netdev - Re: [PATCH net 1/2] ipv6: Dump route exceptions too in rt6_dump

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Date:   Thu, 6 Jun 2019 22:37:11 +0000
From:   Martin Lau <kafai@...com>
To:     Stefano Brivio <sbrivio@...hat.com>
CC:     David Miller <davem@...emloft.net>, Jianlin Shi <jishi@...hat.com>,
        "Wei Wang" <weiwan@...gle.com>, David Ahern <dsahern@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net 1/2] ipv6: Dump route exceptions too in
 rt6_dump_route()

On Fri, Jun 07, 2019 at 12:17:47AM +0200, Stefano Brivio wrote:
> On Thu, 6 Jun 2019 21:44:58 +0000
> Martin Lau <kafai@...com> wrote:
> 
> > > +	if (!(filter->flags & RTM_F_CLONED)) {
> > > +		err = rt6_fill_node(net, arg->skb, rt, NULL, NULL, NULL, 0,
> > > +				    RTM_NEWROUTE,
> > > +				    NETLINK_CB(arg->cb->skb).portid,
> > > +				    arg->cb->nlh->nlmsg_seq, flags);
> > > +		if (err)
> > > +			return err;
> > > +	} else {
> > > +		flags |= NLM_F_DUMP_FILTERED;
> > > +	}
> > > +
> > > +	bucket = rcu_dereference(rt->rt6i_exception_bucket);
> > > +	if (!bucket)
> > > +		return 0;
> > > +
> > > +	for (i = 0; i < FIB6_EXCEPTION_BUCKET_SIZE; i++) {
> > > +		hlist_for_each_entry(rt6_ex, &bucket->chain, hlist) {
> > > +			if (rt6_check_expired(rt6_ex->rt6i))
> > > +				continue;
> > > +
> > > +			err = rt6_fill_node(net, arg->skb, rt,
> > > +					    &rt6_ex->rt6i->dst,
> > > +					    NULL, NULL, 0, RTM_NEWROUTE,
> > > +					    NETLINK_CB(arg->cb->skb).portid,
> > > +					    arg->cb->nlh->nlmsg_seq, flags);  
> > Thanks for the patch.
> > 
> > A question on when rt6_fill_node() returns -EMSGSIZE while dumping the
> > exception bucket here.  Where will the next inet6_dump_fib() start?
> 
> And thanks for reviewing.
> 
> It starts again from the same node, see fib6_dump_node(): w->leaf = rt;
> where rt is the fib6_info where we failed dumping, so we won't skip
> dumping any node.
If the same node will be dumped, does it mean that it will go through this
loop and iterate all exceptions again?

> 
> This also means that to avoid sending duplicates in the case where at
> least one rt6_fill_node() call goes through and one fails, we would
> need to track the last bucket and entry sent, or, alternatively, to
> make sure we can fit the whole node before dumping.
My another concern is the dump may never finish.

> 
> I don't think that can happen in practice, or at least I haven't found a
> way to create enough valid exceptions for the same node.
That I am not sure.  It is not unusual to have many pmtu exceptions in
a gateway node.

> 
> Anyway, I guess that would be nicer, but the fix is going to be much
> bigger, and I don't think we even have to guarantee that. I'd rather
> take care of that as a follow-up. Any preferred solution by the way?
> 
> -- 
> Stefano