linux-kernel - Re: [PATCH v2 3/5] mm: userfaultfd: allow to register continue for guest

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Message-ID: <0c72b9a9-3726-4cc0-809b-3e19413a4476@amazon.com>
Date: Thu, 3 Apr 2025 18:01:57 +0100
From: Nikita Kalyazin <kalyazin@...zon.com>
To: James Houghton <jthoughton@...gle.com>
CC: <akpm@...ux-foundation.org>, <pbonzini@...hat.com>, <shuah@...nel.org>,
	<kvm@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
	<lorenzo.stoakes@...cle.com>, <david@...hat.com>, <ryan.roberts@....com>,
	<quic_eberman@...cinc.com>, <peterx@...hat.com>, <graf@...zon.de>,
	<jgowans@...zon.com>, <roypat@...zon.co.uk>, <derekmn@...zon.com>,
	<nsaenz@...zon.es>, <xmarcalx@...zon.com>
Subject: Re: [PATCH v2 3/5] mm: userfaultfd: allow to register continue for
 guest_memfd



On 02/04/2025 22:25, James Houghton wrote:
> On Wed, Apr 2, 2025 at 9:08 AM Nikita Kalyazin <kalyazin@...zon.com> wrote:
>>
>> Signed-off-by: Nikita Kalyazin <kalyazin@...zon.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
>>