netdev - Re: [PATCH net-next 4/4] act_mirred: use ACT

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Date:   Tue, 17 Jul 2018 10:01:27 +0300
From:   Eyal Birger <eyal.birger@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Paolo Abeni <pabeni@...hat.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        shmulik.ladkani@...il.com
Subject: Re: [PATCH net-next 4/4] act_mirred: use ACT_REDIRECT when possible

Hi,

On Mon, 16 Jul 2018 16:39:55 -0700
Cong Wang <xiyou.wangcong@...il.com> wrote:

> On Fri, Jul 13, 2018 at 2:55 AM Paolo Abeni <pabeni@...hat.com> wrote:
> >
> > When mirred is invoked from the ingress path, and it wants to
> > redirect the processed packet, it can now use the ACT_REDIRECT
> > action, filling the tcf_result accordingly.
> >
> > This avoids a skb_clone() in the TC S/W data path giving a ~10%
> > improvement in forwarding performances. Overall TC S/W performances
> > are now comparable to the kernel openswitch datapath.  
> 
> Avoiding skb_clone() for redirection is cool, but why need to use
> skb_do_redirect() here?
> 
> There is a subtle difference here:
> 
> skb_do_redirect() calls __bpf_rx_skb() which calls
> dev_forward_skb().
> 
> while the current mirred action doesn't scrub packets when
> redirecting to ingress (from egress). Although I forget if it is
> intentionally.
> 
> Also, skb->skb_iif is unset in skb_do_redirect() when
> redirecting to ingress, I recall we have to set it correctly
> for input routing. Probably yet another reason why we
> can't scrub it, unless my memory goes wrong. :)

Also dev_forward_skb() enforces MTU checks on the packet which are not
done at the moment. I am aware of deployments where this would break
things.

Eyal.