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

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Date:   Thu, 3 May 2018 20:50:52 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Alexander Duyck <alexander.duyck@...il.com>,
        netdev@...r.kernel.org, willemb@...gle.com, 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 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.

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 ?