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

David Hildenbrand (Arm)
2026-03-23T11:56:42Z

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