netdev - Re: [PATCH] ipv6: remove the unnecessary statement in find

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Date:	Thu, 31 Oct 2013 12:57:18 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Duan Jiong <duanj.fnst@...fujitsu.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] ipv6: remove the unnecessary statement in find_match()

On Thu, Oct 31, 2013 at 07:09:56PM +0800, Duan Jiong wrote:
> 于 2013年10月31日 16:45, Hannes Frederic Sowa 写道:
> > On Thu, Oct 31, 2013 at 02:02:11PM +0800, Duan Jiong wrote:
> >> 于 2013年10月31日 12:22, David Miller 写道:
> >>> From: Hannes Frederic Sowa <hannes@...essinduktion.org>
> >>> Date: Wed, 30 Oct 2013 22:11:57 +0100
> >>>
> >>>> On Wed, Oct 30, 2013 at 05:08:37PM -0400, David Miller wrote:
> >>>>> From: Duan Jiong <duanj.fnst@...fujitsu.com>
> >>>>> Date: Wed, 30 Oct 2013 15:39:26 +0800
> >>>>>
> >>>>>>
> >>>>>> After reading the function rt6_check_neigh(), we can
> >>>>>> know that the RT6_NUD_FAIL_SOFT can be returned only
> >>>>>> when the IS_ENABLE(CONFIG_IPV6_ROUTER_PREF) is false.
> >>>>>> so in function find_match(), there is no need to execute
> >>>>>> the statement !IS_ENABLED(CONFIG_IPV6_ROUTER_PREF).
> >>>>>>
> >>>>>> Signed-off-by: Duan Jiong <duanj.fnst@...fujitsu.com>
> >>>>>
> >>>>> Applied to net-next, thanks.
> >>>>>
> >>>>> CONFIG_IPV6_ROUTER_PREF is another good candidate for Kconfig
> >>>>> removal.  I know we've had several bugs that only apply when
> >>>>> this option is on vs. off.  We're maintaining two different
> >>>>> code paths, for really no good reason.
> >>>>
> >>>> I agree and actually thought about that yesterday. Do you think a sysctl
> >>>> is a good option?
> >>>
> >>> Every distribution ships with the Kconfig option on, and no sysctl
> >>> exists currently to control it.
> >>>
> >>> So I'd say it's not necessary at all, or at the very least let's have
> >>> someone come forward with a real rather than theoretical use case for
> >>> such a feature before adding it.
> >>>
> >>> Actually, if RFC 4191 has the usual language like "there SHOULD be
> >>> an administrative mechanism to disable blah blah blah" I could
> >>> be convinced to add it now.  Can someone take a look?
> >>
> >> It seems that there is no such an administrative mechanism in RFC 4191.
> >>
> >> By the way, if the sysctl is used, we are still maintaining two different
> >> code paths, isn't it? so i think David's idea is good.
> > 
> > Makes life easier, no objections from me.
> > 
> 
> Removing CONFIG_IPV6_ROUTER_PREF means that the Router Preference is always
> on, is this understanding right?

If we drop support for one of the routing scoring algorithms it will be the
other one. So yes, it will always be on.

> If that's is correct,  i think compatibility issues will arise. For example, when the
> CONFIG_IPV6_ROUTER_PREF option is on, the kernel should not do round-robin during
> default router selection, but when the CONFIG_IPV6_ROUTER_PREF option is off, the
> kernel should do it.

Exactly, but all people use ROUTER_PREF activated given it is enabled
by default by most distributions. I also know about no best practices
or IX-policies where they want a router to deliberately turned router
preference support off.

We can talk about doing RR in the same preference level, I'll have to do more
research on this.

But given that someone wants RR, it is better done by using ECMP routes.

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