MAX_USER_VA_BITS   72 arch/arm64/include/asm/pgtable-hwdef.h #define PTRS_PER_PGD		(1 << (MAX_USER_VA_BITS - PGDIR_SHIFT))
MAX_USER_VA_BITS   12 arch/arm64/include/asm/processor.h #define USER_DS			((UL(1) << MAX_USER_VA_BITS) - 1)