netdev - Re: [PATCH net-next 2/2] ipv4: use dst hint for ipv4 list receive

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Date:   Mon, 18 Nov 2019 17:44:26 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     Edward Cree <ecree@...arflare.com>, netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>
Subject: Re: [PATCH net-next 2/2] ipv4: use dst hint for ipv4 list receive

On Mon, 2019-11-18 at 16:15 +0000, Edward Cree wrote:
> @@ -538,6 +543,7 @@ static void ip_sublist_rcv_finish(struct list_head *head)
> >  static void ip_list_rcv_finish(struct net *net, struct sock *sk,
> >  			       struct list_head *head)
> >  {
> > +	struct ip_route_input_hint _hint, *hint = NULL;
> >  	struct dst_entry *curr_dst = NULL;
> >  	struct sk_buff *skb, *next;
> >  	struct list_head sublist;
> > @@ -554,11 +560,24 @@ static void ip_list_rcv_finish(struct net *net, struct sock *sk,
> >  		skb = l3mdev_ip_rcv(skb);
> >  		if (!skb)
> >  			continue;
> > -		if (ip_rcv_finish_core(net, sk, skb, dev) == NET_RX_DROP)
> > +		if (ip_rcv_finish_core(net, sk, skb, dev, hint) == NET_RX_DROP)
> >  			continue;
> >  
> >  		dst = skb_dst(skb);
> >  		if (curr_dst != dst) {
> > +			struct rtable *rt = (struct rtable *)dst;
> > +
> > +			if (!net->ipv4.fib_has_custom_rules &&
> > +			    rt->rt_type != RTN_BROADCAST) {
> > +				_hint.refdst = skb->_skb_refdst;
> > +				_hint.daddr = ip_hdr(skb)->daddr;
> > +				_hint.tos = ip_hdr(skb)->tos;
> > +				_hint.local = rt->rt_type == RTN_LOCAL;
> > +				hint = &_hint;
> > +			} else {
> > +				hint = NULL;
> > +			}
> Perhaps factor this block out into a function?  Just because it's getting
>  deeply indented and giving it a name would make it more obvious what it's
>  for.  hint = ipv4_extract_route_hint(skb, &_hint)?

yep, I like the idea, will do in the next iteration.
> 
> > +
> >  			/* dispatch old sublist */
> >  			if (!list_empty(&sublist))
> >  				ip_sublist_rcv_finish(&sublist);
> > diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> > index dcc4fa10138d..b0ddff17db80 100644
> > --- a/net/ipv4/route.c
> > +++ b/net/ipv4/route.c
> > @@ -2019,6 +2019,44 @@ static int ip_mkroute_input(struct sk_buff *skb,
> >  	return __mkroute_input(skb, res, in_dev, daddr, saddr, tos);
> >  }
> >  
> > +/* Implements all the saddr-related checks as ip_route_input_slow(),
> > + * assuming daddr is valid and this is not a local broadcast.
> > + * Uses the provided hint instead of performing a route lookup.
> > + */
> > +int ip_route_use_hint(struct sk_buff *skb, __be32 daddr, __be32 saddr,
> > +		      u8 tos, struct net_device *dev,
> > +		      struct ip_route_input_hint *hint)
> Mostly I like the idea of these patches, but it bugs me that this seems
>  to be reimplementing a little bit, and might get out of sync.  Is it
>  possible to factor out the checks from ip_route_input_slow() and just
>  call them here?
> Otherwise maybe stick something in the comment to ip_route_input_slow()
>  reminding to propagate changes to ip_route_use_hint()?
> 
> Or perhaps better still would be to come up with a single function that
>  always takes a hint, that may be NULL, in which case it performs normal
>  routing; and use that in all paths?  (Plumbing the hint through from
>  ip_route_input_noref() etc.)

I experimented a bit with the latter option before restricting to
!RTN_BROADCAST, and it make the code quite uglier. Anyhow, preserving
the !RTN_BROADCAST restriction for hint usage, I think it could work.
Let me try that.

Thank you for the feedback!

Paolo