netdev - Re: [PATCH] Revert "ipv6: add mtu lock check in __ip6_rt_update

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Date:   Thu, 07 May 2020 17:30:04 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     zenczykowski@...il.com
Cc:     maze@...gle.com, netdev@...r.kernel.org, edumazet@...gle.com,
        willemb@...gle.com, lucien.xin@...il.com,
        hannes@...essinduktion.org
Subject: Re: [PATCH] Revert "ipv6: add mtu lock check in
 __ip6_rt_update_pmtu"

From: Maciej Żenczykowski <zenczykowski@...il.com>
Date: Tue,  5 May 2020 11:57:23 -0700

> From: Maciej Żenczykowski <maze@...gle.com>
> 
> This reverts commit 19bda36c4299ce3d7e5bce10bebe01764a655a6d:
> 
> | ipv6: add mtu lock check in __ip6_rt_update_pmtu
> |
> | Prior to this patch, ipv6 didn't do mtu lock check in ip6_update_pmtu.
> | It leaded to that mtu lock doesn't really work when receiving the pkt
> | of ICMPV6_PKT_TOOBIG.
> |
> | This patch is to add mtu lock check in __ip6_rt_update_pmtu just as ipv4
> | did in __ip_rt_update_pmtu.
> 
> The above reasoning is incorrect.  IPv6 *requires* icmp based pmtu to work.
> There's already a comment to this effect elsewhere in the kernel:
> 
>   $ git grep -p -B1 -A3 'RTAX_MTU lock'
>   net/ipv6/route.c=4813=
> 
>   static int rt6_mtu_change_route(struct fib6_info *f6i, void *p_arg)
>   ...
>     /* In IPv6 pmtu discovery is not optional,
>        so that RTAX_MTU lock cannot disable it.
>        We still use this lock to block changes
>        caused by addrconf/ndisc.
>     */
> 
> This reverts to the pre-4.9 behaviour.
> 
> Signed-off-by: Maciej Żenczykowski <maze@...gle.com>
> Fixes: 19bda36c4299 ("ipv6: add mtu lock check in __ip6_rt_update_pmtu")

I've thought about this some more and decided to apply this and
queue it up for -stable, thank you.