balloon: check the number of available pages in leak balloon

The balloon has a special mechanism that is subscribed to the oom
notification which leads to deflation for a fixed number of pages.
The number is always fixed even when the balloon is fully deflated.
But leak_balloon did not expect that the pages to deflate will be more
than taken, and raise a "BUG" in balloon_page_dequeue when page list
will be empty.

So, the simplest solution would be to check that the number of releases
pages is less or equal to the number taken pages.

Cc: stable@vger.kernel.org
Signed-off-by: Konstantin Neumoin <kneumoin@virtuozzo.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 476c0e3a7150694b258e40787f303d6b46c29c17..f6ea8f4ba7cf37341640df7a2334b014c8de847c 100644 (file)
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -202,6 +202,8 @@ static unsigned leak_balloon(struct virtio_balloon *vb, size_t num)
        num = min(num, ARRAY_SIZE(vb->pfns));
 
        mutex_lock(&vb->balloon_lock);
+       /* We can't release more pages than taken */
+       num = min(num, (size_t)vb->num_pages);
        for (vb->num_pfns = 0; vb->num_pfns < num;
             vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
                page = balloon_page_dequeue(vb_dev_info);