SUBSECTION_SHIFT
#define SUBSECTION_SIZE (1UL << SUBSECTION_SHIFT)
#define PFN_SUBSECTION_SHIFT (SUBSECTION_SHIFT - PAGE_SHIFT)
#if SUBSECTION_SHIFT > SECTION_SIZE_BITS
#define SUBSECTIONS_PER_SECTION (1UL << (SECTION_SIZE_BITS - SUBSECTION_SHIFT))