Does similar things to what kernel does - users can mmap file parts into
address space and access them read/write. The manager will be getting
invoked by hardware/OS kernel for cases when there is no page loaded for
read, or when a previousle read-only page is being written to.
Additionally to features provided in kernel, it support to be used to
store back changes in transactional way (see fileh_dirty_writeout()) and
potentially use huge pages for mappings (though this is currently TODO)