linux-ext4 - Problem with external journal and LVM snapshots

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Date:   Fri, 1 Jun 2018 12:47:05 +0200
From:   "Maarten van Malland" <maartenvanmalland@...il.com>
To:     <linux-ext4@...r.kernel.org>
Subject: Problem with external journal and LVM snapshots

I have a not so common setup that IMHO triggers a bug in the Ext4 journal code. I have the following setup:

- A mdadm RAID10 device with Bcache backing and LVM on top. This should actually not matter at all, but perhaps still worth mentioning.
- The Ext4 volume resides on a LVM VG, with an external journal on a NVMe drive.
- I use LVM snapshotting for that volume

Now, when I make the snapshot I do the following:

lvremove /dev/bcache/root-snap
lvcreate -c 512 -I 512 -n root-snap -L 250G -s /dev/bcache/root
tune2fs -O ^has_journal /dev/bcache/root-snap (to get rid of the external journal)
tune2fs -O has_journal /dev/bcache/root-snap (to create a new internal journal)

When finished, I can mount /dev/bcache/root-snap just fine, with the internal journal working. However, when I reboot it's a different issue. For whatever reason the kernel still sees both /dev/bcache/root and /dev/bcache/root-snap with an external journal! The kernel doesn't want to mount root anymore as there are two volumes with the same journal device. For now I've written a little script in the initrd to fix this stuff, but that shouldn't be necessary...