A ReservedSpace can sometimes have an underlying mapping on Linux that uses more than one page sizes. This happens when a mapping is requested to use large pages but the size of the reservation is not aligned to the large page size. This is handled by:
os::Linux::reserve_memory_special_huge_tlbfs_mixed(...)

We should investigate if this functionality is needed or if we could simplify this to guarantee that a ReservedSpace will consist of a mapping with only one page size. If we have that the ReservedSpace could keep the page size it was mapped with and we would not need helpers trying to estimate the page size like:
ReservedSpace::actual_reserved_page_size(...)

There has been a side discussion about this in a recent PR that added the above mentioned helper, for some more background please see:
https://github.com/openjdk/jdk/pull/1161