Re: [RFC PATCH 1/4] mm/damon/core: add DAMOS_FILTER_TYPE_AGE_MS

[SJ Park <sj@kernel.org>]

On Wed, 04 Nov 2026 15:44:08 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [Medium] mm/damon/core: 32-bit overflow calculating age_ms in damos_filter_match()
> --
> 
> commit 8785e74923668744015c21ff7266b94722501bbc
> Author: SJ Park <sj@kernel.org>
> 
> mm/damon/core: add DAMOS_FILTER_TYPE_AGE_MS
> 
> This commit adds a new DAMOS core filter type, DAMOS_FILTER_TYPE_AGE_MS,
> allowing schemes to filter target regions by age in milliseconds using
> damos_filter->range_min and damos_filter->range_max.
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index d36823f4d0928..d631863b034db 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> 
> [ ... ]
> 
> > @@ -2552,7 +2554,7 @@ static bool damos_filter_match(struct damon_ctx *ctx, struct damon_target *t,
> >  	bool matched = false;
> >  	struct damon_target *ti;
> >  	int target_idx = 0;
> > -	unsigned long start, end, wsum;
> > +	unsigned long start, end, wsum, age_ms;
> >  
> >  	*err = 0;
> >  	switch (filter->type) {
> > @@ -2594,6 +2596,11 @@ static bool damos_filter_match(struct damon_ctx *ctx, struct damon_target *t,
> >  		matched = filter->range_min <= wsum &&
> >  			wsum <= filter->range_max;
> >  		break;
> > +	case DAMOS_FILTER_TYPE_AGE_MS:
> > +		age_ms = r->age * ctx->attrs.aggr_interval / USEC_PER_MSEC;
> > +		matched = filter->range_min <= age_ms &&
> > +			age_ms <= filter->range_max;
> > +		break;
> 
> [Severity: Medium]
> Can the multiplication r->age * ctx->attrs.aggr_interval in
> damos_filter_match() overflow on 32-bit architectures?
> 
> Since r->age is unsigned int and ctx->attrs.aggr_interval is unsigned long
> (in microseconds), the multiplication is evaluated in 32-bit unsigned long
> on 32-bit platforms before dividing by USEC_PER_MSEC. Once a region's age
> exceeds ULONG_MAX microseconds (about 4,295 seconds, or r->age >= 42,950 at
> the default 100,000 us aggr_interval), the intermediate product wraps around
> and misclassifies old regions with a small age_ms value.
> 
> By comparison, damon_hot_score() in mm/damon/ops-common.c casts r->age to
> u64 and uses div_u64() to avoid this 32-bit overflow:
> 
> 	age_in_sec = div_u64((u64)r->age * c->attrs.aggr_interval,
> 			     USEC_PER_SEC);
> 
> Should damos_filter_match() similarly use div_u64((u64)r->age *
> ctx->attrs.aggr_interval, USEC_PER_MSEC)?

Good finding.  I will do so in the next spin.

> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20261104153645.82945-1-sj@kernel.org?part=1
> 


Thanks,
SJ