On Wed, Nov 29, 2017 at 09:23:24AM -0500, wexu@xxxxxxxxxx wrote: > From: Wei Xu <wexu@xxxxxxxxxx> > > 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@xxxxxxxxxxxxxxx/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 <wexu@xxxxxxxxxx> > Reported-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxxxxxxx> > --- > 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 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
