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Date: Sun, 20 Sep 2015 15:06:16 +0200
From: Oleg Nesterov <oleg@redhat.com>
Subject: Re: can't oom-kill zap the victim's memory?
Message-ID: <20150920130616.GB2104@redhat.com>
References: <1442512783-14719-1-git-send-email-kwalker@redhat.com> <20150919150316.GB31952@redhat.com> <CA+55aFwkvbMrGseOsZNaxgP3wzDoVjkGasBKFxpn07SaokvpXA@mail.gmail.com> <20150920093332.GA20562@dhcp22.suse.cz>
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To: Michal Hocko <mhocko@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, Kyle Walker <kwalker@redhat.com>, Christoph Lameter <cl@linux.com>, Andrew Morton <akpm@linux-foundation.org>, David Rientjes <rientjes@google.com>, Johannes Weiner <hannes@cmpxchg.org>, Vladimir Davydov <vdavydov@parallels.com>, linux-mm <linux-mm@kvack.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Stanislav Kozina <skozina@redhat.com>, Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>

On 09/20, Michal Hocko wrote:
>
> On Sat 19-09-15 15:24:02, Linus Torvalds wrote:
> > On Sat, Sep 19, 2015 at 8:03 AM, Oleg Nesterov <oleg@redhat.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.

Why? See my reply to Linus's email.

Just in case, yes sure the unconditonal down_read() is suboptimal, but
this is minor compared to other problems we need to solve.

> > 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.

Yes, but see the ->vm_flags check in madvise_dontneed().

Oleg.

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