Re: [PATCH 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 14:09:44 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
> 
> Pre-existing issues:
> - [High] 32-bit arithmetic overflow in sample count and basis points conversions leads to incorrect scaling for large interval ratios.
> - [High] 32-bit integer overflow in `damon_age_for_new_attrs` when calculating region age during attribute updates on 32-bit architectures.
> --
> 
> --- Patch [5]: [PATCH 5/7] mm/damon/core: s/nr_accesses_for_new_attrs/nr_samples_for_new_attrs/ ---
> commit 7659397725f64a348908ab982da60d557f63b8c0
> Author: SJ Park <sj@kernel.org>
> 
> mm/damon/core: s/nr_accesses_for_new_attrs/nr_samples_for_new_attrs/
> 
> This commit renames damon_nr_accesses_for_new_attrs() to
> damon_nr_samples_for_new_attrs() to reflect its general use for any positive
> sample count.
> 
> > 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 is a pre-existing issue, but does this code overflow 32-bit arithmetic
> when the sample count is large?
> 
> In mm/damon/core.c:damon_sample_count_to_bp(), the mult_frac macro multiplies
> the remainder by 10000, which can overflow 32-bit arithmetic.
> 
> Similarly, in damon_sample_bp_to_count(), the 32-bit multiplication overflows
> since both operands are 32-bit unsigned integers:
> 
> mm/damon/core.c:damon_sample_bp_to_count() {
> 	return bp * damon_nr_samples_per_aggr(attrs) / 10000;
> }
> 
> This occurs when the number of samples per aggregation interval exceeds
> roughly 429,496 (e.g., sample_interval=1us and aggr_interval=1s, yielding
> 1,000,000 samples).
> 
> This could cause the region access tracking variables to be corrupted and
> truncated during dynamic attribute updates (damon_set_attrs), rendering DAMOS
> memory management schemes dysfunctional.
> 
> Both intervals are fully configurable by privileged users via DAMON
> sysfs/debugfs interfaces, or dynamically updated by DAMON's auto-tuning
> feature, making the overflow reachable in real-world scenarios.

Known low priority issue.

> 
> [Severity: High]
> This is also a pre-existing issue, but can the region age calculation
> overflow on 32-bit architectures during attribute updates?
> 
> Looking at damon_age_for_new_attrs(), the multiplication is limited to the
> architecture's native unsigned long size:
> 
> mm/damon/core.c:damon_age_for_new_attrs() {
> 	return age * old_attrs->aggr_interval / new_attrs->aggr_interval;
> }
> 
> On 32-bit architectures, aggr_interval (unsigned long) and age (unsigned int)
> are both 32-bit. The product exceeds ULONG_MAX (4,294,967,295) once the
> stable time exceeds ~4,294 seconds.
> 
> This causes an integer overflow before the division, meaning any memory
> region that remains stable for more than ~71.5 minutes will have its age
> wrapped around and reset to an incorrectly small value when DAMON
> attributes are dynamically updated.
> 
> This could break DAMOS schemes that target old pages for reclamation.
> 
> The bug is automatically triggered by DAMON's auto-tuning thread
> (kdamond_tune_intervals), which periodically calls damon_set_attrs, or via
> user-initiated sysfs attribute updates.

Known 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/20260708135359.122587-1-sj@kernel.org?part=5


Thanks,
SJ