On Wed, Nov 29, 2017 at 09:23:24AM -0500, w...@redhat.com wrote:
> From: Wei Xu <w...@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.
>
> Also strengthen the small possibility of leak in case of recvmsg()
> fails by freeing the skb.
>
> Signed-off-by: Wei Xu <w...@redhat.com>
> Reported-by: Matthew Rosato <mjros...@linux.vnet.ibm.com>
> ---
> drivers/vhost/net.c | 23 +++++++++++++----------
> 1 file changed, 13 insertions(+), 10 deletions(-)
>
> v2:
> - add Matthew as the reporter, thanks matthew.
> - moving zero headcount check ahead instead of defer consuming skb
> due to jason and mst's comment.
> - add freeing skb in favor of recvmsg() fails.
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 8d626d7..e302e08 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 +790,18 @@ 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);
> + if (unlikely(err != 1))
Why 1? How is receiving 1 byte special or even possible?
Also, I wouldn't put an unlikely here. It's all error handling code anyway.
> + kfree_skb((struct sk_buff *)msg.msg_control);
You do not need a cast here.
Also, is it really safe to refer to msg_control here?
I'd rather keep a copy of the skb pointer and use it than assume
caller did not change it. But also see below.
> + 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;
> @@ -818,6 +820,7 @@ static void handle_rx(struct vhost_net *net)
> pr_debug("Discarded rx packet: "
> " len %d, expected %zd\n", err, sock_len);
> vhost_discard_vq_desc(vq, headcount);
> + kfree_skb((struct sk_buff *)msg.msg_control);
You do not need a cast here.
Also, we have
ret = tun_put_user(tun, tfile, skb, to);
if (unlikely(ret < 0))
kfree_skb(skb);
else
consume_skb(skb);
return ret;
So it looks like recvmsg actually always consumes the skb.
So I was wrong when I said you need to kfree it after
recv msg, and your original patch was good.
Jason, what do you think?
> continue;
> }
> /* Supply virtio_net_hdr if VHOST_NET_F_VIRTIO_NET_HDR */
> --
> 1.8.3.1
