Re: [PATCH v1 2/2] virtio-balloon: disallow postcopy with VIRTIO_BALLOON_F_FREE_PAGE_HINT

[David Hildenbrand]

On 07.07.21 20:02, Peter Xu wrote:
> On Wed, Jul 07, 2021 at 04:06:55PM +0200, David Hildenbrand wrote:
> > As it never worked properly, let's disable it via the postcopy notifier on
> > the destination. Trying to set "migrate_set_capability postcopy-ram on"
> > on the destination now results in "virtio-balloon: 'free-page-hint' does
> > not support postcopy Error: Postcopy is not supported".
>
> Would it be possible to do this in reversed order?  Say, dynamically disable
> free-page-hinting if postcopy capability is set when migration starts? Perhaps
> it can also be re-enabled automatically when migration completes?

I remember that this might be quite racy. We would have to make sure 
that no hinting happens before we enable the capability.

As soon as we messed with the dirty bitmap (during precopy), postcopy is 
no longer safe. As noted in the patch, the only runtime alternative is 
to disable postcopy as soon as we actually do clear a bit. 
Alternatively, we could ignore any hints if the postcopy capability was 
enabled.

Whatever we do, we have to make sure that a user cannot trick the system 
into an inconsistent state. Like enabling hinting, starting migration, 
then enabling the postcopy capability and kicking of postcopy. I did not 
check if we allow for that, though.

--
Thanks,

David / dhildenb