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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Aug 14, 2012 at 06:34:13PM -0300, Rafael Aquini wrote:
> On Tue, Aug 14, 2012 at 11:49:06PM +0300, Michael S. Tsirkin wrote:
> > 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.
> 
> Sorry. You're right, I'm wrong.
> 
> I'll get back to the scracthpad to overcome this constraint. I believe the way
> this patch was at its v4 revision (wrt this particular case) could possibly
> address this concern of yours.

Almost. We still have a global balloon_mapping. The only reason for
it to exist seems solely to detect balloon mappings, so it
can just be replaced by a flag in the mapping, or in mapping
ops, or elsewhere. Also, please add APIs to mm so we can
avoid doing internal mm stuff like

INIT_RADIX_TREE(&balloon_mapping->page_tree, GFP_ATOMIC | __GFP_NOWARN);

in the driver. It should be
alloc_address_mapping(&virtio_balloon_aops);
free_address_mapping

Make page->mapping use rcu, and sync rcu in
free_address_mapping.

-- 
MST

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