[PATCH v3 4/4] ext4: Do not fallback to buffered-io for DIO atomic write

["Ritesh Harjani (IBM)" <ritesh.list@gmail.com>]

atomic writes is currently only supported for single fsblock and only
for direct-io. We should not return -ENOTBLK for atomic writes since we
want the atomic write request to either complete fully or fail
otherwise. We should not fallback to buffered-io in case of DIO atomic
write requests.
Let's also catch if this ever happens by adding some WARN_ON_ONCE before
buffered-io handling for direct-io atomic writes.

More details of the discussion [1].

[1]: https://lore.kernel.org/linux-xfs/cover.1729825985.git.ritesh.list@gmail.com/T/#m9dbecc11bed713ed0d7a486432c56b105b555f04

Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
---
 fs/ext4/file.c  |  7 +++++++
 fs/ext4/inode.c | 14 +++++++++-----
 2 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 8116bd78910b..61787a37e9d4 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -599,6 +599,13 @@ static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
 		ssize_t err;
 		loff_t endbyte;
 
+		/*
+		 * There is no support for atomic writes on buffered-io yet,
+		 * we should never fallback to buffered-io for DIO atomic
+		 * writes.
+		 */
+		WARN_ON_ONCE(iocb->ki_flags & IOCB_ATOMIC);
+
 		offset = iocb->ki_pos;
 		err = ext4_buffered_write_iter(iocb, from);
 		if (err < 0)
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index fcdee27b9aa2..26b3c84d7f64 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3449,12 +3449,16 @@ static int ext4_iomap_end(struct inode *inode, loff_t offset, loff_t length,
 {
 	/*
 	 * Check to see whether an error occurred while writing out the data to
-	 * the allocated blocks. If so, return the magic error code so that we
-	 * fallback to buffered I/O and attempt to complete the remainder of
-	 * the I/O. Any blocks that may have been allocated in preparation for
-	 * the direct I/O will be reused during buffered I/O.
+	 * the allocated blocks. If so, return the magic error code for
+	 * non-atomic write so that we fallback to buffered I/O and attempt to
+	 * complete the remainder of the I/O.
+	 * For atomic writes we will simply fail the I/O request if we coudn't
+	 * write anything. For non-atomic writes, any blocks that may have been
+	 * allocated in preparation for the direct I/O will be reused during
+	 * buffered I/O.
 	 */
-	if (flags & (IOMAP_WRITE | IOMAP_DIRECT) && written == 0)
+	if (!(flags & IOMAP_ATOMIC) && (flags & (IOMAP_WRITE | IOMAP_DIRECT))
+			&& written == 0)
 		return -ENOTBLK;
 
 	return 0;
-- 
2.46.0