Re: [Bridge] [PATCH net-next 1/5] skbuff: bridge: Add layer 2 miss indication

[Paolo Abeni <pabeni@redhat.com>]

On Tue, 2023-05-23 at 11:10 +0300, Ido Schimmel wrote:
> On Fri, May 19, 2023 at 02:52:18PM -0700, Jakub Kicinski wrote:
> > On Fri, 19 May 2023 16:51:48 +0300 Ido Schimmel wrote:
> > > diff --git a/net/bridge/br_input.c b/net/bridge/br_input.c
> > > index fc17b9fd93e6..274e55455b15 100644
> > > --- a/net/bridge/br_input.c
> > > +++ b/net/bridge/br_input.c
> > > @@ -46,6 +46,8 @@ static int br_pass_frame_up(struct sk_buff *skb)
> > >          */
> > >         br_switchdev_frame_unmark(skb);
> > >  
> > > +       skb->l2_miss = BR_INPUT_SKB_CB(skb)->miss;
> > > +
> > >         /* Bridge is just like any other port.  Make sure the
> > >          * packet is allowed except in promisc mode when someone
> > >          * may be running packet capture.
> > > 
> > > Ran these changes through the selftest and it seems to work.
> > 
> > Can we possibly put the new field at the end of the CB and then have TC
> > look at it in the CB? We already do a bit of such CB juggling in strp
> > (first member of struct sk_skb_cb).
> 
> Using the CB between different layers is very fragile and I would like
> to avoid it. Note that the skb can pass various layers until hitting the
> classifier, each of which can decide to memset() the CB.
> 
> Anyway, I think I have a better alternative. I added the 'l2_miss' bit
> to the tc skb extension and adjusted the bridge to mark packets via this
> extension. The entire thing is protected by the existing 'tc_skb_ext_tc'
> static key, so overhead is kept to a minimum when feature is disabled.
> Extended flower to enable / disable this key when filters that match on
> 'l2_miss' are added / removed.
> 
> bridge change to mark the packet:
> https://github.com/idosch/linux/commit/3fab206492fcad9177f2340680f02ced1b9a0dec.patch
> 
> flow_dissector change to dissect the info from the extension:
> https://github.com/idosch/linux/commit/1533c078b02586547817a4e63989a0db62aa5315.patch
> 
> flower change to enable / disable the key:
> https://github.com/idosch/linux/commit/cf84b277511ec80fe565c41271abc6b2e2f629af.patch
> 
> Advantages compared to the previous approach are that we do not need a
> new bit in the skb and that overhead is kept to a minimum when feature
> is disabled. Disadvantage is that overhead is higher when feature is
> enabled.
> 
> WDYT?

Looks good to me.

I think you would only need to set/add the extension when l2_miss is
true, right? (with no extension l2 hit is assumed). That will avoid
unneeded overhead for br_dev_xmit().

All the others involved paths look like slow(er) one, so the occasional
skb extension overhead should not be a problem.

Cheers,

Paolo