Re: [PATCH v2 2/2] mm/damon/tests/core-kunit: test split above max_nr_regions/2

[SeongJae Park <sj@kernel.org>]

On Fri, 26 Jun 2026 16:58:38 +0800 Jiayuan Chen <jiayuan.chen@linux.dev> wrote:

> From: Jiayuan Chen <jiayuan.chen@shopee.com>
> 
> Add a test that exercises kdamond_split_regions() when the total
> region count is already above max_nr_regions / 2, asserting that the
> function still splits a fraction of the regions (makes progress) and
> does not overshoot max_nr_regions.
> 
> The region size and min_region_sz are picked so the split arithmetic
> does not depend on the page size.
> 
> All tests pass:
>   damon: pass:31 fail:0 skip:0 total:31
>   Totals: pass:31 fail:0 skip:0 total:31

Nice!

> 
> Cc: Jiayuan Chen <jiayuan.chen@linux.dev>
> Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>

Reviewed-by: SeongJae Park <sj@kernel.org>

I have trivial comments below, though.

> ---
>  mm/damon/tests/core-kunit.h | 64 +++++++++++++++++++++++++++++++++++++
>  1 file changed, 64 insertions(+)
> 
> diff --git a/mm/damon/tests/core-kunit.h b/mm/damon/tests/core-kunit.h
> index 29fbe495e532..6fe7abcd670d 100644
> --- a/mm/damon/tests/core-kunit.h
> +++ b/mm/damon/tests/core-kunit.h
> @@ -341,6 +341,69 @@ static void damon_test_split_regions_of(struct kunit *test)
>  	damon_destroy_ctx(c);
>  }
>  
> +/*
> + * When the total region count is already above max_nr_regions / 2,
> + * kdamond_split_regions() must keep refining the resolution by splitting a
> + * fraction of the regions (making progress), without exceeding
> + * max_nr_regions.
> + */
> +static void damon_test_split_above_half_progresses(struct kunit *test)
> +{
> +	struct damon_ctx *c;
> +	struct damon_target *t;
> +	struct damon_region *r;
> +	unsigned long start;
> +	unsigned int nr_before, nr_after, i;
> +	const unsigned int nr_init = 760;
> +	const unsigned long region_sz = 100;
> +
> +	c = damon_new_ctx();
> +	if (!c)
> +		kunit_skip(test, "ctx alloc fail");
> +
> +	/* Keep the split arithmetic independent of the page size */
> +	c->min_region_sz = 1;
> +	c->attrs.min_nr_regions = 10;
> +	c->attrs.max_nr_regions = 1500;
> +
> +	t = damon_new_target();
> +	if (!t) {
> +		damon_destroy_ctx(c);
> +		kunit_skip(test, "target alloc fail");
> +	}
> +
> +	for (i = 0; i < nr_init; i++) {
> +		start = i * region_sz;
> +		r = damon_new_region(start, start + region_sz);
> +		if (!r) {
> +			damon_free_target(t);
> +			damon_destroy_ctx(c);
> +			kunit_skip(test, "region alloc fail");
> +		}
> +		r->nr_accesses = (i & 1) ? 0 : 100;
> +		r->age = 5;
> +		damon_add_region(r, t);
> +	}
> +
> +	damon_add_target(c, t);
> +
> +	nr_before = damon_nr_regions(t);
> +	/* Above max_nr_regions / 2, so the blanket-split path is skipped */
> +	KUNIT_EXPECT_GT(test, (unsigned long)nr_before,
> +			c->attrs.max_nr_regions / 2);

Above KUNIT_EXPECT_GT() feels unnecessary.

> +
> +	kdamond_split_regions(c);
> +
> +	nr_after = damon_nr_regions(t);
> +	/* Still made progress ... */
> +	KUNIT_EXPECT_GT(test, nr_after, nr_before);
> +	/* ... but did not overshoot the configured maximum */
> +	KUNIT_EXPECT_LE(test, (unsigned long)nr_after,
> +			c->attrs.max_nr_regions);

In future, we may do this multiple times.

> +
> +	damon_destroy_ctx(c);
> +}
> +


Thanks,
SJ

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