netdev - Re: [PATCHv4 net 0/8] disable neigh update for tunnels during pmtu update

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Date:   Fri, 20 Dec 2019 09:14:09 -0700
From:   David Ahern <dsahern@...il.com>
To:     Hangbin Liu <liuhangbin@...il.com>, netdev@...r.kernel.org
Cc:     Julian Anastasov <ja@....bg>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Guillaume Nault <gnault@...hat.com>,
        David Miller <davem@...emloft.net>,
        Pablo Neira <pablo@...filter.org>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Alexey Kodanev <alexey.kodanev@...cle.com>
Subject: Re: [PATCHv4 net 0/8] disable neigh update for tunnels during pmtu
 update

On 12/19/19 8:25 PM, Hangbin Liu wrote:
> When we setup a pair of gretap, ping each other and create neighbour cache.
> Then delete and recreate one side. We will never be able to ping6 to the new
> created gretap.
> 
> The reason is when we ping6 remote via gretap, we will call like
> 
> gre_tap_xmit()
>  - ip_tunnel_xmit()
>    - tnl_update_pmtu()
>      - skb_dst_update_pmtu()
>        - ip6_rt_update_pmtu()
>          - __ip6_rt_update_pmtu()
>            - dst_confirm_neigh()
>              - ip6_confirm_neigh()
>                - __ipv6_confirm_neigh()
>                  - n->confirmed = now
> 
> As the confirmed time updated, in neigh_timer_handler() the check for
> NUD_DELAY confirm time will pass and the neigh state will back to
> NUD_REACHABLE. So the old/wrong mac address will be used again.
> 
> If we do not update the confirmed time, the neigh state will go to
> neigh->nud_state = NUD_PROBE; then go to NUD_FAILED and re-create the
> neigh later, which is what IPv4 does.
> 
> We couldn't remove the ip6_confirm_neigh() directly as we still need it
> for TCP flows. To fix it, we have to pass a bool parameter to
> dst_ops.update_pmtu() and only disable neighbor update for tunnels.
> 

seems reasonable to me.

Acked-by: David Ahern <dsahern@...il.com>