damon_feed_loop_next_input() is inefficient and fragile to overflows.
Specifically, 'score_goal_diff_bp' calculation can overflow when 'score'
is high.  The calculation is actually unnecessary at all because 'goal'
is a constant of value 10,000.  Calculation of 'compensation' is again
fragile to overflow.  Final calculation of return value for
under-achiving case is again fragile to overflow when the current score
is under-achieving the target.

Add two corner cases handling at the beginning of the function to make
the body easier to read, and rewrite the body of the function to avoid
overflows and the unnecessary bp value calcuation.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Closes: https://lore.kernel.org/944f3d5b-9177-48e7-8ec9-7f1331a3fea3@roeck-us.net
Fixes: 9294a037c015 ("mm/damon/core: implement goal-oriented feedback-driven quota auto-tuning")
Cc: <stable@vger.kernel.org> # 6.8.x
Signed-off-by: SeongJae Park <sj@kernel.org>
Tested-by: Guenter Roeck <linux@roeck-us.net>
---
Changes from RFC
(https://lore.kernel.org/20240905172405.46995-1-sj@kernel.org)
- Rebase on latest mm-unstable and cleanup code

 mm/damon/core.c | 28 +++++++++++++++++++++-------
 1 file changed, 21 insertions(+), 7 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index a83f3b736d51..27745dcf855f 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -1456,17 +1456,31 @@ static unsigned long damon_feed_loop_next_input(unsigned long last_input,
 		unsigned long score)
 {
 	const unsigned long goal = 10000;
-	unsigned long score_goal_diff = max(goal, score) - min(goal, score);
-	unsigned long score_goal_diff_bp = score_goal_diff * 10000 / goal;
-	unsigned long compensation = last_input * score_goal_diff_bp / 10000;
 	/* Set minimum input as 10000 to avoid compensation be zero */
 	const unsigned long min_input = 10000;
+	unsigned long score_goal_diff, compensation;
+	bool over_achieving = score > goal;
 
-	if (goal > score)
+	if (score == goal)
+		return last_input;
+	if (score >= goal * 2)
+		return min_input;
+
+	if (over_achieving)
+		score_goal_diff = score - goal;
+	else
+		score_goal_diff = goal - score;
+
+	if (last_input < ULONG_MAX / score_goal_diff)
+		compensation = last_input * score_goal_diff / goal;
+	else
+		compensation = last_input / goal * score_goal_diff;
+
+	if (over_achieving)
+		return max(last_input - compensation, min_input);
+	if (last_input < ULONG_MAX - compensation)
 		return last_input + compensation;
-	if (last_input > compensation + min_input)
-		return last_input - compensation;
-	return min_input;
+	return ULONG_MAX;
 }
 
 #ifdef CONFIG_PSI
-- 
2.39.5