netdev - Re: IPv6 PMTU discovery fails with source-specific routing

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Date:   Mon, 13 May 2019 23:12:31 -0700
From:   Wei Wang <weiwan@...gle.com>
To:     David Ahern <dsahern@...il.com>,
        Mikael Magnusson <mikael.kernel@...ts.m7n.se>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>
Subject: Re: IPv6 PMTU discovery fails with source-specific routing

Thanks Mikael for reporting this issue. And thanks David for the bisection.
Let me spend some time to reproduce it and see what is going on.

From: David Ahern <dsahern@...il.com>
Date: Mon, May 13, 2019 at 8:35 PM
To: Mikael Magnusson, <netdev@...r.kernel.org>, Martin KaFai Lau, Wei Wang

> On 5/13/19 1:22 PM, Mikael Magnusson wrote:
> > Hello list,
> >
> > I think I have found a regression in 4.15+ kernels. IPv6 PMTU discovery
> > doesn't seem to work with source-specific routing (AKA source-address
> > dependent routing, SADR).
> >
> > I made a test script (see attachment). It sets up a test environment
> > with three network namespaces (a, b and c) using SADR. The link between
> > b and c is configured with MTU 1280. It then runs a ping test with large
> > packets.
> >
> > I have tested a couple of kernels on Ubuntu 19.04 with the following
> > results.
> >
> > mainline 4.14.117-0414117-generic SUCCESS
> > ubuntu   4.15.0-1036-oem          FAIL
> > mainline 5.1.0-050100-generic     FAIL
> >
>
> git bisect shows
>
> good: 38fbeeeeccdb38d0635398e8e344d245f6d8dc52
> bad:  2b760fcf5cfb34e8610df56d83745b2b74ae1379
>
> Those are back to back commits so
>     2b760fcf5cfb ipv6: hook up exception table to store dst cache
>
> has to be the bad commit.
>
> Your patch may work, but does not seem logical relative to code at the
> time of 4.15 and the commit that caused the failure. cc'ing authors of
> the changes referenced above.