On Wed, Jul 01, 2026 at 11:09:32PM +0300, Mike Rapoport wrote: > 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]>
LGTM, so: Reviewed-by: Lorenzo Stoakes <[email protected]> > --- > I've noticed transient faults of uffd-unit-tests in the CI runs [1] > and found this issue with uffd-unit-tests. Nice work tracking that down! > > 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 > Cheers, Lorenzo
