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

[David Hildenbrand (Arm)]

On 6/24/26 16:08, Denis V. Lunev wrote:
> 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@xxxxxxxxxx>
> Signed-off-by: Denis V. Lunev <den@xxxxxxxxxx>
> ---
>  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));
> -
>  }

We have another uncheck instance in stats_handle_request(), what about that one?

-- 
Cheers,

David