On Wed, Jul 07, 2021 at 04:06:55PM +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.
OK so if source gets a request for a page which is not dirty
it does not respond immediately? Why not just teach it to
respond? It would seem that if destination wants a page we
should just give it to the destination ...
>
> While we could fix 1), for example, by calling
> virtio_balloon_free_page_done() via pre_save callbacks of the
> vmstate, 2) is mostly impossible to fix without additional tracking,
> such that we can actually identify these hinted pages and handle
> them accordingly.
> 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".
> Note 1: We could let qemu_guest_free_page_hint() mark postcopy
> as broken once actually clearing bits on the source. However, it's
> harder to realize as we can race with users starting postcopy
> and we cannot produce an expressive error message easily.
How about the reverse? Ignore qemu_guest_free_page_hint if postcopy
started? Seems better than making it user/guest visible ..
> 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.
>
> Fixes: c13c4153f76d ("virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT")
OK it's not too bad, but I wonder whether above aideas have been
explored.
> Cc: qemu-sta...@nongnu.org
> Cc: Wei Wang <wei.w.w...@intel.com>
> Cc: Michael S. Tsirkin <m...@redhat.com>
> Cc: Philippe Mathieu-Daudé <phi...@redhat.com>
> Cc: Alexander Duyck <alexander.du...@gmail.com>
> Cc: Juan Quintela <quint...@redhat.com>
> Cc: "Dr. David Alan Gilbert" <dgilb...@redhat.com>
> Cc: Peter Xu <pet...@redhat.com>
> Signed-off-by: David Hildenbrand <da...@redhat.com>
> ---
> hw/virtio/virtio-balloon.c | 26 ++++++++++++++++++++++++++
> include/hw/virtio/virtio-balloon.h | 1 +
> 2 files changed, 27 insertions(+)
>
> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> index 4b5d9e5e50..d0c9dc677c 100644
> --- a/hw/virtio/virtio-balloon.c
> +++ b/hw/virtio/virtio-balloon.c
> @@ -30,6 +30,7 @@
> #include "trace.h"
> #include "qemu/error-report.h"
> #include "migration/misc.h"
> +#include "migration/postcopy-ram.h"
>
> #include "hw/virtio/virtio-bus.h"
> #include "hw/virtio/virtio-access.h"
> @@ -692,6 +693,28 @@ virtio_balloon_free_page_hint_notify(NotifierWithReturn
> *n, void *data)
> return 0;
> }
>
> +
> +static int virtio_balloon_postcopy_notify(NotifierWithReturn *n, void
> *opaque)
> +{
> + VirtIOBalloon *dev = container_of(n, VirtIOBalloon, postcopy_notifier);
> + PostcopyNotifyData *pnd = opaque;
> +
> + /* We register the notifier only with 'free-page-hint=on' for now. */
> + g_assert(virtio_has_feature(dev->host_features,
> + VIRTIO_BALLOON_F_FREE_PAGE_HINT));
> +
> + /*
> + * Pages hinted via qemu_guest_free_page_hint() are cleared from the
> dirty
> + * bitmap and will not get migrated, especially also not when the
> postcopy
> + * destination starts using them and requests migration from the source;
> the
> + * faulting thread will stall until postcopy migration finishes and
> + * all threads are woken up.
> + */
> + error_setg(pnd->errp,
> + "virtio-balloon: 'free-page-hint' does not support postcopy");
> + return -ENOENT;
> +}
> +
> static size_t virtio_balloon_config_size(VirtIOBalloon *s)
> {
> uint64_t features = s->host_features;
> @@ -911,6 +934,7 @@ static void virtio_balloon_device_realize(DeviceState
> *dev, Error **errp)
> s->free_page_vq = virtio_add_queue(vdev, VIRTQUEUE_MAX_SIZE,
>
> virtio_balloon_handle_free_page_vq);
> precopy_add_notifier(&s->free_page_hint_notify);
> + postcopy_add_notifier(&s->postcopy_notifier);
>
> object_ref(OBJECT(s->iothread));
> s->free_page_bh = aio_bh_new(iothread_get_aio_context(s->iothread),
> @@ -935,6 +959,7 @@ static void virtio_balloon_device_unrealize(DeviceState
> *dev)
> object_unref(OBJECT(s->iothread));
> virtio_balloon_free_page_stop(s);
> precopy_remove_notifier(&s->free_page_hint_notify);
> + postcopy_remove_notifier(&s->postcopy_notifier);
> }
> balloon_stats_destroy_timer(s);
> qemu_remove_balloon_handler(s);
> @@ -1008,6 +1033,7 @@ static void virtio_balloon_instance_init(Object *obj)
> qemu_cond_init(&s->free_page_cond);
> s->free_page_hint_cmd_id = VIRTIO_BALLOON_FREE_PAGE_HINT_CMD_ID_MIN;
> s->free_page_hint_notify.notify = virtio_balloon_free_page_hint_notify;
> + s->postcopy_notifier.notify = virtio_balloon_postcopy_notify;
>
> object_property_add(obj, "guest-stats", "guest statistics",
> balloon_stats_get_all, NULL, NULL, s);
> diff --git a/include/hw/virtio/virtio-balloon.h
> b/include/hw/virtio/virtio-balloon.h
> index 5139cf8ab6..d0d5b793b9 100644
> --- a/include/hw/virtio/virtio-balloon.h
> +++ b/include/hw/virtio/virtio-balloon.h
> @@ -65,6 +65,7 @@ struct VirtIOBalloon {
> */
> bool block_iothread;
> NotifierWithReturn free_page_hint_notify;
> + NotifierWithReturn postcopy_notifier;
> int64_t stats_last_update;
> int64_t stats_poll_interval;
> uint32_t host_features;
> --
> 2.31.1
