PMD_SHIFT          39 arch/x86/include/asm/pgtable-3level_types.h #define PMD_SHIFT	21
PMD_SHIFT          41 arch/x86/include/asm/pgtable_64_types.h #define PMD_SHIFT	21
PMD_SHIFT          19 include/asm-generic/pgtable-nopmd.h #define PMD_SHIFT	PUD_SHIFT