mm/damon: improve/fixup/update ratio calculation, test and documentation

index cd2d7a8e3fe92..4e931f7380477 100644
@@ -665,7 +665,7 @@ static unsigned int damon_accesses_bp_to_nr_accesses(
 static unsigned int damon_nr_accesses_to_accesses_bp(
 		unsigned int nr_accesses, struct damon_attrs *attrs)
 {
-	return nr_accesses * 10000 / damon_max_nr_accesses(attrs);
+	return mult_frac(nr_accesses, 10000, damon_max_nr_accesses(attrs));
 }
 
 static unsigned int damon_nr_accesses_for_new_attrs(unsigned int nr_accesses,

@@ -1722,7 +1722,7 @@ static unsigned long damon_get_intervals_score(struct damon_ctx *c)
 	}
 	target_access_events = max_access_events * goal_bp / 10000;
 	target_access_events = target_access_events ? : 1;
-	return access_events * 10000 / target_access_events;
+	return mult_frac(access_events, 10000, target_access_events);
 }
 
 static unsigned long damon_feed_loop_next_input(unsigned long last_input,

@@ -2202,7 +2202,7 @@ static __kernel_ulong_t damos_get_node_mem_bp(
 		numerator = i.totalram - i.freeram;
 	else	/* DAMOS_QUOTA_NODE_MEM_FREE_BP */
 		numerator = i.freeram;
-	return numerator * 10000 / i.totalram;
+	return mult_frac(numerator, 10000, i.totalram);
 }
 
 static unsigned long damos_get_node_memcg_used_bp(

@@ -2235,7 +2235,7 @@ static unsigned long damos_get_node_memcg_used_bp(
 		numerator = used_pages;
 	else	/* DAMOS_QUOTA_NODE_MEMCG_FREE_BP */
 		numerator = i.totalram - used_pages;
-	return numerator * 10000 / i.totalram;
+	return mult_frac(numerator, 10000, i.totalram);
 }
 #else
 static __kernel_ulong_t damos_get_node_mem_bp(

@@ -2265,8 +2265,8 @@ static unsigned int damos_get_in_active_mem_bp(bool active_ratio)
 		global_node_page_state(NR_LRU_BASE + LRU_INACTIVE_FILE);
 	total = active + inactive;
 	if (active_ratio)
-		return active * 10000 / total;
-	return inactive * 10000 / total;
+		return mult_frac(active, 10000, total);
+	return mult_frac(inactive, 10000, total);
 }
 
 static void damos_set_quota_goal_current_value(struct damos_quota_goal *goal)

@@ -2309,8 +2309,8 @@ static unsigned long damos_quota_score(struct damos_quota *quota)
 	damos_for_each_quota_goal(goal, quota) {
 		damos_set_quota_goal_current_value(goal);
 		highest_score = max(highest_score,
-				goal->current_value * 10000 /
-				goal->target_value);
+				mult_frac(goal->current_value, 10000,
+					goal->target_value));
 	}
 
 	return highest_score;

@@ -2340,8 +2340,8 @@ static void damos_set_effective_quota(struct damos_quota *quota)
 
 	if (quota->ms) {
 		if (quota->total_charged_ns)
-			throughput = mult_frac(quota->total_charged_sz, 1000000,
-							quota->total_charged_ns);
+			throughput = mult_frac(quota->total_charged_sz,
+					1000000, quota->total_charged_ns);
 		else
 			throughput = PAGE_SIZE * 1024;
 		esz = min(throughput * quota->ms, esz);

index 2289f9e4610c0..e86d4f4fe261a 100644
@@ -1311,6 +1311,28 @@ static void damon_test_apply_min_nr_regions(struct kunit *test)
 	damon_test_apply_min_nr_regions_for(test, 10, 2, 10, 2, 5);
 }
 
+static void damon_test_is_last_region(struct kunit *test)
+{
+	struct damon_region *r;
+	struct damon_target *t;
+	int i;
+
+	t = damon_new_target();
+	if (!t)
+		kunit_skip(test, "target alloc fail\n");
+
+	for (i = 0; i < 4; i++) {
+		r = damon_new_region(i * 2, (i + 1) * 2);
+		if (!r) {
+			damon_free_target(t);
+			kunit_skip(test, "region alloc %d fail\n", i);
+		}
+		damon_add_region(r, t);
+		KUNIT_EXPECT_TRUE(test, damon_is_last_region(r, t));
+	}
+	damon_free_target(t);
+}
+
 static struct kunit_case damon_test_cases[] = {
 	KUNIT_CASE(damon_test_target),
 	KUNIT_CASE(damon_test_regions),

@@ -1339,6 +1361,7 @@ static struct kunit_case damon_test_cases[] = {
 	KUNIT_CASE(damon_test_feed_loop_next_input),
 	KUNIT_CASE(damon_test_set_filters_default_reject),
 	KUNIT_CASE(damon_test_apply_min_nr_regions),
+	KUNIT_CASE(damon_test_is_last_region),
 	{},
 };
 

index 4e931f7380477..7f74982535aca 100644
@@ -751,8 +751,16 @@ static bool damon_valid_intervals_goal(struct damon_attrs *attrs)
  * @ctx:		monitoring context
  * @attrs:		monitoring attributes
  *
- * This function should be called while the kdamond is not running, an access
- * check results aggregation is not ongoing (e.g., from damon_call().
+ * This function updates monitoring results and next monitoring/damos operation
+ * schedules.  Because those are periodically updated by kdamond, this should
+ * be called from a safe contexts.  Such contexts include damon_ctx setup time
+ * while the kdamond is not yet started, and inside of kdamond_fn().
+ *
+ * In detail, all DAMON API callers directly call this function for initial
+ * setup of damon_ctx before calling damon_start().  Some of the API callers
+ * also indirectly call this function via damon_call() -> damon_commit() for
+ * online parameters updates.  Finally, kdamond_fn() itself use this for
+ * applying auto-tuned monitoring intervals.
  *
  * Every time interval is in micro-seconds.
  *

index 9a88cf8d152d8..6bd71546f7b20 100644
@@ -55,6 +55,8 @@ struct damon_size_range {
  * @list:		List head for siblings.
  * @age:		Age of this region.
  *
+ * For any use case, @ar should be non-zero positive size.
+ *
  * @nr_accesses is reset to zero for every &damon_attrs->aggr_interval and be
  * increased for every &damon_attrs->sample_interval if an access to the region
  * during the last sampling interval is found.  The update of this field should