Re: [PATCH] selftests/mm: keep ksm_tests on small pages

[David Hildenbrand (Arm)]

On 10/18/26 06:53, Qiliang Yuan wrote:
> ksm_tests fills regions with identical data and checks KSM's counters
> after merging them. allocate_memory() maps these regions without any
> THP hint.
> 
> With mTHP enabled the regions can be backed by large folios, which KSM
> splits before merging. Splitting an anonymous folio maps its
> zero-filled pages to the shared zeropage, so with use_zero_pages=0
> those pages never become KSM pages and "KSM zero page merging" fails
> its count. "ksm_tests -Z -p 10 -z 0", 20 runs:
> 
>   THP setup                     before    after
>   default                         0/20     0/20
>   every mTHP size "always"       20/20     0/20
> 
> Map the regions with MADV_NOHUGEPAGE, as ksm_functional_tests does.
> ksm_merge_hugepages_time() maps its own THP-backed region and is not
> affected.


That's actually something Nico intends to fix:

https://lore.kernel.org/all/20260609114619.144416-1-npache@xxxxxxxxxx/

> 
> Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
> Signed-off-by: Qiliang Yuan <odys.yuan@xxxxxxxxx>
> ---
>  tools/testing/selftests/mm/ksm_tests.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/tools/testing/selftests/mm/ksm_tests.c b/tools/testing/selftests/mm/ksm_tests.c
> index 5fd7792a0d479..140ea833353b6 100644
> --- a/tools/testing/selftests/mm/ksm_tests.c
> +++ b/tools/testing/selftests/mm/ksm_tests.c
> @@ -178,6 +178,16 @@ static void  *allocate_memory(void *ptr, int prot, int mapping, char data, size_
>  		ksft_perror("mmap");
>  		return NULL;
>  	}
> +	/*
> +	 * Splitting a large folio before merging maps its zero-filled pages
> +	 * to the shared zeropage, so keep the pages small. Ignore EINVAL on
> +	 * kernels without THP.
> +	 */
> +	if (madvise(map_ptr, map_size, MADV_NOHUGEPAGE) && errno != EINVAL) {
> +		ksft_perror("madvise");
> +		munmap(map_ptr, map_size);
> +		return NULL;
> +	}
>  	memset(map_ptr, data, map_size);
>  	if (mprotect(map_ptr, map_size, prot)) {
>  		ksft_perror("mprotect");
> 
> ---
> base-commit: 90201351a862182bdabfc1ad1c97b1e1b6ed5931
> change-id: 20261016-bug-selftests-mm-ksm-tests-nohugepage-1414868db0ef
> 
> Best regards,


-- 
Cheers,

David