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

[Michael S. Tsirkin]

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-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>
> ---
>  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