Re: [PATCH 1/3] vhost: fix skb leak in handle_rx()

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Dec 01, 2017 at 05:10:36AM -0500, wexu@redhat.com wrote:
> From: Wei Xu <wexu@redhat.com>
> 
> Matthew found a roughly 40% tcp throughput regression with commit
> c67df11f(vhost_net: try batch dequing from skb array) as discussed
> in the following thread:
> https://www.mail-archive.com/netdev@vger.kernel.org/msg187936.html
> 
> Eventually we figured out that it was a skb leak in handle_rx()
> when sending packets to the VM. This usually happens when a guest
> can not drain out vq as fast as vhost fills in, afterwards it sets
> off the traffic jam and leaks skb(s) which occurs as no headcount
> to send on the vq from vhost side.
> 
> This can be avoided by making sure we have got enough headcount
> before actually consuming a skb from the batched rx array while
> transmitting, which is simply done by moving checking the zero
> headcount a bit ahead.
> 
> Signed-off-by: Wei Xu <wexu@redhat.com>
> Reported-by: Matthew Rosato <mjrosato@linux.vnet.ibm.com>

Acked-by: Michael S. Tsirkin <mst@redhat.com>

> ---
>  drivers/vhost/net.c | 20 ++++++++++----------
>  1 file changed, 10 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 8d626d7..c7bdeb6 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -778,16 +778,6 @@ static void handle_rx(struct vhost_net *net)
>  		/* On error, stop handling until the next kick. */
>  		if (unlikely(headcount < 0))
>  			goto out;
> -		if (nvq->rx_array)
> -			msg.msg_control = vhost_net_buf_consume(&nvq->rxq);
> -		/* On overrun, truncate and discard */
> -		if (unlikely(headcount > UIO_MAXIOV)) {
> -			iov_iter_init(&msg.msg_iter, READ, vq->iov, 1, 1);
> -			err = sock->ops->recvmsg(sock, &msg,
> -						 1, MSG_DONTWAIT | MSG_TRUNC);
> -			pr_debug("Discarded rx packet: len %zd\n", sock_len);
> -			continue;
> -		}
>  		/* OK, now we need to know about added descriptors. */
>  		if (!headcount) {
>  			if (unlikely(vhost_enable_notify(&net->dev, vq))) {
> @@ -800,6 +790,16 @@ static void handle_rx(struct vhost_net *net)
>  			 * they refilled. */
>  			goto out;
>  		}
> +		if (nvq->rx_array)
> +			msg.msg_control = vhost_net_buf_consume(&nvq->rxq);
> +		/* On overrun, truncate and discard */
> +		if (unlikely(headcount > UIO_MAXIOV)) {
> +			iov_iter_init(&msg.msg_iter, READ, vq->iov, 1, 1);
> +			err = sock->ops->recvmsg(sock, &msg,
> +						 1, MSG_DONTWAIT | MSG_TRUNC);
> +			pr_debug("Discarded rx packet: len %zd\n", sock_len);
> +			continue;
> +		}
>  		/* We don't need to be notified again. */
>  		iov_iter_init(&msg.msg_iter, READ, vq->iov, in, vhost_len);
>  		fixup = msg.msg_iter;
> -- 
> 1.8.3.1