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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 10/20/26 12:11, Mike Rapoport wrote:
> 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?

That would likely even better and we can just simplify the comments as well.

-- 
Cheers,

David