Re: [PATCH v2 1/2] virtio-balloon: don't start free page hinting if postcopy is possible

[Peter Xu <peterx@redhat.com>]

On Thu, Jul 08, 2021 at 11:53:38AM +0200, David Hildenbrand wrote:
> Postcopy never worked properly with 'free-page-hint=on', as there are
> at least two issues:
> 
> 1) With postcopy, the guest will never receive a VIRTIO_BALLOON_CMD_ID_DONE
>    and consequently won't release free pages back to the OS once
>    migration finishes.
> 
>    The issue is that for postcopy, we won't do a final bitmap sync while
>    the guest is stopped on the source and
>    virtio_balloon_free_page_hint_notify() will only call
>    virtio_balloon_free_page_done() on the source during
>    PRECOPY_NOTIFY_CLEANUP, after the VM state was already migrated to
>    the destination.
> 
> 2) Once the VM touches a page on the destination that has been excluded
>    from migration on the source via qemu_guest_free_page_hint() while
>    postcopy is active, that thread will stall until postcopy finishes
>    and all threads are woken up. (with older Linux kernels that won't
>    retry faults when woken up via userfaultfd, we might actually get a
>    SEGFAULT)
> 
>    The issue is that the source will refuse to migrate any pages that
>    are not marked as dirty in the dirty bmap -- for example, because the
>    page might just have been sent. Consequently, the faulting thread will
>    stall, waiting for the page to be migrated -- which could take quite
>    a while and result in guest OS issues.
> 
> While we could fix 1) comparatively easily, 2) is harder to get right and
> might require more involved RAM migration changes on source and destination
> [1].
> 
> As it never worked properly, let's not start free page hinting in the
> precopy notifier if the postcopy migration capability was enabled to fix
> it easily. Capabilities cannot be enabled once migration is already
> running.
> 
> Note 1: in the future we might either adjust migration code on the source
>         to track pages that have actually been sent or adjust
>         migration code on source and destination  to eventually send
>         pages multiple times from the source and and deal with pages
>         that are sent multiple times on the destination.
> 
> Note 2: virtio-mem has similar issues, however, access to "unplugged"
>         memory by the guest is very rare and we would have to be very
>         lucky for it to happen during migration. The spec states
>         "The driver SHOULD NOT read from unplugged memory blocks ..."
>         and "The driver MUST NOT write to unplugged memory blocks".
>         virtio-mem will move away from virtio_balloon_free_page_done()
>         soon and handle this case explicitly on the destination.
> 
> [1] https://lkml.kernel.org/r/e79fd18c-aa62-c1d8-c7f3-ba3fc2c25fc8@redhat.com
> 
> Fixes: c13c4153f76d ("virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT")
> Cc: qemu-stable@nongnu.org
> Cc: Wei Wang <wei.w.wang@intel.com>
> Cc: Michael S. Tsirkin <mst@redhat.com>
> Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
> Cc: Alexander Duyck <alexander.duyck@gmail.com>
> Cc: Juan Quintela <quintela@redhat.com>
> Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> Cc: Peter Xu <peterx@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Reviewed-by: Peter Xu <peterx@redhat.com>

Thanks, David.

-- 
Peter Xu