[PATCH 3/3] exfat: drain in-flight DIO before buffered write fallback

[Jiale Yao]

An asynchronous direct write can remain in flight after the inode lock is
released.  If another direct write falls back to buffered I/O while the
first write is still pending, iomap_file_buffered_write() can dirty pages
before the first write completes its post-I/O page cache invalidation.
The invalidation then finds dirty pages, reports a page cache invalidation
failure, and records -EIO in the mapping error sequence.  A later fsync()
therefore returns -EIO.

Commit 15cdefd0c0522f9d5e12d947fa04f4c11649b699 ("ext4: drain
in-flight DIO before buffered write fallback") fixed the same race in
ext4.  ExFAT has an equivalent fallback after iomap_dio_rw() returns
-ENOTBLK or a short write, but does not drain other in-flight DIO before
dirtying the page cache.

Wait for in-flight DIO before calling iomap_file_buffered_write() in the
fallback path.

A reproducer using concurrent AIO direct writes and buffered fallback
triggered the following warning and made a subsequent fsync() return
-EIO:

  Page cache invalidation failure on direct I/O.  Possible data corruption
  due to collision with buffered I/O!

Fixes: 867b9c96dc83 ("exfat: add iomap direct I/O support")
Link: https://lore.kernel.org/r/20260629113827.4074335-3-libaokun@xxxxxxxxxxxxxxxxx
Signed-off-by: Jiale Yao <yaojiale02@xxxxxxx>
---
 fs/exfat/file.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/exfat/file.c b/fs/exfat/file.c
index a2a9ee1a2004..cf5ccbd54823 100644
--- a/fs/exfat/file.c
+++ b/fs/exfat/file.c
@@ -807,6 +807,12 @@ static ssize_t exfat_fallback_buffered_write(struct kiocb *iocb,
 
 	iocb->ki_flags &= ~IOCB_DIRECT;
 
+	/*
+	 * Prevent concurrent direct I/O and buffered I/O to the same file
+	 * range. Wait for in-flight DIO to finish before dirtying pages.
+	 */
+	inode_dio_wait(file_inode(iocb->ki_filp));
+
 	written = iomap_file_buffered_write(iocb, from, &exfat_write_iomap_ops,
 			NULL, NULL);
 	if (written < 0)
-- 
2.34.1