[PATCH 3.2 052/152] balloon: check the number of available pages in leak balloon

[Ben Hutchings]

3.2.84-rc1 review patch.  If anyone has any objections, please let me know.

------------------

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.

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>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/virtio/virtio_balloon.c | 2 ++
 1 file changed, 2 insertions(+)

--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -142,6 +142,8 @@ static void leak_balloon(struct virtio_b
 	/* We can only do one array worth at a time. */
 	num = min(num, ARRAY_SIZE(vb->pfns));
 
+	/* 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++) {
 		page = list_first_entry(&vb->pages, struct page, lru);
 		list_del(&page->lru);