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

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Message-ID: <20150920093332.GA20562@dhcp22.suse.cz>
Date:	Sun, 20 Sep 2015 11:33:33 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Oleg Nesterov <oleg@...hat.com>, Kyle Walker <kwalker@...hat.com>,
	Christoph Lameter <cl@...ux.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Vladimir Davydov <vdavydov@...allels.com>,
	linux-mm <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Stanislav Kozina <skozina@...hat.com>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Subject: Re: can't oom-kill zap the victim's memory?

On Sat 19-09-15 15:24:02, Linus Torvalds wrote:
> On Sat, Sep 19, 2015 at 8:03 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> > +
> > +static void oom_unmap_func(struct work_struct *work)
> > +{
> > +       struct mm_struct *mm = xchg(&oom_unmap_mm, NULL);
> > +
> > +       if (!atomic_inc_not_zero(&mm->mm_users))
> > +               return;
> > +
> > +       // If this is not safe we can do use_mm() + unuse_mm()
> > +       down_read(&mm->mmap_sem);
> 
> I don't think this is safe.
> 
> What makes you sure that we might not deadlock on the mmap_sem here?
> For all we know, the process that is going out of memory is in the
> middle of a mmap(), and already holds the mmap_sem for writing. No?
> 
> So at the very least that needs to be a trylock, I think.

Agreed.

> And I'm not
> sure zap_page_range() is ok with the mmap_sem only held for reading.
> Normally our rule is that you can *populate* the page tables
> concurrently, but you can't tear the down

Actually mmap_sem for reading should be sufficient because we do not
alter the layout. Both MADV_DONTNEED and MADV_FREE require read mmap_sem
for example.

-- 
Michal Hocko
SUSE Labs
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