Re: [PATCH v3 09/13] selftests/mm: skip uffd-wp-mremap if UFFD write-protect is unsupported

Sayali Patil
2026-04-02T07:00:04Z

On 27/03/26 12:46, Sayali Patil wrote:
> The uffd-wp-mremap test requires the UFFD_FEATURE_PAGEFAULT_FLAG_WP
> capability. On systems where userfaultfd write-protect is
> not supported, uffd_register() fails and the test reports failures.
>
> Check for the required feature at startup and skip the test when the
> UFFD_FEATURE_PAGEFAULT_FLAG_WP capability is not present,
> preventing false failures on unsupported configurations.
>
> Before patch:
> running ./uffd-wp-mremap
> ------------------------
> [INFO] detected THP size: 256 KiB
> [INFO] detected THP size: 512 KiB
> [INFO] detected THP size: 1024 KiB
> [INFO] detected THP size: 2048 KiB
> [INFO] detected hugetlb page size: 2048 KiB
> [INFO] detected hugetlb page size: 1048576 KiB
> 1..24
> [RUN] test_one_folio(size=65536, private=false, swapout=false,
> hugetlb=false)
> not ok 1 uffd_register() failed
> [RUN] test_one_folio(size=65536, private=true, swapout=false,
> hugetlb=false)
> not ok 2 uffd_register() failed
> [RUN] test_one_folio(size=65536, private=false, swapout=true,
> hugetlb=false)
> not ok 3 uffd_register() failed
> [RUN] test_one_folio(size=65536, private=true, swapout=true,
> hugetlb=false)
> not ok 4 uffd_register() failed
> [RUN] test_one_folio(size=262144, private=false, swapout=false,
> hugetlb=false)
> not ok 5 uffd_register() failed
> [RUN] test_one_folio(size=524288, private=false, swapout=false,
> hugetlb=false)
> not ok 6 uffd_register() failed
> .
> .
> .
> Bail out! 24 out of 24 tests failed
> Totals: pass:0 fail:24 xfail:0 xpass:0 skip:0 error:0
> [FAIL]
> not ok 1 uffd-wp-mremap # exit=1
>
> After patch:
> running ./uffd-wp-mremap
> ------------------------
> 1..0 # SKIP uffd-wp feature not supported
> [SKIP]
> ok 1 uffd-wp-mremap # SKIP
>
> Acked-by: Zi Yan <ziy@nvidia.com>
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
> Signed-off-by: Sayali Patil <sayalip@linux.ibm.com>
> ---
> tools/testing/selftests/mm/uffd-wp-mremap.c | 13 +++++++++++++
> 1 file changed, 13 insertions(+)
>
> diff --git a/tools/testing/selftests/mm/uffd-wp-mremap.c b/tools/testing/selftests/mm/uffd-wp-mremap.c
> index 17186d4a4147..6edbd09f0ca6 100644
> --- a/tools/testing/selftests/mm/uffd-wp-mremap.c
> +++ b/tools/testing/selftests/mm/uffd-wp-mremap.c
> @@ -19,6 +19,17 @@ static size_t thpsizes[20];
> static int nr_hugetlbsizes;
> static size_t hugetlbsizes[10];
>
> +static void check_uffd_wp_feature_supported(void)
> +{
> + uint64_t features;
> +
> + if (uffd_get_features(&features) && errno == ENOENT)
> + ksft_exit_skip("failed to get available features (%d)\n", errno);
> +
> + if (!(features & UFFD_FEATURE_PAGEFAULT_FLAG_WP))
> + ksft_exit_skip("uffd-wp feature not supported\n");
> +}
> +
> static int detect_thp_sizes(size_t sizes[], int max)
> {
> int count = 0;
> @@ -336,6 +347,8 @@ int main(int argc, char **argv)
> struct thp_settings settings;
> int i, j, plan = 0;
>
> + check_uffd_wp_feature_supported();
> +
> pagesize = getpagesize();
> nr_thpsizes = detect_thp_sizes(thpsizes, ARRAY_SIZE(thpsizes));
> nr_hugetlbsizes = detect_hugetlb_page_sizes(hugetlbsizes,

AI review question:

> commit 22914640356d1eaf4e0a8a8b13ad71911d12db50
> Author: Sayali Patil <sayalip@linux.ibm.com>
> selftests/mm: skip uffd-wp-mremap if UFFD write-protect is unsupported
> This commit adds a check at startup to verify if the userfaultfd
> write-protect capability is present. It skips the test instead of reporting> false failures on unsupported configurations.
> > diff --git a/tools/testing/selftests/mm/uffd-wp-mremap.c b/tools/testing/selftests/mm/uffd-wp-mremap.c> > index 17186d4a4147..6edbd09f0ca6 100644
> > --- a/tools/testing/selftests/mm/uffd-wp-mremap.c
> > +++ b/tools/testing/selftests/mm/uffd-wp-mremap.c
> > @@ -19,6 +19,17 @@ static size_t thpsizes[20];
> > static int nr_hugetlbsizes;
> > static size_t hugetlbsizes[10];
>
> > +static void check_uffd_wp_feature_supported(void)
> > +{
> > + uint64_t features;
> > +
> > + if (uffd_get_features(&features) && errno == ENOENT)
> > + ksft_exit_skip("failed to get available features (%d)\n", errno);
> If uffd_get_features() fails but errno is something other than ENOENT (for> example, EPERM when unprivileged userfaultfd is disabled), this check will
> evaluate to false and execution will continue.> > +
> > + if (!(features & UFFD_FEATURE_PAGEFAULT_FLAG_WP))
> Could this evaluate uninitialized stack memory?
> If uffd_get_features() fails and is bypassed by the previous check, the
> features variable will remain uninitialized. This might cause the test to
> falsely pass or fail depending on whatever data happens to be on the stack.> Should we initialize features to zero or exit on all uffd_get_features()
> failures instead?
> > + ksft_exit_skip("uffd-wp feature not supported\n");
> > +}

Yes,If uffd_get_features() fails with an error other than ENOENT, the
features variable would remain uninitialized, which could lead to
undefined behavior in the subsequent check.

I will fix this in the next patch version.

Thanks,
Sayali