Re: [RFC PATCH v4 1/7] mm/damon/core: introduce damos_quota_goal->complement

[SJ Park <sj@kernel.org>]

On Sun, 27 Sep 2026 12:17:14 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [Medium] mm: integer overflow in damos_set_psi_current_val on 32-bit
> --
> 
> commit 4f53a2ca54f02309422119354f04050f15950222
> 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 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: Medium]
> Will this calculation overflow on 32-bit platforms?
> 
> When reset_interval is configured to a value > 4,294,967 ms (around 1.19
> hours), s->quota.reset_interval * 1000 will silently overflow when evaluated
> as a 32-bit unsigned long multiplication.
> 
> If this wraps around, could it corrupt the complemented PSI value and lead
> to faulty auto-tuning feedback? Does this need a cast or different typing to
> avoid the overflow?

No blocker.  Sane users ain't set such large reset interval with complment flag
without a test.

> 
> > +			else
> > +				val = 0;
> > +		}
> > +		goal->current_value = val;
> >  		return;
> >  	}
> >  	/* uninitialized last_psi_total; make no effect this round */
> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260927120533.50484-1-sj@kernel.org?part=1


Thanks,
SJ