Re: [PATCH v2 2/2] selftests/mm: cover merging KSM pages aged by smart scan

[David Hildenbrand (Arm)]

On 11/5/26 10:36, Qiliang Yuan wrote:
> ksm_functional_tests checks merging and unmerging with ksmd's default
> settings, but nothing there exercises smart scan, which skips pages KSM
> has failed to merge for a while.
> 
> Smart scan used to time those skips per page, so two identical pages
> that became mergeable some scans apart could end up never being looked
> at in the same scan, and were never merged. "mm/ksm: align smart scan
> skips with the scan number" fixed that.
> 
> Add a case that lets sixteen unique pages age for a dozen scans, then
> makes a duplicate of each mergeable one scan apart, and expects every
> pair to be merged 32 scans later:
> 
>   kernel               failures, two boots
>   before the fix       20/20, 20/20
>   with the fix          0/20,  0/20
> 
> Signed-off-by: Qiliang Yuan <odys.yuan@xxxxxxxxx>
> ---
>  tools/testing/selftests/mm/ksm_functional_tests.c | 108 +++++++++++++++++++++-
>  1 file changed, 107 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/mm/ksm_functional_tests.c b/tools/testing/selftests/mm/ksm_functional_tests.c
> index 31c06c72203fd..71d9b3cbdb8c1 100644
> --- a/tools/testing/selftests/mm/ksm_functional_tests.c
> +++ b/tools/testing/selftests/mm/ksm_functional_tests.c
> @@ -695,6 +695,111 @@ static void test_fork_ksm_merging_page_count(void)
>  	munmap(map, size);
>  }
>  
> +static int ksm_wait_full_scans(long nr)
> +{
> +	long start, cur;
> +
> +	start = ksm_get_full_scans();
> +	if (start < 0)
> +		return start;
> +	do {
> +		cur = ksm_get_full_scans();
> +		if (cur < 0)
> +			return cur;
> +	} while (cur < start + nr);
> +
> +	return 0;
> +}
> +

This looks a lot like the logic we have in ksm_start().

Likely that logic there should be factored out into a common helper.

> +static void test_merge_aged_pairs(void)
> +{
> +	const unsigned int pairs = 16;
> +	const unsigned long size = 2 * pairs * pagesize;
> +	char *map, old_smart_scan = '1';
> +	int smart_scan_fd;
> +	unsigned int i;
> +
> +	ksft_print_msg("[RUN] %s\n", __func__);
> +
> +	smart_scan_fd = open("/sys/kernel/mm/ksm/smart_scan", O_RDWR);
> +	if (smart_scan_fd < 0) {
> +		ksft_test_result_skip("smart_scan not supported\n");
> +		return;
> +	}
> +	if (pread(smart_scan_fd, &old_smart_scan, 1, 0) != 1 ||
> +	    pwrite(smart_scan_fd, "1", 1, 0) != 1) {
> +		ksft_test_result_fail("Enabling smart_scan failed\n");
> +		goto close_fd;
> +	}

Can we have a helper to enable/disable smart scan? Likely belongs into utils as
well.


-- 
Cheers,

David