From: "Mike Rapoport (Microsoft)" <[email protected]>
Non-cooperarive uffd events are inherently racy and can happen in
parallel with other userfaultfd operations.
During event tests in uffd-unit-tests, the uffd monitor calls
UFFDIO_UNREGISTER upon receiving UFFD_EVENT_REMOVE.
In parallel, the faulting_process() verifies that the removed memory is
actually zeroed.
If a verification read wins the race with UFFDIO_UNREGISTER, it causes a
missing fault that uffd monitor would receive after UFFDIO_UNREGISTER is
complete. The monitor resolves the fault using UFFDIO_COPY that fails
with -ENOENT which means that VMA has been changed (see commit
27d02568f529 ("userfaultfd: mcopy_atomic: return -ENOENT when no
compatible VMA found")).
Treat -ENOENT returned by UFFDIO_COPY as non-fatal, the same way
-EEXIST is treated for concurrent faults, and don't fail the test.
Signed-off-by: Mike Rapoport (Microsoft) <[email protected]>
---
I've noticed transient faults of uffd-unit-tests in the CI runs [1]
and found this issue with uffd-unit-tests.
The issue is longstanding and it's not related to or exposed by the
recent uffd refactoring.
I didn't even look for a Fixes: commit, as this is a selftest only and I
don't see a reason to backport it.
[1] https://github.com/linux-mm/linux-mm/actions
tools/testing/selftests/mm/uffd-common.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)
diff --git a/tools/testing/selftests/mm/uffd-common.c
b/tools/testing/selftests/mm/uffd-common.c
index edd02328f77b..f48f5d4594ab 100644
--- a/tools/testing/selftests/mm/uffd-common.c
+++ b/tools/testing/selftests/mm/uffd-common.c
@@ -639,8 +639,13 @@ int __copy_page(uffd_global_test_opts_t *gopts, unsigned
long offset, bool retry
uffdio_copy.mode = 0;
uffdio_copy.copy = 0;
if (ioctl(gopts->uffd, UFFDIO_COPY, &uffdio_copy)) {
- /* real retval in ufdio_copy.copy */
- if (uffdio_copy.copy != -EEXIST)
+ /*
+ * real retval in uffdio_copy.copy
+ *
+ * -EEXIST: the page was faulted in concurrently
+ * -ENOENT: the destination range was concurrently removed
+ */
+ if (uffdio_copy.copy != -EEXIST && uffdio_copy.copy != -ENOENT)
err("UFFDIO_COPY error: %"PRId64,
(int64_t)uffdio_copy.copy);
wake_range(gopts->uffd, uffdio_copy.dst, gopts->page_size);
base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482
--
2.53.0
