MAX_PHYSMEM_BITS  839 include/linux/mmzone.h #define SECTIONS_SHIFT		(MAX_PHYSMEM_BITS - SECTION_SIZE_BITS)
MAX_PHYSMEM_BITS  154 mm/sparse.c    	unsigned long max_sparsemem_pfn = 1UL << (MAX_PHYSMEM_BITS-PAGE_SHIFT);