MAX_PHYSMEM_BITS   57 arch/x86/include/asm/pgtable_64_types.h #define MAXMEM		 _AC(__AC(1, UL) << MAX_PHYSMEM_BITS, UL)
MAX_PHYSMEM_BITS   28 include/linux/page-flags-layout.h #define SECTIONS_SHIFT	(MAX_PHYSMEM_BITS - SECTION_SIZE_BITS)