Re: [PATCH 1/2] mm/damon/core-kunit: test probe_hits handling at region split and merge

On Tue, 18 Aug 2026 03:57:42 +0000 [email protected] wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> 
> Pre-existing issues:
> - [High] This problem wasn't introduced by this patch, but there is an integer overflow in damon_merge_two_regions() when calculating size-weighted averages on 32-bit architectures.
> --
> 
> --- Patch [1]: [PATCH 1/2] mm/damon/core-kunit: test probe_hits handling at region split and merge ---
> commit f275f8ec2d833d0e2eacc7a18ba7c56b04b7e599
> Author: Jason Angelov <[email protected]>
> 
> mm/damon/core-kunit: test probe_hits handling at region split and merge
> 
> Extend damon_test_split_at() and damon_test_merge_two() tests to cover
> handling of probe_hits and last_probe_hits arrays.
> 
> > diff --git a/mm/damon/tests/core-kunit.h b/mm/damon/tests/core-kunit.h
> > index 4a536d41cdb2d..2db94d49c9bae 100644
> > --- a/mm/damon/tests/core-kunit.h
> > +++ b/mm/damon/tests/core-kunit.h
> [ ... ]
> > @@ -204,6 +210,7 @@ static void damon_test_merge_two(struct kunit *test)
> >  	KUNIT_EXPECT_EQ(test, r->ar.start, 0ul);
> >  	KUNIT_EXPECT_EQ(test, r->ar.end, 300ul);
> >  	KUNIT_EXPECT_EQ(test, r->nr_accesses, 16u);
> > +	KUNIT_EXPECT_EQ(test, r->probe_hits[0], 11);
> 
> [Severity: High]
> This is a pre-existing issue, but does the size-weighted average logic
> tested here overflow on 32-bit architectures?
> 
> Looking at mm/damon/core.c:damon_merge_two_regions(), the calculations
> multiply values like nr_accesses and probe_hits by the region size in bytes:
> 
> mm/damon/core.c:damon_merge_two_regions() {
> 	...
> 	l->nr_accesses = (l->nr_accesses * sz_l + r->nr_accesses * sz_r) /
> 			(sz_l + sz_r);
> 	...
> 	for (i = 0; i < DAMON_MAX_PROBES; i++)
> 		l->probe_hits[i] = (l->probe_hits[i] * sz_l + r->probe_hits[i]
> 				* sz_r) / (sz_l + sz_r);
> 	...
> }
> 
> On a 32-bit architecture, unsigned long is 32 bits (max ~4.29GB). If a
> region is just 17MB and probe_hits is 255, the multiplication 
> 255 * 17,000,000 evaluates to ~4.33GB, which would silently truncate.
> 
> Could this wrap-around cause merged regions to inherit drastically incorrect
> statistics, potentially misidentifying hot memory as cold and triggering
> aggressive paging out of actively used pages?

Known low priority issue.  Users could use addr_unit to workaround this.

> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=1


Thanks,
SJ