netdev - Re: [PATCH net,stable] vhost: fix skb leak in handle

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Date:   Wed, 29 Nov 2017 13:06:28 +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:     mjrosato@...ux.vnet.ibm.com, mst@...hat.com
Subject: Re: [PATCH net,stable] vhost: fix skb leak in handle_rx()



On 2017年11月29日 09:53, Jason Wang wrote:
>
>
> On 2017å¹´11æœˆ29æ—¥ 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 

Not a leak actually, but the packet won't be consumed and we will hit 
UIO_MAXIOV forever in this case.

Thanks