[PATCH v3 3/8] exfat: advance valid_size under the folio lock for buffered writes

[Chi Zhiling]

From: Chi Zhiling <chizhiling@xxxxxxxxxx>

Buffered writes currently advance valid_size in
exfat_write_iomap_end(), which runs after the folio has been unlocked.
As a result, a folio beyond valid_size can become uptodate and dirty,
and be observed by writeback or reclaim while valid_size still points
to an earlier position.

If reclaim drops such a folio before valid_size covers its range, a
subsequent read treats the range as unwritten and zero-fills it without
reading from disk. When this zero-filled page is later written back,
the zeros overwrite the data that was just written, silently losing
the write even if valid_size has since been advanced.

Advance valid_size from the buffered write path via an iomap
put_folio callback while the folio is still locked. This ensures
valid_size is updated before the folio becomes visible to writeback,
preventing the written data from being lost. Mark the inode dirty
whenever valid_size advances.

Fixes: 82a81a7352bc ("exfat: add iomap buffered I/O support")
Signed-off-by: Chi Zhiling <chizhiling@xxxxxxxxxx>
---
 fs/exfat/file.c  |  3 +++
 fs/exfat/iomap.c | 13 +++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/fs/exfat/file.c b/fs/exfat/file.c
index aa0e1e47e3d1..49c63364f747 100644
--- a/fs/exfat/file.c
+++ b/fs/exfat/file.c
@@ -929,6 +929,9 @@ static ssize_t exfat_file_write_iter(struct kiocb *iocb, struct iov_iter *iter)
 	if (ret < 0)
 		goto unlock;
 
+	if (exfat_get_valid_size(ei) > valid_size)
+		mark_inode_dirty(inode);
+
 	inode_unlock(inode);
 
 	if (iocb->ki_pos > pos) {
diff --git a/fs/exfat/iomap.c b/fs/exfat/iomap.c
index 32ca90b61b1a..10416335e8ee 100644
--- a/fs/exfat/iomap.c
+++ b/fs/exfat/iomap.c
@@ -232,6 +232,18 @@ const struct iomap_ops exfat_write_iomap_ops = {
 	.iomap_next	= exfat_write_iomap_next,
 };
 
+static void exfat_iomap_put_folio(struct inode *inode, loff_t pos,
+		unsigned int copied, struct folio *folio)
+{
+	struct exfat_inode_info *ei = EXFAT_I(inode);
+
+	if (copied)
+		exfat_advance_valid_size(ei, pos + copied);
+
+	folio_unlock(folio);
+	folio_put(folio);
+}
+
 /*
  * exfat_iomap_read_folio_range - Read a block range during a buffered write
  *
@@ -261,6 +273,7 @@ static int exfat_iomap_read_folio_range(const struct iomap_iter *iter,
 }
 
 const struct iomap_write_ops exfat_iomap_write_ops = {
+	.put_folio	= exfat_iomap_put_folio,
 	.read_folio_range = exfat_iomap_read_folio_range,
 };
 
-- 
2.53.0