netdev - Re: [PATCH net-next] net: udp Allow CHECKSUM

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Date:   Tue, 29 Jan 2019 16:08:14 +0800
From:   maowenan <maowenan@...wei.com>
To:     Tom Herbert <tom@...bertland.com>
CC:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next] net: udp Allow CHECKSUM_UNNECESSARY packets to
 do GRO.



On 2019/1/29 14:24, Tom Herbert wrote:
> On Mon, Jan 28, 2019 at 10:04 PM maowenan <maowenan@...wei.com> wrote:
>>
>>
>>
>> On 2019/1/29 12:01, Tom Herbert wrote:
>>> On Mon, Jan 28, 2019 at 7:00 PM maowenan <maowenan@...wei.com> wrote:
>>>>
>>>> Hi all，
>>>> Do you have any comments about this change?
>>>>
>>>>
>>>> On 2019/1/23 11:33, Mao Wenan wrote:
>>>>> When udp4_gro_receive() get one packet that uh->check=0,
>>>>> skb_gro_checksum_validate_zero_check() will set the
>>>>> skb->ip_summed = CHECKSUM_UNNECESSARY;
>>>>> skb->csum_level = 0;
>>>>> Then udp_gro_receive() will flush the packet which is not CHECKSUM_PARTIAL,
>>>>> It is not our expect,  because check=0 in udp header indicates this
>>>>> packet is no need to caculate checksum, we should go further to do GRO.
>>>>>
>>>>> This patch changes the value of csum_cnt according to skb->csum_level.
>>>>> ---
>>>>>  include/linux/netdevice.h | 1 +
>>>>>  1 file changed, 1 insertion(+)
>>>>>
>>>>> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
>>>>> index 1377d08..9c819f1 100644
>>>>> --- a/include/linux/netdevice.h
>>>>> +++ b/include/linux/netdevice.h
>>>>> @@ -2764,6 +2764,7 @@ static inline void skb_gro_incr_csum_unnecessary(struct sk_buff *skb)
>>>>>                * during GRO. This saves work if we fallback to normal path.
>>>>>                */
>>>>>               __skb_incr_checksum_unnecessary(skb);
>>>>> +             NAPI_GRO_CB(skb)->csum_cnt = skb->csum_level + 1;
>>>
>>> That doesn't look right. This would be reinitializing the GRO
>>> checksums from the beginning.
>>>
>>>>>       }
>>>>>  }
>>>>>
>>>>>
>>>>
>>> I assume the code is bailing on this conditional:
>>>
>>> if (NAPI_GRO_CB(skb)->encap_mark ||
>>>             (skb->ip_summed != CHECKSUM_PARTIAL &&
>>>              NAPI_GRO_CB(skb)->csum_cnt == 0 &&
>>>              !NAPI_GRO_CB(skb)->csum_valid) ||
>>>             !udp_sk(sk)->gro_receive)
>>>                 goto out_unlock;
>>>
>>> I am trying to remember why this needs to check csum_cnt. If there was
>>> a csum_cnt for the UDP csum being zero from checksum-unnecessary, it
>>> was consumed by skb_gro_checksum_validate_zero_check in UDP4 GRO
>>> received.
>>
>> We have met the scene about two VMs in different host with vxlan packets, when udp4_gro_receive receives
>> one packet with ip_summed=CHECKSUM_NONE,csum_cnt=0,csum_valid=0,and udp->check=0, then skb_gro_checksum_validate_zero_check()->
>> skb_gro_incr_csum_unnecessary() validate it and set ip_summed=CHECKSUM_UNNECESSARY,csum_level=0, but csum_cnt and csum_valid
>> keep zero value. Then it will be flushed in udp_gro_receive(), the codes as you have showed.
>>
>> so I think it forgets to modify csum_cnt since csum_level is changed in skb_gro_incr_csum_unnecessary()->__skb_incr_checksum_unnecessary().
>>
> Yes, but the csum_level is changing since we've gone beyond the
> checksums initially reported inc checksum-unnecessary. GRO csum_cnt is
> initialized to skb->csum_level + 1 at the start of GRO processing.
> 
> If I recall, the rule is that UDP GRO requires at least one non-zero
> checksum to be verified. The idea is that if we end up computing
> packet checksums on the host for inner checksums like TCP during GRO,
> then that's negating the performance benefits of GRO. Had UDP check
> not been zero then we would do checksum unnecessary conversion and so
> csum_valid would be set for the remainded of GRO processing. The
> existing code is following the rule I believe, so this may be working
> as intended.

Do you have any suggestion if I need do GRO as udp->check is zero?
My previous modification which works fine as below:
	if (NAPI_GRO_CB(skb)->encap_mark ||
	    (skb->ip_summed != CHECKSUM_PARTIAL &&
+	     skb->ip_summed != CHECKSUM_UNNECESSARY &&
	     NAPI_GRO_CB(skb)->csum_cnt == 0 &&
	     !NAPI_GRO_CB(skb)->csum_valid) ||
	    !udp_sk(sk)->gro_receive)
		goto out_unlock;


> 
> Tom
> 
>>>
>>> .
>>>
>>
> 
> .
>