netdev - Re: [PATCH net-next v1] gso: Support partial splitting at the frag

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Date:   Thu, 25 Aug 2016 09:17:18 -0300
From:   Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
To:     Steffen Klassert <steffen.klassert@...unet.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        Netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Alexander Duyck <alexander.h.duyck@...el.com>
Subject: Re: [PATCH net-next v1] gso: Support partial splitting at the
 frag_list pointer

On Thu, Aug 25, 2016 at 09:31:26AM +0200, Steffen Klassert wrote:
> On Wed, Aug 24, 2016 at 02:25:29PM -0300, Marcelo Ricardo Leitner wrote:
> > Em 24-08-2016 13:27, Alexander Duyck escreveu:
> > >
> > >I'm adding Marcelo as he could probably explain the GSO_BY_FRAGS
> > >functionality better than I could since he is the original author.
> > >
> > >If I recall GSO_BY_FRAGS does something similar to what you are doing,
> > >although I believe it doesn't carry any data in the first buffer other
> > >than just a header.  I believe the idea behind GSO_BY_FRAGS was to
> > >allow for segmenting a frame at the frag_list level instead of having
> > >it done just based on MSS.  That was the only reason why I brought it
> > >up.
> > >
> > 
> > That's exactly it.
> > 
> > On this no data in the first buffer limitation, we probably can
> > allow it have some data in there. It was done this way just because
> > sctp is using skb_gro_receive() to build such skb and this was the
> > way I found to get such frag_list skb generated by it, thus
> > preserving frame boundaries.
> 
> Just to understand what you are doing. You generate MTU sized linear
> buffers in sctp and then, skb_gro_receive() chains up these buffers
> at the frag_list pointer. skb_gro_receive() does this because
> skb_gro_offset is null and skb->head_frag is not set in your case.
> 
> At segmentation, you just need to split at the frag_list pointer
> because you know that the chained buffers fit the MTU, right?
> 

Correct. Just note that these buffers fit the MTU, but not necessary
uses all of it. That is main point in here, variable segmentation size.