linux-kernel - Re: [PATCH v2] virtio_balloon: leak_balloon(): only tell host if we got pages deflated

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Date:	Thu, 6 Jun 2013 10:19:52 -0400
From:	Luiz Capitulino <lcapitulino@...hat.com>
To:	Rafael Aquini <aquini@...hat.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Subject: Re: [PATCH v2] virtio_balloon: leak_balloon(): only tell host if we
 got pages deflated

On Thu, 6 Jun 2013 11:13:58 -0300
Rafael Aquini <aquini@...hat.com> wrote:

> On Wed, Jun 05, 2013 at 09:18:37PM -0400, Luiz Capitulino wrote:
> > The balloon_page_dequeue() function can return NULL. If it does for
> > the first page being freed, then leak_balloon() will create a
> > scatter list with len=0. Which in turn seems to generate an invalid
> > virtio request.
> > 
> > I didn't get this in practice, I found it by code review. On the other
> > hand, such an invalid virtio request will cause errors in QEMU and
> > fill_balloon() also performs the same check implemented by this commit.
> > 
> > Signed-off-by: Luiz Capitulino <lcapitulino@...hat.com>
> > Acked-by: Rafael Aquini <aquini@...hat.com>
> > ---
> > 
> > o v2
> > 
> >  - Improve changelog
> > 
> >  drivers/virtio/virtio_balloon.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> > index bd3ae32..71af7b5 100644
> > --- a/drivers/virtio/virtio_balloon.c
> > +++ b/drivers/virtio/virtio_balloon.c
> > @@ -191,7 +191,8 @@ static void leak_balloon(struct virtio_balloon *vb, size_t num)
> >  	 * virtio_has_feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST);
> >  	 * is true, we *have* to do it in this order
> >  	 */
> > -	tell_host(vb, vb->deflate_vq);
> 
> Luiz, sorry for not being clearer before. I was referring to add a commentary on
> code, to explain in short words why we should not get rid of this check point.

Oh.

> > +	if (vb->num_pfns != 0)
> > +		tell_host(vb, vb->deflate_vq);
> >  	mutex_unlock(&vb->balloon_lock);
> 
> If the comment is regarded as unnecessary, then just ignore my suggestion. I'm
> OK with your patch. :)

IMHO, the code is clear enough.
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