Re: [RFC PATCH v5 1/8] mm/damon/core: introduce damos_quota_goal->complement

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] mm/damon: integer overflow in damos_set_psi_current_val on 32-bit systems
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commit 2c98b9664c0f8cdd559874386842cae96a9ea8cb
Author: SJ Park <sj@kernel.org>
mm/damon/core: introduce damos_quota_goal->complement

This commit introduces the complement flag to damos_quota_goal, allowing the
system to use the complemented value of a target metric. It adds the field
to the data structure and implements support for it during quota goal commits
and metric value retrievals.

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 733025b367457..270ce9c85a2ce 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c

[ ... ]

> @@ -2960,10 +2961,18 @@ static void damos_set_psi_current_val(u64 now_psi_total,
>  		struct damos_quota_goal *goal, struct damos *s)
>  {
>  	u64 last_psi_total = goal->last_psi_total;
> +	unsigned long val;
>  
>  	goal->last_psi_total = now_psi_total;
>  	if (last_psi_total != U64_MAX) {
> -		goal->current_value = now_psi_total - last_psi_total;
> +		val = now_psi_total - last_psi_total;
> +		if (goal->complement) {
> +			if (val < s->quota.reset_interval * 1000)
> +				val = s->quota.reset_interval * 1000 - val;

[Severity: Low]
Does this multiplication risk an integer overflow on 32-bit systems?

On 32-bit systems, unsigned long is 32 bits, with a maximum value of ~4.29
billion. If a user configures reset_interval via sysfs to a value larger
than 4,294,967 (about 71.5 minutes), the multiplication
s->quota.reset_interval * 1000 will overflow unsigned long.

This causes the auto-tuning goal complement metric to evaluate with a
truncated, incorrect value. While 32-bit systems running DAMON with 71+ 
minute intervals are extremely rare, could this cause unexpected behavior
when long intervals are configured?

> +			else
> +				val = 0;
> +		}
> +		goal->current_value = val;
>  		return;
>  	}

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Sashiko AI review · https://sashiko.dev/#/patchset/20260928093342.16584-1-sj@kernel.org?part=1