Subject: Re: [PATCH net,stable] vhost: fix skb leak in handle_rx()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 28 Nov 2017 19:53:33 +0200

On Tue, Nov 28, 2017 at 12:17:16PM -0500, [email-masked] wrote:
> From: Wei Xu <[email-masked]>
> 
> 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/[email-masked]/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 deferring it a moment later
> in this patch.
> 
> Signed-off-by: Wei Xu <[email-masked]>

Given the amount of effort Matthew has put into this,
you definitely want

Reported-by: Matthew Rosato <[email-masked]>

here.

Let's give credit where credit is due.

Thanks a lot Matthew!

> ---
>  drivers/vhost/net.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 8d626d7..e76535e 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -778,8 +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);
> @@ -809,6 +807,8 @@ static void handle_rx(struct vhost_net *net)
>  			 */
>  			iov_iter_advance(&msg.msg_iter, vhost_hlen);
>  		}
> +		if (nvq->rx_array)
> +			msg.msg_control = vhost_net_buf_consume(&nvq->rxq);
>  		err = sock->ops->recvmsg(sock, &msg,
>  					 sock_len, MSG_DONTWAIT | MSG_TRUNC);
>  		/* Userspace might have consumed the packet meanwhile:
> -- 
> 1.8.3.1