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
Signed-off-by: Li Wang
Cc: Aruna Ramakrishna
Cc: Bagas Sanjaya
Cc: Catalin Marinas
Cc: Dave Hansen
Cc: Joey Gouly
Cc: Johannes Weiner
Cc: Keith Lucas
Cc: Ryan Roberts
Cc: Shuah Khan
---
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
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
Signed-off-by: Li Wang
Cc: Aruna Ramakrishna
Cc: Bagas Sanjaya
Cc: Catalin Marinas
Cc: Dave Hansen
Cc: Joey Gouly
Cc: Johannes Weiner
Cc: Keith Lucas
Cc: Ryan Roberts
Cc: Shuah Khan
---
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?
Apart from that
Acked-by: David Hildenbrand
--
Cheers,
David / dhildenb
