Re: [PATCH v3 1/2] mm/khugepaged: Never install PMDs in uffd-minor-registered VMAs

[James Houghton]

On Thu, Sep 10, 2026 at 5:03 PM James Houghton <jthoughton@xxxxxxxxxx> wrote:
>
> On Thu, Sep 10, 2026 at 4:24 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> > Sashiko had a stupid niggle about the selftest change and made quite
> > disturbing claims about the existing try_collapse_pte_mapped_thp() code:
> >
> >         https://sashiko.dev/#/patchset/20260910023411.514987-1-jthoughton@xxxxxxxxxx
>
> Thanks, it looks like the note it wrote about the selftest is genuine; it is
> okay for MADV_COLLAPSE to fail. So I shouldn't have changed uffd_test_skip() to
> uffd_test_fail().
>
> Andrew, if you can, please add this diff as part of the patch to queue. (It's
> a partial revert.) Or I could send the a new version of the patch as a reply
> to patch #2 email. Or a whole v4, whatever's easiest.
>
> diff --git a/tools/testing/selftests/mm/uffd-unit-tests.c b/tools/testing/selftests/mm/uffd-unit-tests.c
> index 6f2360f9b75d..d8e40b770d88 100644
> --- a/tools/testing/selftests/mm/uffd-unit-tests.c
> +++ b/tools/testing/selftests/mm/uffd-unit-tests.c
> @@ -639,7 +639,8 @@ static void uffd_minor_test_common(uffd_global_test_opts_t *gopts, bool test_col
>                 /* MADV_COLLAPSE should succeed with userfaultfd unregistered. */
>                 if (madvise(gopts->area_dst_alias, gopts->nr_pages * gopts->page_size,
>                             MADV_COLLAPSE)) {
> -                       uffd_test_fail("MADV_COLLAPSE failed");
> +                       /* It's fine to fail for this one... */
> +                       uffd_test_skip("MADV_COLLAPSE failed");
>                         return;
>                 }

Ah, the

  /* MADV_COLLAPSE should succeed with userfaultfd unregistered. */

comment at the top of that hunk should also be removed.