MADV_DONTNEED
              Do not expect access in the near future.  (For the time being,
              the application is finished with the given range, so the
              kernel can free resources associated with it.)
  
              After a successful MADV_DONTNEED operation, the semantics of
              memory access in the specified region are changed: subsequent
              accesses of pages in the range will succeed, but will result
              in either repopulating the memory contents from the up-to-date
              contents of the underlying mapped file (for shared file
              mappings, shared anonymous mappings, and shmem-based
              techniques such as System V shared memory segments) or zero-
              fill-on-demand pages for anonymous private mappings.
  
              Note that, when applied to shared mappings, MADV_DONTNEED
              might not lead to immediate freeing of the pages in the range.
              The kernel is free to delay freeing the pages until an
              appropriate moment.  The resident set size (RSS) of the
              calling process will be immediately reduced however.
  
              MADV_DONTNEED cannot be applied to locked pages, Huge TLB
              pages, or VM_PFNMAP pages.  (Pages marked with the kernel-
              internal VM_PFNMAP flag are special memory areas that are not
              managed by the virtual memory subsystem.  Such pages are
              typically created by device drivers that map the pages into
              user space.)