Mike Kravetz [off-list ref] writes:

Two new sysfs files are added to demote hugtlb pages.  These files are
both per-hugetlb page size and per node.  Files are:
  demote_size - The size in Kb that pages are demoted to. (read-write)
  demote - The number of huge pages to demote. (write-only)

By default, demote_size is the next smallest huge page size.  Valid huge
page sizes less than huge page size may be written to this file.  When
huge pages are demoted, they are demoted to this size.

Writing a value to demote will result in an attempt to demote that
number of hugetlb pages to an appropriate number of demote_size pages.

NOTE: Demote interfaces are only provided for huge page sizes if there
is a smaller target demote huge page size.  For example, on x86 1GB huge
pages will have demote interfaces.  2MB huge pages will not have demote
interfaces.

Should we also check if the platform allows for
gigantic_page_runtime_supported() ? 

-aneesh