From mboxrd@z Thu Jan  1 00:00:00 1970
From: Florian Westphal <fw@strlen.de>
Subject: Re: [PATCH 2/2] net: ip, ipv6: handle gso skbs in forwarding path
Date: Wed, 29 Jan 2014 12:00:24 +0100
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Cc: Florian Westphal <fw@strlen.de>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	netdev@vger.kernel.org
To: Eric Dumazet <eric.dumazet@gmail.com>
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Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > > Its still unclear if this is the best strategy.
> > > 
> > > TCP stream not using DF flag are very unlikely to care if we adjust
> > > their MTU (lowering gso_size) at this point ?
> > 
> > Thanks for this suggestion.  It would indeed be nice to avoid sw
> > segmentation.  I tried:
> > 
> > static void ip_gso_adjust_seglen(struct sk_buff *skb)
> > {
> >         unsigned int mtu;
> > 
> >         if (!skb_is_gso(skb))
> >                 return;
> > 
> >         mtu = ip_dst_mtu_maybe_forward(skb_dst(skb), true);
> >         skb_shinfo(skb)->gso_size = mtu - sizeof(struct iphdr);

FWIW Erics idea works fine with:

        headerlen = skb_transport_header(skb) - skb_network_header(skb);
        if (likely(skb_shinfo(skb)->gso_type & (SKB_GSO_TCPV4 | SKB_GSO_TCPV6)))
                headerlen += tcp_hdrlen(skb);
        skb_shinfo(skb)->gso_size = mtu - headerlen;

and disabling 'sg' on outgoing (lower-mtu) interface.  [ else BUG() ]

> > [   28.644776] kernel BUG at net/net/core/skbuff.c:2984!
> 
> Yep, lets CC Herbert Xu, as he 'owns' skb_segment()
> 
> BUG_ON(skb_headlen(fskb));