Re: [PATCH v2 3/5] mm: userfaultfd: allow to register continue for guest_memfd

[Nikita Kalyazin <kalyazin@amazon.com>]

On 02/04/2025 22:25, James Houghton wrote:
> On Wed, Apr 2, 2025 at 9:08 AM Nikita Kalyazin <kalyazin@amazon.com> wrote:
>>
>> Signed-off-by: Nikita Kalyazin <kalyazin@amazon.com>
>> ---
>>   include/linux/userfaultfd_k.h | 13 ++++++++++++-
>>   1 file changed, 12 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
>> index 75342022d144..bc184edfbb85 100644
>> --- a/include/linux/userfaultfd_k.h
>> +++ b/include/linux/userfaultfd_k.h
>> @@ -212,6 +212,10 @@ static inline bool userfaultfd_armed(struct vm_area_struct *vma)
>>          return vma->vm_flags & __VM_UFFD_FLAGS;
>>   }
>>
>> +#ifdef CONFIG_KVM_PRIVATE_MEM
>> +bool kvm_gmem_vma_is_gmem(struct vm_area_struct *vma);
>> +#endif
>> +
>>   static inline bool vma_can_userfault(struct vm_area_struct *vma,
>>                                       unsigned long vm_flags,
>>                                       bool wp_async)
>> @@ -222,7 +226,11 @@ static inline bool vma_can_userfault(struct vm_area_struct *vma,
>>                  return false;
>>
>>          if ((vm_flags & VM_UFFD_MINOR) &&
>> -           (!is_vm_hugetlb_page(vma) && !vma_is_shmem(vma)))
>> +           (!is_vm_hugetlb_page(vma) && !vma_is_shmem(vma))
>> +#ifdef CONFIG_KVM_PRIVATE_MEM
>> +            && !kvm_gmem_vma_is_gmem(vma)
>> +#endif
> 
> Maybe a better way to do this is to add a vm_ops->can_userfault() or
> something, so we could write something like this:
> 
> if (vma->vm_ops && !vma->vm_ops->can_userfault)
>    return false;
> if (vma->vm_ops && !vma->vm_ops->can_userfault(vm_flags))
>    return false;

I like that, thanks!  What do you see passing vm_flags being useful for? 
  Shall we pass the entire vma struct like in most of other callbacks?

> 
> And shmem/hugetlbfs can advertise support for everything they already
> support that way.
> 
>> +           )
>>                  return false;
>>
>>          /*
>> @@ -244,6 +252,9 @@ static inline bool vma_can_userfault(struct vm_area_struct *vma,
>>
>>          /* By default, allow any of anon|shmem|hugetlb */
>>          return vma_is_anonymous(vma) || is_vm_hugetlb_page(vma) ||
>> +#ifdef CONFIG_KVM_PRIVATE_MEM
>> +           kvm_gmem_vma_is_gmem(vma) ||
>> +#endif
>>              vma_is_shmem(vma);
>>   }
>>
>> --
>> 2.47.1
>>