linux-ext4 - Re: [PATCH] ext4: Fix call trace when remounting to read only in data=journal mode

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Message-ID: <gluj62pw5pu7ag2juf5ejwsr3ghvckag7wh4zunwyk57slcrmg@42of57gybigz>
Date: Tue, 3 Feb 2026 15:50:43 +0100
From: Jan Kara <jack@...e.cz>
To: Gerald Yang <gerald.yang@...onical.com>
Cc: Jan Kara <jack@...e.cz>, tytso@....edu, adilger.kernel@...ger.ca, 
	linux-ext4@...r.kernel.org, gerald.yang.tw@...il.com
Subject: Re: [PATCH] ext4: Fix call trace when remounting to read only in
 data=journal mode

Hello,

On Fri 30-01-26 19:38:55, Gerald Yang wrote:
> Thanks for sharing the findings, I'd also like to share some findings:
> I tried to figure out why the buffer is dirty after calling sync_filesystem,
> in mpage_prepare_extent_to_map, first I printed folio_test_dirty(folio):
> 
> while (index <= end)
>     ...
>     for (i = 0; i < nr_folios; i++) {
>         ...
>         (print if folio is dirty here)
> 
> and actually all folios are clean:
> if (!folio_test_dirty(folio) ||
>     ...
>     folio_unlock(folio);
>     continue;       <==== continue here without writing anything
> 
> Because the call trace happens before going into the above while loop:
> 
> if (ext4_should_journal_data(mpd->inode)) {
>     handle = ext4_journal_start(mpd->inode, EXT4_HT_WRITE_PAGE,
> 
> it checks if the file system is read only and dumps the call trace in
> ext4_journal_check_start, but it doesn't check if there are any real writes
> that will happen later in the loop.
> 
> To confirm this, first I added 2 more lines in the reproduce script before
> remounting read only:
> sync      <==== it calls ext4_sync_fs to flush all dirty data same as what's
>                          called during remount read only
> echo 1 > /proc/sys/vm/drop_caches       <==== drop clean page cache
> mount -o remount,ro ext4disk mnt
> 
> Then I can no longer reproduce the call trace.

OK, but ext4_do_writepages() has a check at the beginning:

        if (!mapping->nrpages || !mapping_tagged(mapping, PAGECACHE_TAG_DIRTY))
                goto out_writepages;

So if there are no dirty pages, mapping_tagged(mapping, PAGECACHE_TAG_DIRTY)
should be false and so we shouldn't go further?

It all looks like some kind of a race because I'm not always able to
reproduce the problem... I'll try to look more into this.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR