Re: [RFC PATCH v5 8/8] Docs/ABI/damon: update for quota goal metric complement sysfs file

[SJ Park <sj@kernel.org>]

On Mon, 28 Sep 2026 09:43:59 +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 5d7327c0afcb5fe144677a675b1eea685917f61e
> Author: SJ Park <sj@kernel.org>
> Docs/ABI/damon: update for quota goal metric complement sysfs file
> 
> This commit introduces a complement flag for the quota auto-tuning goal metric
> and updates the sysfs ABI documentation to reflect this addition. It also
> modifies the core DAMON tuning logic to support the complement calculation.
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index 733025b367457..86f13411b91fa 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> 
> [ ... ]
> 
> > @@ -2960,10 +2962,18 @@ 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]
> Could this multiplication in mm/damon/core.c:damos_set_psi_current_val()
> overflow on 32-bit systems?  If reset_interval is configured to a value
> greater than 4,294,967 (about 71 minutes) via sysfs,
> s->quota.reset_interval * 1000 will exceed the maximum value of a 32-bit
> unsigned long.
> 
> This could cause the complement calculation to yield corrupted values or
> premature zeros, breaking the auto-tuning logic.

It is an extreme setup.  Also, sane users would test it before using the new
feature.  No blocker of this series in my opinion.

> 
> > +			else
> > +				val = 0;
> > +		}
> > +		goal->current_value = val;
> >  		return;
> >  	}
> >  	/* uninitialized last_psi_total; make no effect this round */
> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260928093342.16584-1-sj@kernel.org?part=8
> 


Thanks,
SJ