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Message-ID: <dfd7650d-1154-467d-ae70-c126610413f6@redhat.com>
Date: Tue, 24 Jun 2025 10:07:29 +0200
From: David Hildenbrand <david@...hat.com>
To: Li Wang <liwang@...hat.com>, akpm@...ux-foundation.org,
linux-kselftest@...r.kernel.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Cc: Aruna Ramakrishna <aruna.ramakrishna@...cle.com>,
Bagas Sanjaya <bagasdotme@...il.com>,
Catalin Marinas <catalin.marinas@....com>,
Dave Hansen <dave.hansen@...ux.intel.com>, Joey Gouly <joey.gouly@....com>,
Johannes Weiner <hannes@...xchg.org>, Keith Lucas <keith.lucas@...cle.com>,
Ryan Roberts <ryan.roberts@....com>, Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH v2] selftests/mm: Fix UFFDIO_API usage with proper
two-step feature negotiation
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@...hat.com>
> Signed-off-by: Li Wang <liwang@...hat.com>
> Cc: Aruna Ramakrishna <aruna.ramakrishna@...cle.com>
> Cc: Bagas Sanjaya <bagasdotme@...il.com>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Joey Gouly <joey.gouly@....com>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Keith Lucas <keith.lucas@...cle.com>
> Cc: Ryan Roberts <ryan.roberts@....com>
> Cc: Shuah Khan <shuah@...nel.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));
> +
Not sure if that is really required. If UFFDIO_API failed after
__NR_userfaultfd worked something unexpected is happening.
> 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);
Is that actually required?
The man page explicitly documents:
" EINVAL A previous UFFDIO_API call already enabled one or more
features for this userfaultfd. Calling UFF‐
DIO_API twice, the first time with no features set, is
explicitly allowed as per the two-step feature
detection handshake.
"
So if that doesn't work, something might be broken.
--
Cheers,
David / dhildenb
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