[PATCH 1/3] ext2: 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, generic_perform_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.  Ext2 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 generic_perform_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: fb5de4358e1a ("ext2: Move direct-io to use iomap")
Link: https://lore.kernel.org/r/20260629113827.4074335-3-libaokun@xxxxxxxxxxxxxxxxx
Signed-off-by: Jiale Yao <yaojiale02@xxxxxxx>
---
 fs/ext2/file.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/ext2/file.c b/fs/ext2/file.c
index b9020df7d89e..67fe423c3828 100644
--- a/fs/ext2/file.c
+++ b/fs/ext2/file.c
@@ -135,6 +135,13 @@ static ssize_t ext2_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
 		int ret2;
 
 		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(inode);
+
 		pos = iocb->ki_pos;
 		status = generic_perform_write(iocb, from);
 		if (unlikely(status < 0)) {
-- 
2.34.1