[PATCH v2 4/4] virtio_balloon: warn on failed buffer add in tell_host()

["Denis V. Lunev" <den@openvz.org>]

tell_host() ignores the return value of virtqueue_add_outbuf() and goes
on to kick the queue and wait_event() for the host's ack. The comment
claims "We should always be able to add one buffer to an empty queue",
but that does not hold once the virtqueue has been broken (e.g. on
device shutdown): the add then fails with -EIO and the following
wait_event() would block forever on a buffer the host can never return.

Warn and bail out on failure, mirroring virtballoon_free_page_report().

Suggested-by: David Hildenbrand <david@kernel.org>
Signed-off-by: Denis V. Lunev <den@openvz.org>
---
 drivers/virtio/virtio_balloon.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 26fc3c40d5b2..0866a8781f0b 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -184,16 +184,18 @@ static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
 {
 	struct scatterlist sg;
 	unsigned int len;
+	int err;
 
 	sg_init_one(&sg, vb->pfns, sizeof(vb->pfns[0]) * vb->num_pfns);
 
 	/* We should always be able to add one buffer to an empty queue. */
-	virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
+	err = virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
+	if (WARN_ON_ONCE(err))
+		return;
 	virtqueue_kick(vq);
 
 	/* When host has read buffer, this completes via balloon_ack */
 	wait_event(vb->acked, virtqueue_get_buf(vq, &len));
-
 }
 
 static int virtballoon_free_page_report(struct page_reporting_dev_info *pr_dev_info,
-- 
2.53.0