netdev - Re: [net-next PATCH 3/5] udp: Add support for software checksum and GSO

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Date:   Fri, 4 May 2018 07:22:17 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Netdev <netdev@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        David Miller <davem@...emloft.net>,
        Steffen Klassert <steffen.klassert@...unet.com>
Subject: Re: [net-next PATCH 3/5] udp: Add support for software checksum and
 GSO_PARTIAL with GSO offload

On Thu, May 3, 2018 at 8:50 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
> On 05/03/2018 05:33 PM, Alexander Duyck wrote:
>> From: Alexander Duyck <alexander.h.duyck@...el.com>
>>
>> This patch adds support for a software provided checksum and GSO_PARTIAL
>> segmentation support. With this we can offload UDP segmentation on devices
>> that only have partial support for tunnels.
>>
>> Since we are no longer needing the hardware checksum we can drop the checks
>> in the segmentation code that were verifying if it was present.
>>
>> Signed-off-by: Alexander Duyck <alexander.h.duyck@...el.com>
>> ---
>>  net/ipv4/udp_offload.c |   28 ++++++++++++++++++----------
>>  net/ipv6/udp_offload.c |   11 +----------
>>  2 files changed, 19 insertions(+), 20 deletions(-)
>>
>> diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
>> index 946d06d2aa0c..fd94bbb369b2 100644
>> --- a/net/ipv4/udp_offload.c
>> +++ b/net/ipv4/udp_offload.c
>> @@ -217,6 +217,13 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
>>               return segs;
>>       }
>>
>> +     /* GSO partial and frag_list segmentation only requires splitting
>> +      * the frame into an MSS multiple and possibly a remainder, both
>> +      * cases return a GSO skb. So update the mss now.
>> +      */
>> +     if (skb_is_gso(segs))
>> +             mss *= skb_shinfo(segs)->gso_segs;
>> +
>>
>
> I do not understand this code.

In the case of GSO partial or frag_list segmentation we only split
along certain MSS sized boundaries and we leave the resulting frame as
a GSO skb. As a result the actual frame size is a multiple of MSS so
we were updating the mss by that value.

> I am also seeing it in tcp, after commit 07b26c9454a2a ("gso: Support partial splitting at the frag_list pointer")
>
> Presumably this broke tcp_gso_tstamp() , right ?

I'm not sure it did. The MSS multiplication is there because the frame
now spans multiple MSS chunks in one shot. So tcp_gso_tstamp should
still have the correct behavior, I think.

- Alex