[Devel] Re: [PATCHSET] 2.6.20-lxc8

[Eric W. Biederman]

Kirill Korotaev <dev at openvz.org> writes:

> we have the hack below in ip_forward() to avoid skb_cow(),
> Banjamin, can you check whether it helps in your case please?
> (NOTE: you will need to replace check for NETIF_F_VENET with something else
>  or introduce the same flag on etun device).

Ugh.  The thing is skb_cow should be free.  It only has a cost when the skb
is too small or there is a second copy of the skb.  I don't there is a technical
reason for either of those to be the case when we are going over ethernet.

And since the hardware header needs to change as well your hack is actually broken
if the incoming network interface is not ethernet. 

So while I can see this hack for testing I'd much rather see if we can actually
fix this one cleanly.

Unless you understand what is triggering the skb_cow to actually perform
the copy.

Eric

> diff -upr linux-2.6.18-rhel5.orig/net/ipv4/ip_forward.c
> linux-2.6.18-rhel5-028stab023/net/ipv4/ip_forward.c
> --- linux-2.6.18-rhel5.orig/net/ipv4/ip_forward.c 2006-09-20 07:42:06.000000000
> +0400
> +++ linux-2.6.18-rhel5-028stab023/net/ipv4/ip_forward.c 2007-03-20
> 17:22:45.000000000 +0300
> @@ -86,6 +86,24 @@ int ip_forward(struct sk_buff *skb)
>         if (opt->is_strictroute && rt->rt_dst != rt->rt_gateway)
>                 goto sr_failed;
>
> +       /*
> +        * We try to optimize forwarding of VE packets:
> +        * do not decrement TTL (and so save skb_cow)
> +        * during forwarding of outgoing pkts from VE.
> +        * For incoming pkts we still do ttl decr,
> +        * since such skb is not cloned and does not require
> +        * actual cow. So, there is at least one place
> +        * in pkts path with mandatory ttl decr, that is
> +        * sufficient to prevent routing loops.
> +        */
> +       iph = skb->nh.iph;
> +       if (
> +#ifdef CONFIG_IP_ROUTE_NAT
> + (rt->rt_flags & RTCF_NAT) == 0 && /* no NAT mangling expected */
> +#endif                                           /* and */
> +           (skb->dev->features & NETIF_F_VENET)) /* src is VENET device */
> +               goto no_ttl_decr;
> +
>         /* We are about to mangle packet. Copy it! */
>         if (skb_cow(skb, LL_RESERVED_SPACE(rt->u.dst.dev)+rt->u.dst.header_len))
>                 goto drop;
> @@ -94,6 +112,8 @@ int ip_forward(struct sk_buff *skb)
>         /* Decrease ttl after skb cow done */
>         ip_decrease_ttl(iph);
>
> +no_ttl_decr:
> +
>         /*
>          *      We now generate an ICMP HOST REDIRECT giving the route
>          *      we calculated.
> @@ -121,3 +141,5 @@ drop:
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