netdev - BUG: jumbo frames broken after commit xen-netfront: Fix race between device setup and open

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Message-ID: <CAK=Wzwcjz5JKzfrdGpWNXhb6ZmTrpbNjSJxiivZt3HF8cKYi3Q@mail.gmail.com>
Date:   Wed, 6 Jun 2018 09:29:03 -0700
From:   Andrew Jeddeloh <andrew.jeddeloh@...hat.com>
To:     netdev@...r.kernel.org
Subject: BUG: jumbo frames broken after commit xen-netfront: Fix race between
 device setup and open

Hi all,

The patch "xen-netfront: Fix race between device setup and open" seems
to have introduced a regression preventing setting MTU's larger than
1500. We experienced this downstream with Container Linux and
confirmed with Fedora 28 as well.

It's commit f599c64fdf7d9c108e8717fb04bc41c680120da4 in the linux-stable tree.
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/commit/?id=f599c64fdf7d9c108e8717fb04bc41c680120da4

Downstream bugs:
https://github.com/coreos/bugs/issues/2443
https://bugzilla.redhat.com/show_bug.cgi?id=1584216

We've confirmed that reverting that commit fixes the bug. It be
reliably can be reproduced on AWS with t2.micro instances (and
presumably other systems using the same driver). Both using
systemd-networkd to set the mtu and manual ip link commands cause the
link to repsond with "Invalid argument" when trying to set the MTU >
1500.

I'm not sure why that commit introduced the regression.

Please let me know if there's any more information that would be helpful.

- Andrew