linux-kernel - Re: [PATCH v7 2/4] virtio_balloon: introduce migration primitives to balloon pages

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Date:	Tue, 14 Aug 2012 23:49:06 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Rafael Aquini <aquini@...hat.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mel@....ul.ie>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH v7 2/4] virtio_balloon: introduce migration primitives to
 balloon pages

On Tue, Aug 14, 2012 at 05:29:50PM -0300, Rafael Aquini wrote:
> On Tue, Aug 14, 2012 at 11:24:01PM +0300, Michael S. Tsirkin wrote:
> > On Tue, Aug 14, 2012 at 05:08:31PM -0300, Rafael Aquini wrote:
> > > On Tue, Aug 14, 2012 at 10:59:16PM +0300, Michael S. Tsirkin wrote:
> > > > > > > What if there is more than one balloon device?
> > > > > > 
> > > > > > Is it possible to load this driver twice, or are you foreseeing a future case
> > > > > > where this driver will be able to manage several distinct memory balloons for
> > > > > > the same guest?
> > > > > > 
> > > > > 
> > > > > Second.
> > > > > It is easy to create several balloons they are just
> > > > > pci devices.
> > > >  
> > > > 
> > > > 
> > > > and it might not be too important to make it work but
> > > > at least would be nice not to have a crash in this
> > > > setup.
> > > >
> > > Fair enough. For now, as I believe it's safe to assume we are only inflating one
> > > balloon per guest, I'd like to propose this as a future enhancement. Sounds
> > > good?
> > >  
> > 
> > Since guest crashes when it's not the case, no it doesn't, sorry :(.
> >
> Ok, but right now this driver only takes care of 1 balloon per guest,

It does? Are you sure? There is no global state as far as I can see. So
I can create 2 devices and driver will happily create two instances,
each one can be inflated/deflated independently.

> so how
> could this approach crash it? 

Add device. inflate. Add another device. inflate. deflate. unplug.
Now you have pointer to freed memory and when mm touches
page from first device, you ge use after free.

> Your point is a good thing to be on a to-do list for future enhancements, but
> it's not a dealbreaker for the present balloon driver implementation, IMHO.
> 

Yes it looks like a dealbreaker to me.

-- 
MST 
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