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

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Date:   Mon, 13 May 2019 21:35:37 -0600
From:   David Ahern <dsahern@...il.com>
To:     Mikael Magnusson <mikael.kernel@...ts.m7n.se>,
        netdev@...r.kernel.org, Martin KaFai Lau <kafai@...com>,
        Wei Wang <weiwan@...gle.com>
Subject: Re: IPv6 PMTU discovery fails with source-specific routing

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.