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

[Jason Wang <jasowang@redhat.com>]

On 2017年12月01日 22:37, Michael S. Tsirkin wrote:
> On Fri, Dec 01, 2017 at 03:11:05PM +0800, Jason Wang wrote:
>>
>> On 2017年12月01日 13:54, wexu@redhat.com wrote:
>>> From: Wei Xu <wexu@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.
>>>
>>> Signed-off-by: Wei Xu <wexu@redhat.com>
>>> Reported-by: Matthew Rosato <mjrosato@linux.vnet.ibm.com>
>>> ---
>>>    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