linux-kernel - Re: RO mount of ext4 filesystem causes writes

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Message-ID: <ZIa5P1HqE62rmzqu@debian.me>
Date:   Mon, 12 Jun 2023 13:20:47 +0700
From:   Bagas Sanjaya <bagasdotme@...il.com>
To:     Sean Greenslade <sean@...ngreenslade.com>,
        linux-ext4@...r.kernel.org, Ye Bin <yebin10@...wei.com>,
        Theodore Ts'o <tytso@....edu>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Regressions <regressions@...ts.linux.dev>
Subject: Re: RO mount of ext4 filesystem causes writes

On Sun, Jun 11, 2023 at 10:32:53PM -0700, Sean Greenslade wrote:
> 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.
> 

Can you show dmesg when regression happens?

Ye: It looks like this regression is caused by your commit. Would you like
to take a look on it?

Anyway, thanks for the bug report. I'm adding it to regzbot:

#regzbot ^introduced: eee00237fa5ec8
#regzbot title: commit super block writes even in read-only filesystems

Thanks.

-- 
An old man doll... just what I always wanted! - Clara

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