Commit 4401e9d1 authored by SeongJae Park's avatar SeongJae Park Committed by Andrew Morton
Browse files

mm/damon/core: avoid overflow in damon_feed_loop_next_input() 

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.

Link: https://lkml.kernel.org/r/20241031161203.47751-1-sj@kernel.org


Fixes: 9294a037 ("mm/damon/core: implement goal-oriented feedback-driven quota auto-tuning")
Signed-off-by: default avatarSeongJae Park <sj@kernel.org>
Reported-by: default avatarGuenter Roeck <linux@roeck-us.net>
Closes: https://lore.kernel.org/944f3d5b-9177-48e7-8ec9-7f1331a3fea3@roeck-us.net


Tested-by: default avatarGuenter Roeck <linux@roeck-us.net>
Cc: <stable@vger.kernel.org>	[6.8.x]
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 8e7bde61

mm/damon/core.c

+21 −7
Original line number Diff line number Diff line
@@ -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)

		return last_input + compensation;

	if (last_input > compensation + min_input)

		return last_input - compensation;

	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;

	return ULONG_MAX;

}



#ifdef CONFIG_PSI