SWP_PFN_BITS
BUILD_BUG_ON(SWP_TYPE_SHIFT < SWP_PFN_BITS);
#define SWP_PFN_MASK (BIT(SWP_PFN_BITS) - 1)
#define SWP_MIG_YOUNG_BIT (SWP_PFN_BITS)
#define SWP_MIG_DIRTY_BIT (SWP_PFN_BITS + 1)
#define SWP_MIG_TOTAL_BITS (SWP_PFN_BITS + 2)