Re: [RFC PATCH 7/7] Docs/ABI/damon: update for quota goal metric complement sysfs file

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] mm: damon: unsigned integer underflow in damos_set_quota_goal_current_value()
--

commit 94b0fa41235165f1d082ca5735dcefbf9f65199b
Author: SJ Park <sj@kernel.org>

mm/damon/core: introduce damos_quota_goal->complement

This commit introduces a complement flag for DAMOS quota auto-tuning goals.
It allows the system to target the complement value of a given metric by
updating the current metric value during retrieval.

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 2258b72da7a78..25a8221dd2a40 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c

[ ... ]

> @@ -3238,6 +3239,19 @@ static void damos_set_quota_goal_current_value(struct damon_ctx *c,
>  	default:
>  		break;
>  	}
> +	if (!goal->complement)
> +		return;
> +
> +	/* updte current_value to complemented value */
> +
> +	/* for user_input, users set complemented value on their own */
> +	if (goal->metric == DAMOS_QUOTA_USER_INPUT)
> +		return;
> +	if (goal->metric == DAMOS_QUOTA_SOME_MEM_PSI_US)
> +		goal->current_value = s->quota.reset_interval * 1000 -
> +			goal->current_value;

[Severity: High]
Can this underflow in damos_set_quota_goal_current_value() if
goal->current_value exceeds s->quota.reset_interval * 1000?

For DAMOS_QUOTA_SOME_MEM_PSI_US, the background kdamond thread calculates the
elapsed PSI time in microseconds. Due to scheduler delays or heavy system
load, the actual elapsed time between evaluations can slightly exceed
reset_interval milliseconds, meaning goal->current_value can legitimately
exceed s->quota.reset_interval * 1000.

An underflow would wrap goal->current_value to a massive positive number near
ULONG_MAX. The DAMON auto-tuning loop would interpret this as the goal being
severely over-achieved, causing it to erroneously throttle its aggressiveness
to the minimum precisely when memory pressure is highly severe.

> +	else
> +		goal->current_value = 10000 - goal->current_value;

[Severity: High]
Similarly, can this also underflow for BP metrics like
DAMOS_QUOTA_HUGEPAGE_MEM_BP?

Per-CPU counter drift can cause the calculated ratio to exceed 10000,
triggering an underflow here that similarly inverts the DAMON logic.

>  }
>  
>  /* Return the highest score since it makes schemes least aggressive */

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260918142827.85303-1-sj@kernel.org?part=7