When the "modules" file is opened in ImageFileReader::open(), and the contents are mmap()-ed, the size to be mmap()-ed is derived from map_size().

399     // Memory map image (minimally the index.)
400     _index_data = (u1*)osSupport::map_memory(_fd, _name, 0, (size_t)map_size());

Which could be _file_size or _index_size, and for 64 bit processes, it would be _file_size. (about 140 MB)

488     // Retrieve the size of the mapped image.
489     inline u8 map_size() const {
490         return (u8)(memory_map_image ? _file_size : _index_size);
491     }

But when the contents are unmapped in ImageFileReader::close(), the amount of memory unmapped is only _index_size (which is considerably lesser than _file_size).

427 // Close image file.
428 void ImageFileReader::close() {
429     // Deallocate the index.
430     if (_index_data) {
431         osSupport::unmap_memory((char*)_index_data, _index_size);
432         _index_data = NULL;
433     }

The amount of memory unmapped should also be map_size().  

Mail thread here: 

http://mail.openjdk.java.net/pipermail/jigsaw-dev/2018-December/014043.html