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

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Message-ID: <20160825073126.GQ3735@gauss.secunet.com>
Date:   Thu, 25 Aug 2016 09:31:26 +0200
From:   Steffen Klassert <steffen.klassert@...unet.com>
To:     Marcelo Ricardo Leitner <marcelo.leitner@...il.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 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?