Re: [PATCH 6.12.y v2] mm/damon/core: handle uninitialized damos_quota_goal->last_psi_total

[SJ Park <sj@kernel.org>]

On Mon, 09 Nov 2026 01:28:24 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential
> issue to consider:
> - [Medium] mm/damon: integer overflow in damos_quota_score() on first
>   PSI round
> --
> 
> commit 5e9b1f636c184ecf41e05d717ad5e0116b0fca20
> Author: SJ Park <sj@kernel.org>
> 
> mm/damon/core: handle uninitialized damos_quota_goal->last_psi_total
> 
> This commit initializes damos_quota_goal->last_psi_total to U64_MAX when
> creating or committing a DAMOS_QUOTA_SOME_MEM_PSI_US goal, and sets
> goal->current_value to goal->target_value on the first tuning round so the
> effective quota remains unchanged until a baseline PSI reading is recorded.
> 
> Link: https://lore.kernel.org/20260902002725.108635-3-sj@kernel.org
> Link: https://lore.kernel.org/20260718005316.89585-1-sj@kernel.org [1]
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index 3142776e2e0ee..420ef0adaa7e3 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> 
> [ ... ]
> 
> > @@ -1616,7 +1619,12 @@ static void damos_set_quota_goal_current_value(struct damos_quota_goal *goal)
> >  		break;
> >  	case DAMOS_QUOTA_SOME_MEM_PSI_US:
> >  		now_psi_total = damos_get_some_mem_psi_total();
> > -		goal->current_value = now_psi_total - goal->last_psi_total;
> > +		/* uninitialized last_psi_total; make no effect this round */
> > +		if (goal->last_psi_total == U64_MAX)
> > +			goal->current_value = goal->target_value;
> > +		else
> > +			goal->current_value = now_psi_total -
> > +				goal->last_psi_total;
> >  		goal->last_psi_total = now_psi_total;
> >  		break;
> >  	default:
> 
> [Severity: Medium]
> Can setting goal->current_value = goal->target_value here overflow in
> damos_quota_score() when goal->target_value exceeds ULONG_MAX / 10000?
> 
> Unlike mainline where damos_quota_score() uses mult_frac(), this branch
> computes the score with direct unsigned long multiplication:
> 
> mm/damon/core.c:damos_quota_score() {
> 	damos_for_each_quota_goal(goal, quota) {
> 		damos_set_quota_goal_current_value(goal);
> 		highest_score = max(highest_score,
> 				goal->current_value * 10000 /
> 				goal->target_value);
> 	}
> }
> 
> When goal->target_value > ULONG_MAX / 10000 (for example, above 429,496 us
> on 32-bit architectures), goal->current_value * 10000 wraps around and
> produces a score below 10000, causing damon_feed_loop_next_input() to
> increase quota->esz_bp on the first round instead of keeping the current
> quota unchanged.

The issue impacts only DAMOS quota auto-tuning quality under certain setups.
No blocker for this patch.

> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20261109011347.92467-1-sj@kernel.org?part=1
> 


Thanks,
SJ