Re: [RFC PATCH v1.3 5/7] mm/damon/core: s/nr_accesses_for_new_attrs/nr_samples_for_new_attrs/

[SJ Park <sj@kernel.org>]

On Wed, 08 Jul 2026 03:01:54 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> 
> Pre-existing issues:
> - [High] 32-bit integer overflow in sample scaling functions `damon_sample_bp_to_count` and `damon_sample_count_to_bp`.
> --
> 
> --- Patch [5]: [RFC PATCH v1.3 5/7] mm/damon/core: s/nr_accesses_for_new_attrs/nr_samples_for_new_attrs/ ---
> commit 9db3c90e1f930b4b71612a504362560074396989
> Author: SJ Park <sj@kernel.org>
> 
> mm/damon/core: s/nr_accesses_for_new_attrs/nr_samples_for_new_attrs/
> 
> damon_nr_accesses_for_new_attrs() can be used for not only nr_accesses
> but also any positive sample count, like probe_hits.  Rename to be able
> to be used for such general uses without confusion.
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index 718268df27fa7..800ff6b6d2473 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> > @@ -890,22 +890,21 @@ static unsigned int damon_sample_count_to_bp(
> >  	return mult_frac(count, 10000, damon_nr_samples_per_aggr(attrs));
> >  }
> 
> [Severity: High]
> This isn't a bug introduced by this patch, but can the 32-bit unsigned
> integer multiplication here and in damon_sample_bp_to_count() overflow?
> 
> When a privileged user configures an aggregation to sample interval ratio
> greater than ~429,496 via sysfs (e.g., 10s aggregation and 10us sampling),
> the mult_frac() macro performs a 32-bit multiplication of the remainder
> by 10000. This could overflow when the remainder exceeds 429,496.
> 
> Similarly, in damon_sample_bp_to_count():
> 
> mm/damon/core.c:damon_sample_bp_to_count() {
> 	...
> 	return bp * damon_nr_samples_per_aggr(attrs) / 10000;
> }
> 
> The expression bp * damon_nr_samples_per_aggr(attrs) is performed as a
> 32-bit unsigned int multiplication, which could overflow 4.29B.
> 
> Could this overflow silently corrupt region access counts, causing DAMON to
> make incorrect memory management decisions like paging out actively used
> memory?

Knowwn low priority issue.

> 
> >  
> > -static unsigned int damon_nr_accesses_for_new_attrs(unsigned int nr_accesses,
> > +static unsigned int damon_nr_samples_for_new_attrs(unsigned int nr,
> >  		struct damon_attrs *old_attrs, struct damon_attrs *new_attrs)
> >  {
> >  	return damon_sample_bp_to_count(
> > -			damon_sample_count_to_bp(nr_accesses, old_attrs),
> > -			new_attrs);
> > +			damon_sample_count_to_bp(nr, old_attrs), new_attrs);
> >  }
> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260708024855.116614-1-sj@kernel.org?part=5


Thanks,
SJ