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

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Message-ID: <tmtgzmvkfag4r6lbt4i2ej5ad3bfudezcm35l27ybit25r7l4d@5o2i4cuymh5j>
Date: Thu, 5 Feb 2026 10:25:39 +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

On Tue 03-02-26 15:50:43, Jan Kara wrote:
> 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.

OK, the race is with checkpointing code writing the buffers while flush
worker tries to writeback the pages. I've posted a patch which fixes the
issue for me.

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