linux-ext4 - RO mount of ext4 filesystem causes writes

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Message-ID: <ZIauBR7YiV3rVAHL@glitch>
Date:   Sun, 11 Jun 2023 22:32:53 -0700
From:   Sean Greenslade <sean@...ngreenslade.com>
To:     linux-ext4@...r.kernel.org
Cc:     Ye Bin <yebin10@...wei.com>
Subject: RO mount of ext4 filesystem causes writes

Hello, folks.

I noticed a change in behavior of ext4 in recent kernels. I make use of
several luks loopback images formatted as ext4 that I mount read-only
most of the time. I use rsync to synchronize the backing images between
machines. In the past, mouning the images as read-only would not touch
the backing image contents at all, but recently this changed. Every
mount, even ones that are RO from the start, will cause some small
writes to the backing image and thus force rsync to scan the whole file.

I confirmed that the issue is still present on v6.4.rc6, so I performed
a bisect and landed on the following commit:

> eee00237fa5ec8f704f7323b54e48cc34e2d9168 is the first bad commit
> commit eee00237fa5ec8f704f7323b54e48cc34e2d9168
> Author: Ye Bin <yebin10@...wei.com>
> Date:   Tue Mar 7 14:17:02 2023 +0800
> 
>     ext4: commit super block if fs record error when journal record without error

That certainly looks like a likely cause of my issue, but I'm not
familiar enough with the ext4 code to diagnose any further. Please let
me know if you need any additional information, or if you would like me
to test anything.

Thanks,

--Sean