netdev - Re: IPv6 regression introduced by commit 3b6761d18bc11f2af2a6fc494e9026d39593f22c

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Date:   Sun, 27 Sep 2020 20:38:28 -0700
From:   David Ahern <dsahern@...il.com>
To:     Baptiste Jonglez <baptiste@...sofnetworks.org>
Cc:     Alarig Le Lay <alarig@...rdarmor.fr>, netdev@...r.kernel.org,
        jack@...ilfillan.uk, Vincent Bernat <bernat@...ian.org>,
        Oliver <bird-o@...net.de>
Subject: Re: IPv6 regression introduced by commit
 3b6761d18bc11f2af2a6fc494e9026d39593f22c

On 9/27/20 9:10 AM, Baptiste Jonglez wrote:
> On 27-09-20, Baptiste Jonglez wrote:
>> 1) failing IPv6 neighbours, what Alarig reported.  We are seeing this
>>    on a full-view BGP router with rather low amount of IPv6 traffic
>>    (around 10-20 Mbps)
> 
> Ok, I found a quick way to reproduce this issue:
> 
>     # for net in {1..9999}; do ip -6 route add 2001:db8:ffff:${net}::/64 via fe80::4242 dev lo; done
> 
> and then:
> 
>     # for net in {1..9999}; do ping -c1 2001:db8:ffff:${net}::1; done
> 
> This quickly gets to a situation where ping fails early with:
> 
>     ping: connect: Network is unreachable
> 
> At this point, IPv6 connectivity is broken.  The kernel is no longer
> replying to IPv6 neighbor solicitation from other hosts on local
> networks.
> 
> When this happens, the "fib_rt_alloc" field from /proc/net/rt6_stats
> is roughly equal to net.ipv6.route.max_size (a bit more in my tests).
> 
> Interestingly, the system appears to stay in this broken state
> indefinitely, even without trying to send new IPv6 traffic.  The
> fib_rt_alloc statistics does not decrease.
> 

fib_rt_alloc is incremented by calls to ip6_dst_alloc. Each of your
9,999 pings is to a unique address and hence causes a dst to be
allocated and the counter to be incremented. It is never decremented.
That is standard operating procedure.