Re: [PATCH] dev: selftests/mm/uffd: don't treat UFFDIO_COPY -ENOENT as a failure

[Mike Rapoport <rppt@kernel.org>]

Oops, the subject came out wrong, will resend.

Sorry for the noise.

On Wed, Jul 01, 2026 at 11:01:43PM +0300, Mike Rapoport wrote:
> From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>
> 
> 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) <rppt@kernel.org>
> ---
> 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
> 

-- 
Sincerely yours,
Mike.