Re: [PATCH] selftests/mm: Fix soft-dirty kselftest supported check

["David Hildenbrand (Arm)" <david@kernel.org>]

On 3/20/26 19:39, Audra Mitchell wrote:
> On architectures with separate user address space, such as s390 or
> those without an MMU, the soft-dirty kselftest may fail when checking
> to see if the feature is supported.
> 
>   # --------------------
>   # running ./soft-dirty
>   # --------------------
>   # TAP version 13
>   # 1..15
>   # Bail out! PAGEMAP_SCAN succeeded unexpectedly
>   # # Totals: pass:0 fail:0 xfail:0 xpass:0 skip:0 error:0
>   # [FAIL]
>   not ok 1 soft-dirty # exit=1
>   # SUMMARY: PASS=0 SKIP=0 FAIL=1
> 
> The soft-dirty test will initate an ioctl with the PAGEMAP_SCAN flag with
> an invalid address for the page_region. This is done intentionally to
> have the ioctl return with an expected EFAULT and with the correct
> categories returned.
> 
> However, on architectures with separate user address space, such as s390 or
> those without an MMU, the call to __access_ok (used to validate the
> variables provided with the ioctl) will always return true and we will not
> fail as expected.
> 
>         if (IS_ENABLED(CONFIG_ALTERNATE_USER_ADDRESS_SPACE) ||
>             !IS_ENABLED(CONFIG_MMU))
>                 return true;
> 
> Let's simplify the check for PAGEMAP_SCAN and provide a valid page_region
> address so that we get a non-errno return if it is supported.
> 
> Fixes: 600bca580579 ("selftests/mm: check that PAGEMAP_SCAN returns correct categories")
> 
> Signed-off-by: Audra Mitchell <audra@redhat.com>
> ---
>  tools/testing/selftests/mm/vm_util.c | 24 +++++++++++++++---------
>  1 file changed, 15 insertions(+), 9 deletions(-)
> 
> diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
> index a6d4ff7dfdc0..82998406b335 100644
> --- a/tools/testing/selftests/mm/vm_util.c
> +++ b/tools/testing/selftests/mm/vm_util.c
> @@ -67,20 +67,26 @@ static uint64_t pagemap_scan_get_categories(int fd, char *start)
>  }
>  
>  /* `start` is any valid address. */
> -static bool pagemap_scan_supported(int fd, char *start)
> +static bool pagemap_scan_supported(int fd)
>  {
> +	const size_t pagesize = getpagesize();
>  	static int supported = -1;
> -	int ret;
> +	struct page_region r;
> +	void *test_area;
>  
>  	if (supported != -1)
>  		return supported;
>  
> -	/* Provide an invalid address in order to trigger EFAULT. */
> -	ret = __pagemap_scan_get_categories(fd, start, (struct page_region *) ~0UL);
> -	if (ret == 0)
> -		ksft_exit_fail_msg("PAGEMAP_SCAN succeeded unexpectedly\n");
> -
> -	supported = errno == EFAULT;
> +	test_area = mmap(0, pagesize, PROT_READ | PROT_WRITE,
> +			MAP_ANONYMOUS | MAP_PRIVATE, 0, 0);
> +	if (test_area == MAP_FAILED) {
> +		ksft_print_msg("WARN: mmap() failed: %s\n", strerror(errno));
> +		supported = 0;
> +	} else {
> +		supported = __pagemap_scan_get_categories(fd, test_area, &r) >= 0;

You have to cast it to a (long) first before comparing, like we do in
pagemap_scan_get_categories().


Maybe best written as

	long ret = __pagemap_scan_get_categories(fd, test_area, &r);

	if (ret >= 0)
		supported = 1;

> +		ksft_print_msg("errno: %d\n", errno);

I guess we should drop that, was mostly for testing.

Thanks!

-- 
Cheers,

David