netdev - Re: [PATCH net] ipv6: make ECMP route replacement less greedy

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Date:   Mon, 13 Mar 2017 18:47:53 +0100
From:   Michal Kubecek <mkubecek@...e.cz>
To:     Sabrina Dubroca <sd@...asysnail.net>
Cc:     netdev@...r.kernel.org,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Xin Long <lucien.xin@...il.com>
Subject: Re: [PATCH net] ipv6: make ECMP route replacement less greedy

On Mon, Mar 13, 2017 at 01:28:09PM +0100, Sabrina Dubroca wrote:
> Commit 27596472473a ("ipv6: fix ECMP route replacement") introduced a
> loop that removes all siblings of an ECMP route that is being
> replaced. However, this loop doesn't stop when it has replaced
> siblings, and keeps removing other routes with a higher metric.
> We also end up triggering the WARN_ON after the loop, because after
> this nsiblings < 0.
> 
> Instead, stop the loop when we have taken care of all routes with the
> same metric as the route being replaced.
> 
>   Reproducer:
>   ===========
>     #!/bin/sh
>     
>     ip netns add ns1
>     ip netns add ns2
>     ip -net ns1 link set lo up
>     
>     for x in 0 1 2 ; do
>         ip link add veth$x netns ns2 type veth peer name eth$x netns ns1
>         ip -net ns1 link set eth$x up
>         ip -net ns2 link set veth$x up
>     done
>     
>     ip -net ns1 -6 r a 2000::/64 nexthop via fe80::0 dev eth0 \
>             nexthop via fe80::1 dev eth1 nexthop via fe80::2 dev eth2
>     ip -net ns1 -6 r a 2000::/64 via fe80::42 dev eth0 metric 256
>     ip -net ns1 -6 r a 2000::/64 via fe80::43 dev eth0 metric 2048
>     
>     echo "before replace, 3 routes"
>     ip -net ns1 -6 r | grep -v '^fe80\|^ff00'
>     echo
>     
>     ip -net ns1 -6 r c 2000::/64 nexthop via fe80::4 dev eth0 \
>             nexthop via fe80::5 dev eth1 nexthop via fe80::6 dev eth2
>     
>     echo "after replace, only 2 routes, metric 2048 is gone"
>     ip -net ns1 -6 r | grep -v '^fe80\|^ff00'
> 
> Fixes: 27596472473a ("ipv6: fix ECMP route replacement")
> Signed-off-by: Sabrina Dubroca <sd@...asysnail.net>
> ---
>  net/ipv6/ip6_fib.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
> index e4266746e4a2..d4bf2c68a545 100644
> --- a/net/ipv6/ip6_fib.c
> +++ b/net/ipv6/ip6_fib.c
> @@ -923,6 +923,8 @@ static int fib6_add_rt2node(struct fib6_node *fn, struct rt6_info *rt,
>  			ins = &rt->dst.rt6_next;
>  			iter = *ins;
>  			while (iter) {
> +				if (iter->rt6i_metric > rt->rt6i_metric)
> +					break;
>  				if (rt6_qualify_for_ecmp(iter)) {
>  					*ins = iter->dst.rt6_next;
>  					fib6_purge_rt(iter, fn, info->nl_net);
> -- 
> 2.12.0
> 

Good catch, thank you. The metric comparison could be merged into the
cycle condition but this way it doesn't seem any worse.

Reviewed-by: Michal Kubecek <mkubecek@...e.cz>