Re: [PATCH 1/8] mm/damon/core: introduce damos_quota_goal->complement

[SJ Park <sj@kernel.org>]

On Tue, 29 Sep 2026 08:12:39 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [Low] damon: integer overflow in psi complement calculation on 32-bit
> --
> 
> commit 194cde60eb24e43a85afddadebba1bd09c82f1e0
> Author: SJ Park <sj@kernel.org>
> 
> mm/damon/core: introduce damos_quota_goal->complement
> 
> This commit introduces the complement flag to the damos_quota_goal structure
> to specify whether to use a complemented value of the goal target metric. It
> also implements the core logic to handle this flag during quota goal commit
> and metric retrieval.
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > --- 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: Low]
> Does this calculation overflow on 32-bit systems?
> 
> On 32-bit architectures, s->quota.reset_interval is a 32-bit unsigned long.
> Multiplying it by the integer literal 1000 will overflow before the
> comparison and subtraction if the user configures a reset_interval_ms
> greater than 4,294,967 (around 71.5 minutes).
> 
> While such a high interval is atypical for DAMON's continuous monitoring,
> it is technically reachable via sysfs since the sysfs store function does
> not bound the value beyond the maximum unsigned long value.

Yes, it is atypical and extreme setup.  Sane users ain't use such setup without
testing.  I think it wouldn't be too late to revisit this after a report from a
user is arrived.  I will also consider making a better handling of the corner
case.  But not a blocker of this series.

> 
> > +			else
> > +				val = 0;
> > +		}
> > +		goal->current_value = val;
> >  		return;
> >  	}
> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260929080113.41708-1-sj@kernel.org?part=1
> 


Thanks,
SJ