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

[Li Wang <liwang@redhat.com>]

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Hi David,

On Mon, Jun 23, 2025 at 4:33 PM David Hildenbrand <david@redhat.com> wrote:

> 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.
>

Thanks for pointing this out, very helpful.

Will send a patch v2 soon.



>
> 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
>
>

-- 
Regards,
Li Wang

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