Re: [PATCH V2 2/3] mm/hugetlb: disable PMD sharing for UFFD missing mode

[Mike Rapoport <rppt@kernel.org>]

On Mon, Oct 19, 2026 at 07:30:33PM +0800, Li Zhe wrote:
> From: "David Hildenbrand (Arm)" <david@kernel.org>
> 
> For UFFDIO_POISON to install a poison marker in a hugetlb mapping, it is
> sufficient to have the VMA registered for missing mode.
> 
> However, UFFD missing mode does currently not block PMD table sharing, so
> installing the marker will be visible to everyone using the shared PMD
> table, which is wrong as UFFDIO_POISON must stay local to the process.
> 
> To fix it, include VM_UFFD_MISSING in uffd_disable_huge_pmd_share(). Add
> a debug check on the marker installation path.
> 
> The existing registration path will then unshare PMD tables, and
> subsequent faults will not share them while the mapping remains
> registered.
> 
> The alternative would be to unshare PMD tables selectively when
> installing poison markers, which results in way more complexity. Or
> alternatively, simply failing the marker installation, which might break
> some use cases.
> 
> This effectively disables PMD sharing for missing-mode registrations
> that never use UFFDIO_POISON.
> 
> Suggested-by: Li Zhe <lizhe.67@bytedance.com>
> Fixes: 8a13897fb0da ("mm: userfaultfd: support UFFDIO_POISON for hugetlbfs")
> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
> Signed-off-by: Li Zhe <lizhe.67@bytedance.com>
> ---
>  include/linux/userfaultfd_k.h | 13 +++++++++----
>  mm/hugetlb.c                  |  1 +
>  2 files changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> index a4351cffc60c..fe78ba14dc68 100644
> --- a/include/linux/userfaultfd_k.h
> +++ b/include/linux/userfaultfd_k.h
> @@ -169,7 +169,10 @@ static inline bool is_mergeable_vm_userfaultfd_ctx(struct vm_area_struct *vma,
>  }
>  
>  /*
> - * Never enable huge pmd sharing on some uffd registered vmas:
> + * Never enable huge pmd sharing on the following VMAs:
> + *
> + * - VM_UFFD_MISSING VMAs, because UFFDIO_POISON may install markers and we
> + *   want PMD tables to be unshared when installing these markers already.
>   *
>   * - VM_UFFD_WP and VM_UFFD_RWP VMAs, because the write protect / access
>   *   tracking information is per pgtable entry.
> @@ -182,9 +185,11 @@ static inline bool is_mergeable_vm_userfaultfd_ctx(struct vm_area_struct *vma,
>   */
>  static inline bool uffd_disable_huge_pmd_share(struct vm_area_struct *vma)
>  {
> -	return vma_test_any_mask(vma,
> -		mk_vma_flags_from_masks(VMA_UFFD_WP, VMA_UFFD_RWP,
> -					VMA_UFFD_MINOR));
> +	const vma_flags_t uffd_flags =
> +		mk_vma_flags_from_masks(VMA_UFFD_MISSING, VMA_UFFD_WP,
> +					VMA_UFFD_RWP, VMA_UFFD_MINOR);

Since this essentially disable pmd sharing for any uffd mode, maybe replace
the flags check with the check for non-NULL userfaultfd_ctx?

> +
> +	return vma_test_any_mask(vma, uffd_flags);
>  }
>  
>  /*
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 2c64ca985dfa..fdd65e019a55 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6304,6 +6304,7 @@ int hugetlb_mfill_atomic_pte(pte_t *dst_pte,
>  			return -EEXIST;
>  		}
>  
> +		VM_WARN_ON_ONCE(size == PMD_SIZE && hugetlb_pmd_shared(dst_pte));
>  		_dst_pte = make_pte_marker(PTE_MARKER_POISONED);
>  		set_huge_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte, size);
>  
> -- 
> 2.20.1

-- 
Sincerely yours,
Mike.