Re: [PATCH 0/2] mm/userfaultfd: recheck WP registration in the hugetlb UFFDIO_COPY retry

[Mike Rapoport <rppt@kernel.org>]

Hi Jose,

On Thu, Oct 29, 2026 at 11:11:00PM +0100, Jose A. Perez de Azpillaga wrote:
> The urgent fixes made hugetlb_fault() tolerate uffd bits left in VMAs
> that are no longer registered for them, and asked for follow-ups that
> remove the sources [1]. This is one: a hugetlb UFFDIO_COPY_MODE_WP copy
> that has to fault in its source page doesn't recheck the WP registration
> before retrying, so re-registering the range without WP in that window
> leaves a uffd-wp PTE behind.
> 
> Patch 1 adds the check the non-hugetlb retry already gets from
> mfill_get_vma(). Patch 2 adds a uffd-unit-tests case for it, which fails
> on hugetlb without patch 1.
> 
> Question:
> 
> - The non-hugetlb retry compares the whole mfill_retry_state. hugetlb
>   copies into a freshly allocated folio, so only WP seemed to matter
>   there. Mike, would you rather have the full comparison on both paths?

Let's keep things simple and easily backportable :)

Since hugetlb does not leave a folio from the original VMA behind there's
no inconsistency we need to worry about.
 
> The other source 6/10 of [1] mentions, UFFDIO_COPY in MODE_WP racing
> with UFFDIO_UNREGISTER [2], is closed by "mm/userfaultfd: write-lock the
> VMA before waking waiters when unregistering" [3].
> 
> Based on mm-unstable.
> 
> Tested on x86_64 in QEMU.
> 
> [1] https://lore.kernel.org/20261015-urgent_fixes-v1-0-42796911043b@kernel.org
> [2] https://lore.kernel.org/20261015-urgent_fixes-v1-6-42796911043b@kernel.org
> [3] https://lore.kernel.org/20261027-fixes_core-v1-23-6f74bec9a5bb@kernel.org (v1, now 23/47 in v2)
> 
> Jose A. Perez de Azpillaga (2):
>   mm/userfaultfd: recheck WP registration in the hugetlb UFFDIO_COPY
>     retry
>   selftests/mm: add a uffd test for re-registering during UFFDIO_COPY
> 
>  mm/userfaultfd.c                             |   4 +
>  tools/testing/selftests/mm/uffd-unit-tests.c | 105 +++++++++++++++++++
>  2 files changed, 109 insertions(+)
> 
> 
> base-commit: 24d45a1475dd4c671feb1df705f67ef2fc293969
> -- 
> 2.55.0
> 

-- 
Sincerely yours,
Mike.