Re: [PATCH v3 5/9] selftests/mm: check that FORCE_READ() succeeded

[Usama Anjum <usama.anjum@foss.arm.com>]

On 22/01/2026 5:02 pm, Kevin Brodsky wrote:
> Many cow tests rely on FORCE_READ() to populate pages. Introduce a
> helper to make sure that the pages are actually populated, and fail
> otherwise.
> 
> Suggested-by: David Hildenbrand (Red Hat) <david@kernel.org>
> Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
> ---
>   tools/testing/selftests/mm/cow.c | 43 ++++++++++++++++++++++++--------
>   1 file changed, 33 insertions(+), 10 deletions(-)
> 
> diff --git a/tools/testing/selftests/mm/cow.c b/tools/testing/selftests/mm/cow.c
> index 83b3563be26b..d9c69c04b67d 100644
> --- a/tools/testing/selftests/mm/cow.c
> +++ b/tools/testing/selftests/mm/cow.c
> @@ -75,6 +75,18 @@ static bool range_is_swapped(void *addr, size_t size)
>   	return true;
>   }
>   
> +static bool populate_page_checked(char *addr)
> +{
> +	bool ret;
> +
> +	FORCE_READ(*addr);
> +	ret = pagemap_is_populated(pagemap_fd, addr);
Very useful. Maybe extending FORCE_READ can help the entire suite?

> +	if (!ret)
> +		ksft_print_msg("Failed to populate page\n");
> +
> +	return ret;
> +}
> +
>   struct comm_pipes {
>   	int child_ready[2];
>   	int parent_ready[2];
> @@ -1549,8 +1561,10 @@ static void run_with_zeropage(non_anon_test_fn fn, const char *desc)
>   	}
>   
>   	/* Read from the page to populate the shared zeropage. */
> -	FORCE_READ(*mem);
> -	FORCE_READ(*smem);
> +	if (!populate_page_checked(mem) || !populate_page_checked(smem)) {
> +		log_test_result(KSFT_FAIL);
> +		goto munmap;
> +	}
>   
>   	fn(mem, smem, pagesize);
>   munmap:
> @@ -1612,8 +1626,11 @@ static void run_with_huge_zeropage(non_anon_test_fn fn, const char *desc)
>   	 * the first sub-page and test if we get another sub-page populated
>   	 * automatically.
>   	 */
> -	FORCE_READ(*mem);
> -	FORCE_READ(*smem);
> +	if (!populate_page_checked(mem) || !populate_page_checked(smem)) {
> +		log_test_result(KSFT_FAIL);
> +		goto munmap;
> +	}
> +
>   	if (!pagemap_is_populated(pagemap_fd, mem + pagesize) ||
>   	    !pagemap_is_populated(pagemap_fd, smem + pagesize)) {
>   		ksft_test_result_skip("Did not get THPs populated\n");
> @@ -1663,8 +1680,10 @@ static void run_with_memfd(non_anon_test_fn fn, const char *desc)
>   	}
>   
>   	/* Fault the page in. */
> -	FORCE_READ(*mem);
> -	FORCE_READ(*smem);
> +	if (!populate_page_checked(mem) || !populate_page_checked(smem)) {
> +		log_test_result(KSFT_FAIL);
> +		goto munmap;
> +	}
>   
>   	fn(mem, smem, pagesize);
>   munmap:
> @@ -1719,8 +1738,10 @@ static void run_with_tmpfile(non_anon_test_fn fn, const char *desc)
>   	}
>   
>   	/* Fault the page in. */
> -	FORCE_READ(*mem);
> -	FORCE_READ(*smem);
> +	if (!populate_page_checked(mem) || !populate_page_checked(smem)) {
> +		log_test_result(KSFT_FAIL);
> +		goto munmap;
> +	}
>   
>   	fn(mem, smem, pagesize);
>   munmap:
> @@ -1773,8 +1794,10 @@ static void run_with_memfd_hugetlb(non_anon_test_fn fn, const char *desc,
>   	}
>   
>   	/* Fault the page in. */
> -	FORCE_READ(*mem);
> -	FORCE_READ(*smem);
> +	if (!populate_page_checked(mem) || !populate_page_checked(smem)) {
> +		log_test_result(KSFT_FAIL);
> +		goto munmap;
> +	}
>   
>   	fn(mem, smem, hugetlbsize);
>   munmap: