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

[sashiko-bot@kernel.org]

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?

>  
> -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