Re: [RFC PATCH v1.1 1/7] mm/damon/core: remove obsolete comment for nr_to_bp() divide-by-zero

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Removing the `!attrs.aggr_interval` skip guard causes `damon_test_nr_accesses_to_accesses_bp` to fail deterministically on 32-bit architectures.
--

--- Patch [1]: [RFC PATCH v1.1 1/7] mm/damon/core: remove obsolete comment for nr_to_bp() divide-by-zero ---
commit a31fcddef9e05dc323b42f64143ffe58dc702dee
Author: SJ Park <sj@kernel.org>

mm/damon/core: remove obsolete comment for nr_to_bp() divide-by-zero

This commit removes obsolete comments about division-by-zero in
damon_nr_accesses_to_accesses_bp() and its related test code. A previous
commit modified the underlying calculation to always return a non-zero value,
preventing the division-by-zero issue.

> diff --git a/mm/damon/tests/core-kunit.h b/mm/damon/tests/core-kunit.h
> index 0124f83b39b83..224a3ecaa0701 100644
> --- a/mm/damon/tests/core-kunit.h
> +++ b/mm/damon/tests/core-kunit.h
[ ... ]
> @@ -582,18 +582,6 @@ static void damon_test_nr_accesses_to_accesses_bp(struct kunit *test)
>  		.aggr_interval = ((unsigned long)UINT_MAX + 1) * 10
>  	};
>  
> -	/*
> -	 * In some cases such as 32bit architectures where UINT_MAX is
> -	 * ULONG_MAX, attrs.aggr_interval becomes zero.  Calling
> -	 * damon_nr_accesses_to_accesses_bp() in the case will cause
> -	 * divide-by-zero.  Such case is prohibited in normal execution since
> -	 * the caution is documented on the comment for the function, and
> -	 * damon_update_monitoring_results() does the check.  Skip the test in
> -	 * the case.
> -	 */
> -	if (!attrs.aggr_interval)
> -		kunit_skip(test, "aggr_interval is zero.");
> -
>  	KUNIT_EXPECT_EQ(test, damon_nr_accesses_to_accesses_bp(123, &attrs), 0);

[Severity: Medium]
Will removing this skip guard cause a deterministic test failure on 32-bit
architectures?

When the test runs on a 32-bit architecture, attrs.aggr_interval is initialized
to ((unsigned long)UINT_MAX + 1) * 10, which overflows to 0. With the division
by zero fix in place, damon_max_nr_accesses(&attrs) will return 1 instead of 0.

This causes damon_nr_accesses_to_accesses_bp(123, &attrs) to evaluate to
(123 * 10000) / 1 = 1230000. Since the test unconditionally expects the result
to be 0, the KUNIT_EXPECT_EQ assertion will fail.

>  }

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260707135038.90068-1-sj@kernel.org?part=1