Re: [PATCH] mm/selftests: improve UFFD-WP feature detection in KSM test

[David Hildenbrand <david@redhat.com>]

On 22.06.25 10:10, Li Wang wrote:
> The current implementation of test_unmerge_uffd_wp() explicitly sets
> `uffdio_api.features = UFFD_FEATURE_PAGEFAULT_FLAG_WP` before calling
> UFFDIO_API. This can cause the ioctl() call to fail with EINVAL on kernels
> that do not support UFFD-WP, leading the test to fail unnecessarily:
> 
>    # ------------------------------
>    # running ./ksm_functional_tests
>    # ------------------------------
>    # TAP version 13
>    # 1..9
>    # # [RUN] test_unmerge
>    # ok 1 Pages were unmerged
>    # # [RUN] test_unmerge_zero_pages
>    # ok 2 KSM zero pages were unmerged
>    # # [RUN] test_unmerge_discarded
>    # ok 3 Pages were unmerged
>    # # [RUN] test_unmerge_uffd_wp
>    # not ok 4 UFFDIO_API failed     <-----
>    # # [RUN] test_prot_none
>    # ok 5 Pages were unmerged
>    # # [RUN] test_prctl
>    # ok 6 Setting/clearing PR_SET_MEMORY_MERGE works
>    # # [RUN] test_prctl_fork
>    # # No pages got merged
>    # # [RUN] test_prctl_fork_exec
>    # ok 7 PR_SET_MEMORY_MERGE value is inherited
>    # # [RUN] test_prctl_unmerge
>    # ok 8 Pages were unmerged
>    # Bail out! 1 out of 8 tests failed
>    # # Planned tests != run tests (9 != 8)
>    # # Totals: pass:7 fail:1 xfail:0 xpass:0 skip:0 error:0
>    # [FAIL]
> 
> This patch improves compatibility and error handling by:
> 
> 1. Changes the feature check to first query supported features (features=0)
>     rather than specifically requesting WP support.
> 
> 2. Gracefully skipping the test if:
>     - UFFDIO_API fails with EINVAL (feature not supported), or
>     - UFFD_FEATURE_PAGEFAULT_FLAG_WP is not advertised by the kernel.
> 
> 3. Providing better diagnostics by distinguishing expected failures (e.g.,
>     EINVAL) from unexpected ones and reporting them using strerror().
> 
> The updated logic makes the test more robust across different kernel versions
> and configurations, while preserving existing behavior on systems that do
> support UFFD-WP.
> 
> Signed-off-by: Li Wang <liwang@redhat.com>
> Cc: Aruna Ramakrishna <aruna.ramakrishna@oracle.com>
> Cc: Bagas Sanjaya <bagasdotme@gmail.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Joey Gouly <joey.gouly@arm.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Keith Lucas <keith.lucas@oracle.com>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Shuah Khan <shuah@kernel.org>
> ---
>   tools/testing/selftests/mm/ksm_functional_tests.c | 8 ++++++--
>   1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/mm/ksm_functional_tests.c b/tools/testing/selftests/mm/ksm_functional_tests.c
> index b61803e36d1c..f3db257dc555 100644
> --- a/tools/testing/selftests/mm/ksm_functional_tests.c
> +++ b/tools/testing/selftests/mm/ksm_functional_tests.c
> @@ -393,9 +393,13 @@ static void test_unmerge_uffd_wp(void)
>   
>   	/* See if UFFD-WP is around. */
>   	uffdio_api.api = UFFD_API;
> -	uffdio_api.features = UFFD_FEATURE_PAGEFAULT_FLAG_WP;
> +	uffdio_api.features = 0;
>   	if (ioctl(uffd, UFFDIO_API, &uffdio_api) < 0) {
> -		ksft_test_result_fail("UFFDIO_API failed\n");
> +		if (errno == EINVAL)
> +			ksft_test_result_skip("UFFDIO_API not supported (EINVAL)\n");
> +		else
> +			ksft_test_result_fail("UFFDIO_API failed: %s\n", strerror(errno));
> +
>   		goto close_uffd;
>   	}
>   	if (!(uffdio_api.features & UFFD_FEATURE_PAGEFAULT_FLAG_WP)) {


The man page (man UFFDIO_API) documents:

        Since  Linux  4.11,  applications should use the features field to perform a two-step handshake.
        First, UFFDIO_API is called with the features field set to zero.  The kernel responds by setting
        all supported feature bits.

        Applications which do not require any specific features can begin using the userfaultfd  immedi‐
        ately.   Applications which do need specific features should call UFFDIO_API again with a subset
        of the reported feature bits set to enable those features.

So likely, what you want in this patch here is something like:

diff --git a/tools/testing/selftests/mm/ksm_functional_tests.c b/tools/testing/selftests/mm/ksm_functional_tests.c
index b61803e36d1cf..5cf819ac958d0 100644
--- a/tools/testing/selftests/mm/ksm_functional_tests.c
+++ b/tools/testing/selftests/mm/ksm_functional_tests.c
@@ -393,7 +393,7 @@ static void test_unmerge_uffd_wp(void)
  
         /* See if UFFD-WP is around. */
         uffdio_api.api = UFFD_API;
-       uffdio_api.features = UFFD_FEATURE_PAGEFAULT_FLAG_WP;
+       uffdio_api.features = 0;
         if (ioctl(uffd, UFFDIO_API, &uffdio_api) < 0) {
                 ksft_test_result_fail("UFFDIO_API failed\n");
                 goto close_uffd;
@@ -403,6 +403,14 @@ static void test_unmerge_uffd_wp(void)
                 goto close_uffd;
         }
  
+       /* Now, enable it ("two-step handshake") */
+       uffdio_api.api = UFFD_API;
+       uffdio_api.features = UFFD_FEATURE_PAGEFAULT_FLAG_WP;
+       if (ioctl(uffd, UFFDIO_API, &uffdio_api) < 0) {
+               ksft_test_result_fail("UFFDIO_API failed\n");
+               goto close_uffd;
+       }
+
         /* Register UFFD-WP, no need for an actual handler. */
         if (uffd_register(uffd, map, size, false, true, false)) {
                 ksft_test_result_fail("UFFDIO_REGISTER_MODE_WP failed\n");


-- 
Cheers,

David / dhildenb