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

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Date:   Tue, 17 Jul 2018 10:24:53 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     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>,
        Eyal Birger <eyal.birger@...il.com>
Subject: Re: [PATCH net-next 4/4] act_mirred: use ACT_REDIRECT when possible

On Tue, Jul 17, 2018 at 2:16 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> Hi,
>
> On Mon, 2018-07-16 at 16:39 -0700, Cong Wang 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.
>
> Thank you for the feedback.
>
> > Avoiding skb_clone() for redirection is cool, but why need to use
> > skb_do_redirect() here?
>
> Well, it's not needed. I tried to reduce code duplication, and I tried
> to avoid adding another TC_ACT_* value.


If you consider dev_forward_skb(), it is not a duplication.


>
> > 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.
>
> Understood.
>
> A possible option out of this issues would be adding another action
> value - TC_ACT_MIRRED ? - and handle it in sch_handle_egress()[1] with
> the appropriate semantic. That should address also Daniel and Eyal
> concerns.
>
> Would you consider the above acceptable?

If you goal is to get rid of skb_clone(), why not just do the following?

        if (tcf_mirred_is_act_redirect(m_eaction)) {
                skb2 = skb;
        } else {
                skb2 = skb_clone(skb, GFP_ATOMIC);
                if (!skb2)
                        goto out;
        }

For redirect, we return TC_ACT_SHOT, so upper layer should not
touch the skb after that.

What am I missing here?