrote:
> > On 09/22, Tetsuo Handa wrote:
> > > 	rcu_read_lock();
> > > 	for_each_process_thread(g, p) {
> > > 		if (likely(!fatal_signal_pending(p)))
> > > 			continue;
> > > 		task_lock(p);
> > > 		mm = p->mm;
> > > 		if (mm && mm->mmap && !mm->mmap_zapped && down_read_trylock(&mm->mmap_sem)) {
> >                                        ^^^^^^^^^^^^^^^
> >
> > We do not want mm->mmap_zapped, it can't work. We need mm->needs_zap
> > set by oom_kill_process() and cleared after zap_page_range().
> >
> > Because otherwise we can not handle CLONE_VM correctly. Suppose that
> > an innocent process P does vfork() and the child is killed but not
> > exited yet. mm_zapper() can find the child, do zap_page_range(), and
> > surprise its alive parent P which uses the same ->mm.
>
> kill(P's-child, SIGKILL) does not kill P sharing the same ->mm.
> Thus, mm_zapper() can be used for only OOM-kill case

Yes, and only if we know for sure that all tasks which can use
this ->mm were killed.

> and
> test_tsk_thread_flag(p, TIF_MEMDIE) should be used than
> fatal_signal_pending(p).

No. For example, just look at mark_oom_victim() at the start of
out_of_memory().

> > Tetsuo, can't we do something simple which "obviously can't hurt at
> > least" and then discuss the potential improvements?
>
> No problem. I can wait for your version.

All I wanted to say is that this all is a bit more complicated than it
looks at first glance.

Oleg.

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