BITS_PER_PGD
#define PTRS_PER_PGD (1UL << BITS_PER_PGD)
#define MAX_ADDRBITS (PGDIR_SHIFT + BITS_PER_PGD)
DEFINE(ASM_BITS_PER_PGD, BITS_PER_PGD);
BUILD_BUG_ON(PAGE_SHIFT + BITS_PER_PTE + BITS_PER_PMD + BITS_PER_PGD