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

From: Alexander Duyck
Subject: Re: [PATCH v1 2/2] virtio-balloon: disallow postcopy with VIRTIO_BALLOON_F_FREE_PAGE_HINT
Date: Wed, 7 Jul 2021 14:22:32 -0700

On Wed, Jul 7, 2021 at 1:08 PM Peter Xu <peterx@redhat.com> wrote:
> On Wed, Jul 07, 2021 at 08:57:29PM +0200, David Hildenbrand wrote:
> > 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.
> Logically migration capabilities are applied at VM starts, and these
> capabilities should be constant during migration (I didn't check if there's a
> hard requirement; easy to add that if we want to assure it), and in most cases
> for the lifecycle of the vm.

Would it make sense to maybe just look at adding a postcopy value to
the PrecopyNotifyData that you could populate with
migration_in_postcopy() in precopy_notify()?

Then all you would need to do is check for that value and if it is set
you shut down the page hinting or don't start it since I suspect it
wouldn't likely add any value anyway since I would think flagging
unused pages doesn't add much value in a postcopy environment anyway.

> >
> > 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.
> We could turn free page hinting off when migration starts with 
> postcopy-ram=on,
> then re-enable it after migration finishes.  That looks very safe to me.  And 
> I
> don't even worry on user trying to mess it up - as that only put their own VM
> at risk; that's mostly fine to me.

We wouldn't necessarily even need to really turn it off, just don't
start it. I wonder if we couldn't just get away with adding a check to
the existing virtio_balloon_free_page_hint_notify to see if we are in
the postcopy state there and just shut things down or not start them.

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