Re: [PATCH v2] selftests/mm: Fix UFFDIO_API usage with proper two-step feature negotiation

[Li Wang <liwang@redhat.com>]

[-- Attachment #1: Type: text/plain, Size: 5334 bytes --]

On Tue, Jun 24, 2025 at 11:18 PM David Hildenbrand <david@redhat.com> wrote:

> On 24.06.25 06:24, 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 robustness of the UFFD-WP test
> > (test_unmerge_uffd_wp) by correctly implementing the UFFDIO_API
> > two-step handshake as recommended by the userfaultfd(2) man page.
> >
> > Key changes:
> >
> > 1. Use features=0 in the initial UFFDIO_API call to query supported
> >     feature bits, rather than immediately requesting WP support.
> >
> > 2. Skip the test gracefully if:
> >     - UFFDIO_API fails with EINVAL (e.g. unsupported API version), or
> >     - UFFD_FEATURE_PAGEFAULT_FLAG_WP is not advertised by the kernel.
> >
> > 3. Close the initial userfaultfd and create a new one before enabling
> >     the required feature, since UFFDIO_API can only be called once per
> fd.
> >
> > 4. Improve diagnostics by distinguishing between expected and unexpected
> >     failures, using strerror() to report errors.
> >
> > This ensures the test behaves correctly across a wider range of kernel
> > versions and configurations, while preserving the intended behavior on
> > kernels that support UFFD-WP.
> >
> > Suggestted-by: David Hildenbrand <david@redhat.com>
> > 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: 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>
> > ---
> >
> > Notes:
> >      v1 --> v2:
> >       * Close the original userfaultfd and open a new one before
> enabling features
> >       * Reworked UFFDIO_API negotiation to follow the official two-step
> handshake
> >
> >   .../selftests/mm/ksm_functional_tests.c       | 28 +++++++++++++++++--
> >   1 file changed, 26 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..19e5b741893a 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("The API version requested
> is not supported\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)) {
> > @@ -403,6 +407,26 @@ static void test_unmerge_uffd_wp(void)
> >               goto close_uffd;
> >       }
> >
> > +     /*
> > +      * UFFDIO_API must only be called once to enable features.
> > +      * So we close the old userfaultfd and create a new one to
> > +      * actually enable UFFD_FEATURE_PAGEFAULT_FLAG_WP.
> > +      */
> > +     close(uffd);
> > +     uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
> > +     if (uffd < 0) {
> > +             ksft_test_result_skip("__NR_userfaultfd failed\n");
>
> If it now suddenly fails (after it working above), this sure is a fail,
> right?
>

Yes, you're right, here should ksft_test_result_fail().



>
> Apart from that
>
> Acked-by: David Hildenbrand <david@redhat.com>
>

Thanks for the review!


-- 
Regards,
Li Wang

[-- Attachment #2: Type: text/html, Size: 7784 bytes --]