mm/mglru: frequency guided promotion (MGLRU-FG)

static inline int lru_tier_from_refs(unsigned int refs)
 {
-	VM_WARN_ON_ONCE(refs > BIT(LRU_REFS_WIDTH));
-	/* see the comment on MAX_NR_TIERS */
-	return workingset ? MAX_NR_TIERS - 1 : order_base_2(refs);
+	VM_WARN_ON_ONCE(refs > LRU_REFS_MAX);
+	if (refs < LRU_REFS_WORKINGSET)
+		return 0;
+	return fls(refs - 1);
 }

 static inline int lru_refs_from_flags(unsigned long flags)
 {
-	if (!(flags & BIT(PG_referenced)))
-		return 0;
+	int refs;
 	/* Return the total number of accesses. Also see the comment on
 	 * LRU_REFS_FLAGS. */
-	return ((flags & LRU_REFS_MASK) >> LRU_REFS_PGOFF) + 1;
+	refs = (flags & BIT(PG_referenced)) ? BIT(0) : 0;
+	refs += (flags & BIT(PG_workingset)) ? BIT(1) : 0;
+	refs += ((flags & LRU_REFS_MASK) >> LRU_REFS_PGOFF) << 2;
+	return refs;
 }

/* To promote frequently used folios, prevent isolation
 * first, it's a lazy promotion so no LRU lock needed. */
if (!isolated) {
	if (!folio_test_clear_lru(folio))
		goto out;
	isolated = true;
	...
	max_seq = READ_ONCE(lrugen->max_seq);
	max_gen = lru_gen_from_seq(max_seq);
	if (gen == max_gen)
		goto out;   /* 已在最年轻代，无需再提 */
	if (refs <= LRU_REFS_MAX) {
		type = folio_is_file_lru(folio);
		min_gen = lru_gen_from_seq(READ_ONCE(lrugen->min_seq[type]));
		if (gen != min_gen)
			goto out;   /* 只从最旧代提升，防过度保护缓存 */
	} else {
		refs = LRU_REFS_PROTECTED;
	}
	new_gen = (gen + 1UL) % MAX_NR_GENS;
	lru_gen_set_flags(&new_flags, new_gen);
...
out:
	lru_refs_set_flags(&new_flags, min(refs, LRU_REFS_MAX));