[PATCH 4/6] fs/ntfs3: write back folios that raced with $DATA conversion

[Kamil Chojnowski via B4 Relay]

From: Kamil Chojnowski <contact@xxxxxxxxx>

ntfs_writepages() takes the resident path when is_resident() is true on
entry, then calls attr_data_write_resident() for each dirty folio.  If
$DATA was moved out of the MFT record in between, it returns the
internal code E_NTFS_NONRESIDENT.  writeback_iter() has already cleared
the folio's dirty bit, so its data is never written.

Commit 2727ac2ba87a ("fs/ntfs3: prevent positive E_NTFS_NONRESIDENT from
reaching writeback error handling") turned the code into -EIO, which
stops the errseq and writeback warnings, but the folio is still dropped
and the writeback reports an I/O error for data that could be written.

Redirty the folio and finish through iomap_writepages(), which writes
it to the clusters the attribute now has.  ntfs_writepage() handled the
same race by falling back to block_write_full_page() until commit
d4428bad14dd ("ntfs3: stop using generic_writepages").

Reproducer: grow small files with fallocate(2) while another thread
reads them, with the first patch of this series applied so that the
deadlock does not hide the race.  Without the -EIO conversion this
warned in lib/errseq.c and mm/page-writeback.c within seconds.

Fixes: d4428bad14dd ("ntfs3: stop using generic_writepages")
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Kamil Chojnowski <contact@xxxxxxxxx>
---
 fs/ntfs3/inode.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/fs/ntfs3/inode.c b/fs/ntfs3/inode.c
index 913ebd2a2196..8651b9e4ba06 100644
--- a/fs/ntfs3/inode.c
+++ b/fs/ntfs3/inode.c
@@ -1267,6 +1267,7 @@ static int ntfs_writepages(struct address_space *mapping,
 
 	if (is_resident(ni)) {
 		struct folio *folio = NULL;
+		bool converted = false;
 		err = 0;
 
 		while ((folio = writeback_iter(mapping, wbc, folio, &err))) {
@@ -1276,6 +1277,18 @@ static int ntfs_writepages(struct address_space *mapping,
 			err2 = attr_data_write_resident(ni, folio);
 			ni_unlock(ni);
 
+			if (err2 == E_NTFS_NONRESIDENT) {
+				/*
+				 * $DATA was moved out of the MFT record after
+				 * the is_resident() check above.  The folio is
+				 * no longer dirty: redirty it and write it
+				 * through iomap below.
+				 */
+				folio_redirty_for_writepage(wbc, folio);
+				converted = true;
+				err2 = 0;
+			}
+
 			folio_unlock(folio);
 			if (err2) {
 				/*
@@ -1293,7 +1306,8 @@ static int ntfs_writepages(struct address_space *mapping,
 			}
 		}
 
-		return err;
+		if (!converted || err)
+			return err;
 	}
 
 	return iomap_writepages(&wpc);

-- 
2.56.0