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Date:   Wed, 29 Nov 2017 09:53:37 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     wexu@...hat.com, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     mst@...hat.com, mjrosato@...ux.vnet.ibm.com
Subject: Re: [PATCH net,stable] vhost: fix skb leak in handle_rx()



On 2017年11月29日 01:17, wexu@...hat.com wrote:
> From: Wei Xu <wexu@...hat.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 deferring it a moment later
> in this patch.
>
> Signed-off-by: Wei Xu <wexu@...hat.com>
> ---
>   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)) {

You need do msg.msg_control = vhost_net_buf_consume() here too, 
otherwise we may still get it leaked.

Thanks

>   			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:

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