fuse: Invalidate the page cache after FOPEN_DIRECT_IO write

generic_file_direct_write() also does this and has a large
comment about.

Reproducer here is xfstest's generic/209, which is exactly to
have competing DIO write and cached IO read.

Signed-off-by: Bernd Schubert <[email protected]>
Signed-off-by: Miklos Szeredi <[email protected]>

fs/fuse/file.c

@@ -1693,6 +1693,15 @@ ssize_t fuse_direct_io(struct fuse_io_priv *io, struct iov_iter *iter,
 	if (res > 0)
 		*ppos = pos;
 
+	if (res > 0 && write && fopen_direct_io) {
+		/*
+		 * As in generic_file_direct_write(), invalidate after the
+		 * write, to invalidate read-ahead cache that may have competed
+		 * with the write.
+		 */
+		invalidate_inode_pages2_range(mapping, idx_from, idx_to);
+	}
+
 	return res > 0 ? res : err;
 }
 EXPORT_SYMBOL_GPL(fuse_direct_io);