Re: [RFC PATCH v1.2 2/4] mm/damon/core: handle uninitialized damos_quota_goal->last_psi_total

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Integer underflow when initializing last_psi_total causes severe quota throttling instead of maintaining the quota.
--

commit 216061cc4d3a0304d1961ab1798b65cbf748aa6c
Author: SJ Park <sj@kernel.org>

mm/damon/core: handle uninitialized damos_quota_goal->last_psi_total

This commit handles an uninitialized last_psi_total for PSI metric DAMOS
quota goals by initializing it to U64_MAX. During the first tuning round,
it detects this state and attempts to set last_psi_total such that the
first iteration has no unexpected effect on the quota.

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 644daf5a16560..b0c8e22ab5404 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c

[ ... ]

> @@ -3018,6 +3022,10 @@ static void damos_set_quota_goal_current_value(struct damon_ctx *c,
>  		break;
>  	case DAMOS_QUOTA_SOME_MEM_PSI_US:
>  		now_psi_total = damos_get_some_mem_psi_total();
> +		/* uninitialized last_psi_total; make no effect this round */
> +		if (goal->last_psi_total == U64_MAX)
> +			goal->last_psi_total = now_psi_total +
> +				goal->target_value;
>  		goal->current_value = now_psi_total - goal->last_psi_total;

[Severity: High]
Does this arithmetic result in an integer underflow on the first tuning
round?

If goal->last_psi_total is set to (now_psi_total + goal->target_value), the
calculation on the very next line becomes:

goal->current_value = now_psi_total - (now_psi_total + goal->target_value)

This evaluates to -goal->target_value. Since current_value is an unsigned
long, this underflows to an astronomically large value. Will the feedback
loop interpret this as a massive over-achievement and drastically throttle
the quota to its minimum, causing the exact opposite of the intended
"no effect this round" behavior?

>  		goal->last_psi_total = now_psi_total;
>  		break;

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260828014659.121058-1-sj@kernel.org?part=2