Re: [RFC PATCH v2 1/7] mm/damon/core: introduce damos_quota_goal->complement

[SJ Park <sj@kernel.org>]

On Sat, 19 Sep 2026 01:25:09 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [High] mm: Integer underflow in DAMOS_QUOTA_SOME_MEM_PSI_US complement calculation
> --
> 
> commit f2de2d4493473c2c847f3ad213eaef640f0e1600
> Author: SJ Park <sj@kernel.org>
> 
> mm/damon/core: introduce damos_quota_goal->complement
> 
> Introduce damos_quota_goal->complement for specifying whether to use a
> complemented value of the given goal target metric. Add the field to
> the data structure and implement essential core support. Handle the
> flag in the quota goal commit and current quota goal metric value
> retrieval.
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index 2258b72da7a78..9fd08cbfacd8c 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> [ ... ]
> > @@ -3238,6 +3239,23 @@ 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]
> Does this unsigned subtraction underflow if goal->current_value exceeds
> s->quota.reset_interval * 1000?
> 
> If goal->current_value exceeds the nominal reset interval, which can happen on
> the first iteration when initialized to an arbitrarily large user-provided
> target_value, or on subsequent iterations if kernel scheduling delays cause the
> actual elapsed time between callbacks to exceed the nominal reset_interval,
> this subtraction will underflow and produce a massive current_value near
> ULONG_MAX.
> 
> This large value is then passed to damos_quota_score(), which computes a
> normalized score that maxes out the feedback loop, forcing the tuned quota
> limit to drop aggressively.

It's an unlikely corner case.  But I agree it would be nice to handle it.  I
will do so in the next revision.

> 
> > +		return;
> > +	}
> > +	if (goal->current_value < 10000)
> > +		goal->current_value = 10000 - goal->current_value;
> > +	else
> > +		goal->current_value = 0;
> >  }
> >  
> >  /* Return the highest score since it makes schemes least aggressive */
> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260919011359.88921-1-sj@kernel.org?part=1


Thanks,
SJ