[zasdffaa]

> With single level store, the program pages are mapped in, not copied. Writing to the page alters the disk image. All processes running the same program share the memory pages.

Umm, mmap'ed files on linux allow both https://www.man7.org/linux/man-pages/man2/mmap.2.html

       MAP_SHARED
              Share this mapping.  Updates to the mapping are visible to
              other processes mapping the same region, and (in the case
              of file-backed mappings) are carried through to the
              underlying file.  (To precisely control when updates are
              carried through to the underlying file requires the use of
              msync(2).)

       MAP_PRIVATE
              Create a private copy-on-write mapping.  Updates to the
              mapping are not visible to other processes mapping the
              same file, and are not carried through to the underlying
              file.  It is unspecified whether changes made to the file
              after the mmap() call are visible in the mapped region.

and AFAIK on Windows the same.