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

From: David Hildenbrand
Subject: Re: [PATCH v1 2/2] virtio-balloon: disallow postcopy with VIRTIO_BALLOON_F_FREE_PAGE_HINT
Date: Wed, 7 Jul 2021 20:57:29 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0

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.


David / dhildenb

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