[PATCH 3.10 166/180] balloon: check the number of available pages in leak balloon

[Willy Tarreau]

From: Konstantin Neumoin <kneumoin@xxxxxxxxxxxxx>

commit 37cf99e08c6fb4dcea0f9ad2b13b6daa8c76a711 upstream.

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@xxxxxxxxxxxxxxx
Signed-off-by: Konstantin Neumoin <kneumoin@xxxxxxxxxxxxx>
Signed-off-by: Denis V. Lunev <den@xxxxxxxxxx>
CC: Michael S. Tsirkin <mst@xxxxxxxxxx>
Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Signed-off-by: Willy Tarreau <w@xxxxxx>
---
 drivers/virtio/virtio_balloon.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 7d7add5..148e8ea 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -177,6 +177,8 @@ static void 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);
-- 
2.8.0.rc2.1.gbe9624a