netdev - Re: [PATCH net v3] ipv6: fix multipath route replace error recovery

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Date:	Mon, 7 Sep 2015 14:37:56 +0200
From:	Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:	Roopa Prabhu <roopa@...ulusnetworks.com>, davem@...emloft.net
Cc:	mkubecek@...e.cz, Mazziesaccount@...il.com,
	hannes@...essinduktion.org, kuznet@....inr.ac.ru,
	jmorris@...ei.org, yoshfuji@...ux-ipv6.org, netdev@...r.kernel.org
Subject: Re: [PATCH net v3] ipv6: fix multipath route replace error recovery

Le 06/09/2015 22:46, Roopa Prabhu a écrit :
> From: Roopa Prabhu <roopa@...ulusnetworks.com>
I've sent you some comments about the v2, so please keep me in CC for the next
versions.

>
> Problem:
> The ecmp route replace support for ipv6 in the kernel, deletes the
> existing ecmp route too early, ie when it installs the first nexthop.
> If there is an error in installing the subsequent nexthops, its too late
> to recover the already deleted existing route
>
> This patch fixes the problem with the following:
It does not really 'fix' the problem, it only reduces the probability to have
an error. This is really different. The status is much better after this patch,
but it could be good to reword a bit the commitlog to reflect that.

> a) Changes the existing multipath route add code to a two stage process:
>    build rt6_infos + insert them
> 	ip6_route_add rt6_info creation code is moved into
> 	ip6_route_info_create.
> b) This ensures that all errors are caught during building rt6_infos
>    and we fail early
> c) Separates multipath add and del code. Because add needs the special
>    two stage mode in a) and delete essentially does not care.
> d) In any event if the code fails during inserting a route again, a
>    warning is printed (This should be unlikely)
>
> Before the patch:
> $ip -6 route show
> 3000:1000:1000:1000::2 via fe80::202:ff:fe00:b dev swp49s0 metric 1024
> 3000:1000:1000:1000::2 via fe80::202:ff:fe00:d dev swp49s1 metric 1024
> 3000:1000:1000:1000::2 via fe80::202:ff:fe00:f dev swp49s2 metric 1024
>
> /* Try replacing the route with a duplicate nexthop */
> $ip -6 route change 3000:1000:1000:1000::2/128 nexthop via
> fe80::202:ff:fe00:b dev swp49s0 nexthop via fe80::202:ff:fe00:d dev
> swp49s1 nexthop via fe80::202:ff:fe00:d dev swp49s1
> RTNETLINK answers: File exists
>
> $ip -6 route show
> /* previously added ecmp route 3000:1000:1000:1000::2 dissappears from
>   * kernel */
>
> After the patch:
> $ip -6 route show
> 3000:1000:1000:1000::2 via fe80::202:ff:fe00:b dev swp49s0 metric 1024
> 3000:1000:1000:1000::2 via fe80::202:ff:fe00:d dev swp49s1 metric 1024
> 3000:1000:1000:1000::2 via fe80::202:ff:fe00:f dev swp49s2 metric 1024
>
> /* Try replacing the route with a duplicate nexthop */
> $ip -6 route change 3000:1000:1000:1000::2/128 nexthop via
> fe80::202:ff:fe00:b dev swp49s0 nexthop via fe80::202:ff:fe00:d dev
> swp49s1 nexthop via fe80::202:ff:fe00:d dev swp49s1
> RTNETLINK answers: File exists
>
> $ip -6 route show
> 3000:1000:1000:1000::2 via fe80::202:ff:fe00:b dev swp49s0 metric 1024
> 3000:1000:1000:1000::2 via fe80::202:ff:fe00:d dev swp49s1 metric 1024
> 3000:1000:1000:1000::2 via fe80::202:ff:fe00:f dev swp49s2 metric 1024
>
> Fixes: 27596472473a ("ipv6: fix ECMP route replacement")
As said in the v2 thread, I still don't agree with this tag.

[snip]
> +static void ip6_print_replace_route_err(struct list_head *rt6_nh_list)
> +{
> +	struct rt6_nh *nh;
> +
> +	list_for_each_entry(nh, rt6_nh_list, next) {
> +		pr_warn("IPV6: replace premature del %pI6 nexthop %pI6 ifi %d\n",
I don't think that a user (who didn't read the code) can understand this
sentence. Another suggestion:
"ECMPv6: route replacement failed (check the consistency of the installed 
route)". Not sure that the nexthops should be listed after.
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