From: Kunwu Chan <kunwu.chan@gmail.com>
To: SJ Park <sj@kernel.org>
Cc: Kunwu Chan <kunwu.chan@linux.dev>,
	Andrew Morton <akpm@linux-foundation.org>,
	Brendan Higgins <brendan.higgins@linux.dev>,
	David Gow <davidgow@davidgow.net>,
	damon@lists.linux.dev,
	kunit-dev@googlegroups.com,
	linux-kernel@vger.kernel.org,
	linux-kselftest@vger.kernel.org,
	linux-mm@kvack.org
Subject: Re: [PATCH 09/12] mm/damon/tests/core-kunit: test damon_nr_samples_per_aggr()
Date: Wed,  2 Sep 2026 23:08:41 +0800
Message-ID: <20260902150842.4033320-1-kunwu.chan@linux.dev>
In-Reply-To: <20260902054747.99370-10-sj@kernel.org>
References: 
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On Tue,  1 Sep 2026 22:47:42 -0700 SJ Park <sj@kernel.org> wrote:

> damon_max_nr_accesses(), which is a previous version of
> damon_nr_samples_per_aggr() before the renaming, was wrongly returning
> zero or random overflowed values for extreme intervals setup.  Commit
> 35d4a3cf70a8 ("mm/damon/ops-common: handle extreme intervals in
> damon_hot_score()") updated the function to return correct or more valid
> values.  Add a kunit test to ensure it is working as expected.
> 
> Signed-off-by: SJ Park <sj@kernel.org>
> ---
>  mm/damon/tests/core-kunit.h | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/mm/damon/tests/core-kunit.h b/mm/damon/tests/core-kunit.h
> index a4164ac489a61..d0bed01a5b7fe 100644
> --- a/mm/damon/tests/core-kunit.h
> +++ b/mm/damon/tests/core-kunit.h
> @@ -625,6 +625,27 @@ static void damon_test_set_regions(struct kunit *test)
>  			(struct damon_addr_range[]){}, 0, -EINVAL);
>  }
>  
> +static void damon_test_nr_samples_per_aggr(struct kunit *test)
> +{
> +	struct damon_attrs attrs = {
> +		.sample_interval = 0,
> +		.aggr_interval = 0,
> +	};
> +

Hi SJ,

A small question about the first test case.

Both `sample_interval` and `aggr_interval` are zero here.  Since
`sample_interval` is the denominator in `damon_nr_samples_per_aggr()`,
would it be better to keep `aggr_interval` non-zero when testing the
zero `sample_interval` case?

This would make the test explicitly cover the divide-by-zero protection.
If the `aggr_interval == 0` behavior is also worth covering, perhaps it
could be tested separately.

Thanks,
Kunwu


> +	/* Zero aggregation interval doesn't cause division by zero */
> +	KUNIT_EXPECT_EQ(test, damon_nr_samples_per_aggr(&attrs), 1);
> +
> +	/*
> +	 * Too large aggregation interval on 64 bit system doesn't cause
> +	 * overflow
> +	 */
> +	if (ULONG_MAX > UINT_MAX) {
> +		attrs.aggr_interval = (unsigned long)UINT_MAX + 1;
> +		KUNIT_EXPECT_EQ(test, damon_nr_samples_per_aggr(&attrs),
> +				UINT_MAX);
> +	}
> +}
> +
>  static void damon_test_update_monitoring_result(struct kunit *test)
>  {
>  	struct damon_attrs old_attrs = {
> @@ -1736,6 +1757,7 @@ static struct kunit_case damon_test_cases[] = {
>  	KUNIT_CASE(damon_test_split_above_half_progresses),
>  	KUNIT_CASE(damon_test_ops_registration),
>  	KUNIT_CASE(damon_test_set_regions),
> +	KUNIT_CASE(damon_test_nr_samples_per_aggr),
>  	KUNIT_CASE(damon_test_update_monitoring_result),
>  	KUNIT_CASE(damon_test_set_attrs),
>  	KUNIT_CASE(damon_test_mvsum),
> -- 
> 2.47.3
> 

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