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Re: [PATCH v2 1/2] virtio-balloon: don't start free page hinting if post
From: |
Peter Xu |
Subject: |
Re: [PATCH v2 1/2] virtio-balloon: don't start free page hinting if postcopy is possible |
Date: |
Thu, 8 Jul 2021 14:51:55 -0400 |
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] e79fd18c-aa62-c1d8-c7f3-ba3fc2c25fc8@redhat.com">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