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    Subject[PATCH v4 12/12] virtio: balloon: Add freeze, restore handlers to support S4
    Date
    Handling balloon hibernate / restore is tricky.  If the balloon was
    inflated before going into the hibernation state, upon resume, the host
    will not have any memory of that. Any pages that were passed on to the
    host earlier would most likely be invalid, and the host will have to
    re-balloon to the previous value to get in the pre-hibernate state.

    So the only sane thing for the guest to do here is to discard all the
    pages that were put in the balloon. When to discard the pages is the
    next question.

    One solution is to deflate the balloon just before writing the image to
    the disk (in the freeze() PM callback). However, asking for pages from
    the host just to discard them immediately after seems wasteful of
    resources. Hence, it makes sense to do this by just fudging our
    counters soon after wakeup. This means we don't deflate the balloon
    before sleep, and also don't put unnecessary pressure on the host.

    This also helps in the thaw case: if the freeze fails for whatever
    reason, the balloon should continue to remain in the inflated state.
    This was tested by issuing 'swapoff -a' and trying to go into the S4
    state. That fails, and the balloon stays inflated, as expected. Both
    the host and the guest are happy.

    Now to not race with a host issuing ballooning requests while we are in
    the process of freezing, we just exit from the vballoon kthread when the
    processes are asked to freeze. Upon thaw and restore, we re-start the
    thread.

    Finally, in the restore() callback, we empty the list of pages that were
    previously given off to the host, add the appropriate number of pages to
    the totalram_pages counter, reset the num_pages counter to 0, and
    all is fine.

    As a last step, delete the vqs on the freeze callback to prepare for
    hibernation, and re-create them in the restore and thaw callbacks to
    resume normal operation.

    Signed-off-by: Amit Shah <amit.shah@redhat.com>
    ---
    drivers/virtio/virtio_balloon.c | 79 ++++++++++++++++++++++++++++++++++++++-
    1 files changed, 78 insertions(+), 1 deletions(-)

    diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
    index 8bf99be..10ec638 100644
    --- a/drivers/virtio/virtio_balloon.c
    +++ b/drivers/virtio/virtio_balloon.c
    @@ -258,7 +258,13 @@ static int balloon(void *_vballoon)
    while (!kthread_should_stop()) {
    s64 diff;

    - try_to_freeze();
    + /*
    + * On suspend, we want to exit this thread. We will
    + * start a new thread on resume.
    + */
    + if (freezing(current))
    + break;
    +
    wait_event_interruptible(vb->config_change,
    (diff = towards_target(vb)) != 0
    || vb->need_stats_update
    @@ -365,6 +371,72 @@ static void __devexit virtballoon_remove(struct virtio_device *vdev)
    kfree(vb);
    }

    +#ifdef CONFIG_PM
    +static int virtballoon_freeze(struct virtio_device *vdev)
    +{
    + /* Ensure we don't get any more requests from the host */
    + vdev->config->reset(vdev);
    +
    + /*
    + * The kthread is already gone as a result of the PM code
    + * issuing a freeze request.
    + */
    +
    + vdev->config->del_vqs(vdev);
    + return 0;
    +}
    +
    +static int restore_common(struct virtio_device *vdev)
    +{
    + struct virtio_balloon *vb = vdev->priv;
    + int err;
    +
    + /*
    + * If init_vqs below fails, a subsequent module removal
    + * shouldn't cause us to dereference invalid pointers!
    + */
    + vb->thread = NULL;
    +
    + err = init_vqs(vdev->priv);
    + if (err)
    + return err;
    +
    + vb->thread = kthread_run(balloon, vb, "vballoon");
    + if (IS_ERR(vb->thread)) {
    + err = PTR_ERR(vb->thread);
    + vb->thread = NULL;
    + }
    + return err;
    +}
    +
    +static int virtballoon_thaw(struct virtio_device *vdev)
    +{
    + return restore_common(vdev);
    +}
    +
    +static int virtballoon_restore(struct virtio_device *vdev)
    +{
    + struct virtio_balloon *vb = vdev->priv;
    + struct page *page, *page2;
    +
    + /* We're starting from a clean slate */
    + vb->num_pages = 0;
    +
    + /*
    + * If a request wasn't complete at the time of freezing, this
    + * could have been set.
    + */
    + vb->need_stats_update = 0;
    +
    + /* We don't have these pages in the balloon anymore! */
    + list_for_each_entry_safe(page, page2, &vb->pages, lru) {
    + list_del(&page->lru);
    + totalram_pages++;
    + }
    + return restore_common(vdev);
    +}
    +#endif
    +
    static unsigned int features[] = {
    VIRTIO_BALLOON_F_MUST_TELL_HOST,
    VIRTIO_BALLOON_F_STATS_VQ,
    @@ -379,6 +451,11 @@ static struct virtio_driver virtio_balloon_driver = {
    .probe = virtballoon_probe,
    .remove = __devexit_p(virtballoon_remove),
    .config_changed = virtballoon_changed,
    +#ifdef CONFIG_PM
    + .freeze = virtballoon_freeze,
    + .restore = virtballoon_restore,
    + .thaw = virtballoon_thaw,
    +#endif
    };

    static int __init init(void)
    --
    1.7.7.3


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