Re: [PATCH v2 net-next] ipv6: prevent useless neigh alloc on PTP or lo routes

[Eric Dumazet <eric.dumazet@gmail.com>]

On Thu, 2012-09-13 at 17:13 -0400, David Miller wrote:
> From: Eric Dumazet <eric.dumazet@gmail.com>
> Date: Thu, 13 Sep 2012 05:15:58 +0200
> 
> > From: Eric Dumazet <edumazet@google.com>
> > 
> > We have special handling of SIT devices in addrconf_prefix_route()
> > to avoid allocating a neighbour for each destination.
> > 
> > If routing entry is :
> > 
> > ip -6 route add 2001:db8::/64 dev sit1
> > 
> > Then the kernel will create a new route and neighbour for every new
> > address under 2001:db8::/64 that we send a packet to 
> > (potentially, 2^64 routes and neighbours).
> > 
> > Under load, we immediately get the infamous "Neighbour table overflow"
> > message and machine eventually crash.
> > 
> > This does not happen if we specify a next-hop explicitly, like so:
> > 
> > ip -6 route add 2001:db8::/64 via fe80:: dev sit1
> > 
> > Same problem happens if we use routes to loopback.
> > 
> > Idea of this patch is to move existing SIT related code from
> > addrconf_prefix_route() to a more generic one in ip6_route_add(). 
> > 
> > This permits ip6_pol_route() to clone route instead of calling
> > rt6_alloc_cow() and allocate a neighbour.
> > 
> > Many thanks to Lorenzo for his help and suggestions.
> > 
> > Reported-by: Lorenzo Colitti <lorenzo@google.com>
> > Signed-off-by: Eric Dumazet <edumazet@google.com>
> 
> This patch lacks the desired effect without your clone-caching-removal
> patch, which I will not apply.
> 
> Therefore it doesn't make any sense to apply this either, as it won't
> fix the stated problem.
> 
> Doing a proper conversion of ipv6 to ref-count-less neigh's will solve
> this problem and allow all of the clone/cow caching code to be elided
> for the majority of cases and is the correct approach to these problems.


This seems quite different patches to me. Addressing two problems.

This patch is about not allocating a neighbour for a given route, but
reusing one existing neighbour. What could be possibly wrong with this,
since all neighbours are exactly the sames ?

I understand we can make it better with big surgery later, but right now
it seems quite reasonable.

This is already done (in part) for SIT devices, which are a subclass of
PtP device.

For the other patch, it seems problem was introduced in 2.6.38 when
CLONE_OFFLINK_ROUTE was removed in commit d80bc0fd26.