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[118.163.61.247]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-845b8655227sm1536396b3a.5.2026.06.25.09.01.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 25 Jun 2026 09:01:50 -0700 (PDT) From: Gerald Yang To: tytso@mit.edu, jack@suse.cz Cc: linux-ext4@vger.kernel.org, gerald.yang.tw@gmail.com Subject: [PATCH v2] ext4: clear stale xarray tags on folios skipped during writeback Date: Fri, 26 Jun 2026 00:01:23 +0800 Message-ID: <20260625160127.162272-1-gerald.yang@canonical.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-ext4@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit In data=journal mode, the writeback thread can hit the WARN_ON_ONCE(sb_rdonly(sb)) in ext4_journal_check_start() while the superblock is being remounted read-only during reboot: Workqueue: writeback wb_workfn (flush-253:0) RIP: 0010:ext4_journal_check_start+0x8b/0xd0 Call Trace: __ext4_journal_start_sb+0x3c/0x1e0 mpage_prepare_extent_to_map+0x4af/0x580 ext4_do_writepages+0x3c0/0x1080 ext4_writepages+0xc8/0x1a0 do_writepages+0xc4/0x180 __writeback_single_inode+0x45/0x2f0 writeback_sb_inodes+0x26b/0x5d0 __writeback_inodes_wb+0x54/0x100 wb_writeback+0x1ac/0x320 wb_workfn+0x394/0x470 And followed by the warning: EXT4-fs warning (device vda1): ext4_evict_inode:195: inode #6263: comm (sd-umount): data will be lost This issue is not reproduced every time, but frequently. The reproduction step is to create a VM with 8 CPUs, 16G memory and setup data=journal: sudo tune2fs -o journal_data /dev/vda1 Run fio: rm -f fiotest fio --name=fiotest --rw=randwrite --bs=4k --runtime=6 --ioengine=libaio --iodepth=256 --numjobs=8 --filename=fiotest --filesize=30G --group_reporting Reboot the VM, and check the console output from: virsh console testvm But there is no dirty inode, folio_clear_dirty_for_io clears PG_dirty but leaves tags PAGECACHE_TAG_DIRTY and PAGECACHE_TAG_TOWRITE set which are only cleared by __folio_start_writeback. In data=journal mode, jbd2 checkpoints the journalled data to its final location and clears its own dirty flag without touching folio PG_dirty or xarray dirty flags. The commit f4a2b42e7891 ("ext4: fix stale xarray tags after writeback") fixes when PG_dirty is still set but there is no dirty page. Another case is PG_dirty is cleared, but PAGECACHE_TAG_DIRTY and PAGECACHE_TAG_TOWRITE is still set. In this case, writeback thread checks clean folio and skips it in mpage_prepare_extent_to_map: if (!folio_test_dirty(folio) || ... folio_unlcok(folio); continue And never reaches ext4_bio_write_folio where the commit f4a2b42e7891 clears the stale xarray tags. Print debug logs after the filesystem is remounted read-only: writepages RDONLY nrpages=2048 dirtytag=1 wbtag=0 towrite=1 sync=0 And all folios are actually clean: folio idx=3 dirty=0 wb=0 checked=0 dirtybuf=0 jbddirty=0 mapped=1 ... We need to clear the xarray stale tags for such clean folios by cycling them through writeback in the skip path, the same way f4a2b42e7891 does in ext4_bio_write_folio. Fixes: dff4ac75eeee ("ext4: move keep_towrite handling to ext4_bio_write_page()") Signed-off-by: Gerald Yang --- Changes in v2: Split the top level condition based on Jan's suggestion fs/ext4/inode.c | 18 +++++++++++++++--- 1 file changed, 15 insertions(+), 3 deletions(-) diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c index ce99807c5f5b..150f8789f0aa 100644 --- a/fs/ext4/inode.c +++ b/fs/ext4/inode.c @@ -2694,13 +2694,25 @@ static int mpage_prepare_extent_to_map(struct mpage_da_data *mpd) * page is already under writeback and we are not doing * a data integrity writeback, skip the page */ - if (!folio_test_dirty(folio) || - (folio_test_writeback(folio) && - (mpd->wbc->sync_mode == WB_SYNC_NONE)) || + if ((folio_test_writeback(folio) && + mpd->wbc->sync_mode == WB_SYNC_NONE) || unlikely(folio->mapping != mapping)) { folio_unlock(folio); continue; } + /* + * If the folio is clean, skip writing it back. + * Cycle the folio through the writeback state + * though, to clear stale xarray tags. + */ + if (!folio_test_dirty(folio)) { + if (!folio_test_writeback(folio)) { + __folio_start_writeback(folio, false); + folio_end_writeback(folio); + } + folio_unlock(folio); + continue; + } folio_wait_writeback(folio); BUG_ON(folio_test_writeback(folio)); -- 2.43.0