Re: [Qemu-devel] [PATCH-for-4.1 v2 3/3] virtio-balloon: reset pbp on device resets

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

On Wed, Jul 17, 2019 at 01:06:29PM +0200, David Hildenbrand wrote:
> On 17.07.19 12:48, Michael S. Tsirkin wrote:
> > On Wed, Jul 17, 2019 at 12:35:50PM +0200, David Hildenbrand wrote:
> >> When a guest reboots (ordinary reboots, but also via kexec), it will
> >> happily reuse any system memory, including previously inflated memory.
> >>
> >> We could have tracking data for a pbp (PartiallyBalloonedPage). It could
> >> happen that a new inflation request from the guest will result in a
> >> discard of such a pbp, although the guest is (again) reusing some
> >> memory.
> >>
> >> We should reset the pbp on any device resets.
> >>
> >> Fixes: ed48c59875b6 ("virtio-balloon: Safely handle BALLOON_PAGE_SIZE <
> >>                      host page size")
> >> Cc: qemu-stable@nongnu.org #v4.0.0
> >> Cc: Stefan Hajnoczi <stefanha@redhat.com>
> >> Cc: David Gibson <david@gibson.dropbear.id.au>
> >> Cc: Michael S. Tsirkin <mst@redhat.com>
> >> Cc: Igor Mammedov <imammedo@redhat.com>
> >> Signed-off-by: David Hildenbrand <david@redhat.com>
> > 
> > Can't something else remove a ramblock besides a reset?
> 
> Yes, however this patch is not about ramblocks getting removed.
> 
> Take a close look, "balloon->pbp->rb" is only used as a token, it is
> never used besides for comparisons.


You are right but that's still not safe :)

E.g. the bit we are going to set could be out of range of the bitmap because
the backing page size changed.



> -- 
> 
> Thanks,
> 
> David / dhildenb