ext4: Check for atomic writes support in write iter

Let's validate the given constraints for atomic write request.
Otherwise it will fail with -EINVAL. Currently atomic write is only
supported on DIO, so for buffered-io it will return -EOPNOTSUPP.

Reviewed-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>

Author: riteshharjani

fs/ext4/file.c

@@ -692,6 +692,20 @@ ext4_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	if (IS_DAX(inode))
 		return ext4_dax_write_iter(iocb, from);
 #endif
+
+	if (iocb->ki_flags & IOCB_ATOMIC) {
+		size_t len = iov_iter_count(from);
+		int ret;
+
+		if (len < EXT4_SB(inode->i_sb)->s_awu_min ||
+		    len > EXT4_SB(inode->i_sb)->s_awu_max)
+			return -EINVAL;
+
+		ret = generic_atomic_write_valid(iocb, from);
+		if (ret)
+			return ret;
+	}
+
 	if (iocb->ki_flags & IOCB_DIRECT)
 		return ext4_dio_write_iter(iocb, from);
 	else