[PATCH] nilfs2: clear folio dirty flag when copying back from the shadow map

[Jiaming Zhang <r772577952@gmail.com>]

While garbage collection runs, nilfs2 keeps a shadow copy of the DAT
metadata file's page cache so that it can roll the file back if GC
fails.  The rollback has two steps: nilfs_clear_dirty_pages() drops the
dirty state of the folios in the DAT cache, then nilfs_copy_back_pages()
overwrites them with the saved contents.  The second step warns if it
still finds a dirty folio, because the first step is supposed to have
cleared every one of them:

	/* overwrite existing folio in the destination cache */
	WARN_ON(folio_test_dirty(dfolio));

Clearing has been best-effort since commit ca76bb226bf4 ("nilfs2: do not
force clear folio if buffer is referenced"): nilfs_clear_folio_dirty()
leaves a folio dirty if a buffer head under it is still busy.  Reading
metadata creates such buffers.  nilfs_mdt_read_block() submits
read-ahead for the blocks following the one it was asked for and waits
only for that one, so the read-ahead buffers are still locked when it
returns.  When the block size is smaller than the page size, several
metadata blocks share a folio, so a single folio can hold both a dirty
block and a locked read-ahead buffer.  Such a folio survives the
clearing step, and the copy-back warns on it.

Use __nilfs_clear_folio_dirty() to clear the dirty flag of the
destination folio before overwriting it, rather than making
nilfs_clear_folio_dirty() force-clear busy buffer heads again.

Fixes: ca76bb226bf4 ("nilfs2: do not force clear folio if buffer is referenced")
Closes: https://lore.kernel.org/lkml/CANypQFZSYrtcshnUzOPiqatyLd-M8_OReOewQoAi_V5yY0dTtg@mail.gmail.com/
Cc: stable@vger.kernel.org
Assisted-by: Claude Code:claude-opus-5
Signed-off-by: Jiaming Zhang <r772577952@gmail.com>
---
 fs/nilfs2/page.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/nilfs2/page.c b/fs/nilfs2/page.c
index cf4f1c6798f5..b26d9c3bda6d 100644
--- a/fs/nilfs2/page.c
+++ b/fs/nilfs2/page.c
@@ -328,7 +328,8 @@ void nilfs_copy_back_pages(struct address_space *dmap,
 		dfolio = filemap_lock_folio(dmap, index);
 		if (!IS_ERR(dfolio)) {
 			/* overwrite existing folio in the destination cache */
-			WARN_ON(folio_test_dirty(dfolio));
+			if (unlikely(folio_test_dirty(dfolio)))
+				__nilfs_clear_folio_dirty(dfolio);
 			nilfs_copy_folio(dfolio, folio, false);
 			folio_unlock(dfolio);
 			folio_put(dfolio);
-- 
2.43.0