direct-io: add flag to allow aio writes beyond i_size

Some filesystems can handle direct I/O writes beyond i_size safely,
so allow them to opt into receiving them.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/direct-io.c

@@ -1194,13 +1194,19 @@ do_blockdev_direct_IO(int rw, struct kiocb *iocb, struct inode *inode,
 	}
 
 	/*
-	 * For file extending writes updating i_size before data
-	 * writeouts complete can expose uninitialized blocks. So
-	 * even for AIO, we need to wait for i/o to complete before
-	 * returning in this case.
+	 * For file extending writes updating i_size before data writeouts
+	 * complete can expose uninitialized blocks in dumb filesystems.
+	 * In that case we need to wait for I/O completion even if asked
+	 * for an asynchronous write.
 	 */
-	dio->is_async = !is_sync_kiocb(iocb) && !((rw & WRITE) &&
-		(end > i_size_read(inode)));
+	if (is_sync_kiocb(iocb))
+		dio->is_async = false;
+	else if (!(dio->flags & DIO_ASYNC_EXTEND) &&
+            (rw & WRITE) && end > i_size_read(inode))
+		dio->is_async = false;
+	else
+		dio->is_async = true;
+
 	dio->inode = inode;
 	dio->rw = rw;
 

include/linux/fs.h

@@ -2527,6 +2527,9 @@ enum {
 
 	/* filesystem does not support filling holes */
 	DIO_SKIP_HOLES	= 0x02,
+
+	/* filesystem can handle aio writes beyond i_size */
+	DIO_ASYNC_EXTEND = 0x04,
 };
 
 void dio_end_io(struct bio *bio, int error);