netdev - Re: xdp and fragments with virtio

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Date:   Thu, 17 May 2018 17:24:25 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     David Ahern <dsahern@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: xdp and fragments with virtio



On 2018年05月17日 10:55, David Ahern wrote:
> On 5/16/18 1:24 AM, Jason Wang wrote:
>>
>> On 2018年05月16日 11:51, David Ahern wrote:
>>> Hi Jason:
>>>
>>> I am trying to test MTU changes to the BPF fib_lookup helper and seeing
>>> something odd. Hoping you can help.
>>>
>>> I have a VM with multiple virtio based NICs and tap backends. I install
>>> the xdp program on eth1 and eth2 to do forwarding. In the host I send a
>>> large packet to eth1:
>>>
>>> $ ping -s 1500 9.9.9.9
>>>
>>>
>>> The tap device in the host sees 2 packets:
>>>
>>> $ sudo tcpdump -nv -i vm02-eth1
>>> 20:44:33.943160 IP (tos 0x0, ttl 64, id 58746, offset 0, flags [+],
>>> proto ICMP (1), length 1500)
>>>       10.100.1.254 > 9.9.9.9: ICMP echo request, id 17917, seq 1,
>>> length 1480
>>> 20:44:33.943172 IP (tos 0x0, ttl 64, id 58746, offset 1480, flags
>>> [none], proto ICMP (1), length 48)
>>>       10.100.1.254 > 9.9.9.9: ip-proto-1
>>>
>>>
>>> In the VM, the XDP program only sees the first packet, not the fragment.
>>> I added a printk to the program (see diff below):
>>>
>>> $ cat trace_pipe
>>>             <idle>-0     [003] ..s2   254.436467: 0: packet length 1514
>>>
>>>
>>> Anything come to mind in the virtio xdp implementation that affects
>>> fragment packets? I see this with both IPv4 and v6.
>> Not yet. But we do turn of tap gso when virtio has XDP set, but it
>> shouldn't matter this case.
>>
>> Will try to see what's wrong.
>>
> I added this to the command line for the NICs and it works:
>
> "mrg_rxbuf=off,guest_tso4=off,guest_tso6=off,guest_ecn=off,guest_ufo=off"
>
> XDP program sees the full size packet and the fragment.
>
> Fun fact: only adding mrg_rxbuf=off so that mergeable_rx_bufs is false
> but big_packets is true generates a panic when it receives large packets.

It looks like we wrongly drop packets after linearizing the packets 
during XDP_REDIRECT.

Please try the patch (but I do spot some other issues, will post a series):

Thanks

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index f34794a..59702f9 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -800,7 +800,7 @@ static struct sk_buff *receive_mergeable(struct 
net_device *dev,
                         }
                         *xdp_xmit = true;
                         if (unlikely(xdp_page != page))
-                               goto err_xdp;
+                               put_page(page);
                         rcu_read_unlock();
                         goto xdp_xmit;
                 default: