EVICTION_SHIFT
#define EVICTION_MASK (~0UL >> EVICTION_SHIFT)
BUILD_BUG_ON(LRU_GEN_WIDTH + LRU_REFS_WIDTH > BITS_PER_LONG - EVICTION_SHIFT);
BUILD_BUG_ON(BITS_PER_LONG < EVICTION_SHIFT);
timestamp_bits = BITS_PER_LONG - EVICTION_SHIFT;