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

[Jason Wang]

On 2017å12æ01æ 22:37, Michael S. Tsirkin wrote:
> On Fri, Dec 01, 2017 at 03:11:05PM +0800, Jason Wang wrote:
> > On 2017å12æ01æ 13:54, 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.
> > >
> > > Signed-off-by: Wei Xu <wexu@xxxxxxxxxx>
> > > Reported-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxxxxxxx>
> > > ---
> > >    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;
> > I suggest to reorder this patch to 3/3.
> >
> > Thanks
> Why? This doesn't cause any new leaks, does it?

It doesn't, just think it can ease the downstream back porting in case 
patch 2-3 were missed if somebody did a bisect and just backport patch 1.

Thanks