BITS_PER_PGD_ENTRY
#define PGD_ENTRY_SIZE (1UL << BITS_PER_PGD_ENTRY)
#if (PGDIR_SHIFT + PAGE_SHIFT + PGD_TABLE_ORDER - BITS_PER_PGD_ENTRY) > BITS_PER_LONG
#define BITS_PER_PGD (PAGE_SHIFT + PGD_TABLE_ORDER - BITS_PER_PGD_ENTRY)