Re: [PATCH v12 05/12] KVM: guest_memfd: Enforce NUMA mempolicy using shared policy

["Garg, Shivank" <shivankg@amd.com>]

On 10/11/2025 3:27 AM, Ackerley Tng wrote:
> Sean Christopherson <seanjc@google.com> writes:
> 
>> On Fri, Oct 10, 2025, Shivank Garg wrote:
>>>>> @@ -112,6 +114,19 @@ static int kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slot,
>>>>>  	return r;
>>>>>  }
>>>>>  
>>>>> +static struct mempolicy *kvm_gmem_get_folio_policy(struct gmem_inode *gi,
>>>>> +						   pgoff_t index)
>>>>
>>>> How about kvm_gmem_get_index_policy() instead, since the policy is keyed
>>>> by index?
>>
>> But isn't the policy tied to the folio?  I assume/hope that something will split
>> folios if they have different policies for their indices when a folio contains
>> more than one page.  In other words, how will this work when hugepage support
>> comes along?
>>
>> So yeah, I agree that the lookup is keyed on the index, but conceptually aren't
>> we getting the policy for the folio?  The index is a means to an end.
>>
> 
> I think the policy is tied to the index.
> 
> When we mmap(), there may not be a folio at this index yet, so any folio
> that gets allocated for this index then is taken from the right NUMA
> node based on the policy.
> 
> If the folio is later truncated, the folio just goes back to the NUMA
> node, but the memory policy remains for the next folio to be allocated
> at this index.
> 
>>>>> +{
>>>>> +#ifdef CONFIG_NUMA
>>>>> +	struct mempolicy *mpol;
>>>>> +
>>>>> +	mpol = mpol_shared_policy_lookup(&gi->policy, index);
>>>>> +	return mpol ? mpol : get_task_policy(current);
>>>>
>>>> Should we be returning NULL if no shared policy was defined?
>>>>
>>>> By returning NULL, __filemap_get_folio_mpol() can handle the case where
>>>> cpuset_do_page_mem_spread().
>>>>
>>>> If we always return current's task policy, what if the user wants to use
>>>> cpuset_do_page_mem_spread()?
>>>>
>>>
>>> I initially followed shmem's approach here.
>>> I agree that returning NULL maintains consistency with the current default
>>> behavior of cpuset_do_page_mem_spread(), regardless of CONFIG_NUMA.
>>>
>>> I'm curious what could be the practical implications of cpuset_do_page_mem_spread()
>>> v/s get_task_policy() as the fallback?
>>
>> Userspace could enable page spreading on the task that triggers guest_memfd
>> allocation.  I can't conjure up a reason to do that, but I've been surprised
>> more than once by KVM setups.
>>
>>> Which is more appropriate for guest_memfd when no policy is explicitly set
>>> via mbind()?
>>
>> I don't think we need to answer that question?  Userspace _has_ set a policy,
>> just through cpuset, not via mbind().  So while I can't imagine there's a sane
>> use case for cpuset_do_page_mem_spread() with guest_memfd, I also don't see a
>> reason why KVM should effectively disallow it.
>>
>> And unless I'm missing something, allocation will eventually fallback to
>> get_task_policy() (in alloc_frozen_pages_noprof()), so by explicitly getting the
>> task policy in guest_memfd, KVM is doing _more_ work than necessary _and_ is
>> unnecessarily restricting usersepace.
>>
>> Add in that returning NULL would align this code with the ->get_policy hook (and
>> could be shared again, I assume), and my vote is definitely to return NULL and
>> not get in the way.
> 
> ... although if we are going to return NULL then we can directly use
> mpol_shared_policy_lookup(), so the first discussion is moot.
> 
> 
> Though looking slightly into the future, shareability (aka memory
> attributes or shared/private state within guest_memfd inodes) are also
> keyed by index, and is a property of the index and not the folio (since
> shared/private state is defined even before folios are allocated for a
> given index.

Thanks Ackerley and Sean for catching this and giving suggestions.
With directly using mpol_shared_policy_lookup(), the code looks much cleaner.

 virt/kvm/guest_memfd.c | 26 +++++++-------------------
 1 file changed, 7 insertions(+), 19 deletions(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 95267c92983b..409cbfc6db13 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -114,19 +114,6 @@ static int kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slot,
 	return r;
 }
 
-static struct mempolicy *kvm_gmem_get_folio_policy(struct gmem_inode *gi,
-						   pgoff_t index)
-{
-#ifdef CONFIG_NUMA
-	struct mempolicy *mpol;
-
-	mpol = mpol_shared_policy_lookup(&gi->policy, index);
-	return mpol ? mpol : get_task_policy(current);
-#else
-	return NULL;
-#endif
-}
-
 /*
  * Returns a locked folio on success.  The caller is responsible for
  * setting the up-to-date flag before the memory is mapped into the guest.
@@ -151,7 +138,7 @@ static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index)
 	if (!IS_ERR(folio))
 		return folio;
 
-	policy = kvm_gmem_get_folio_policy(GMEM_I(inode), index);
+	policy = mpol_shared_policy_lookup(&GMEM_I(inode)->policy, index);
 	folio = __filemap_get_folio_mpol(inode->i_mapping, index,
 					 FGP_LOCK | FGP_ACCESSED | FGP_CREAT,
 					 mapping_gfp_mask(inode->i_mapping), policy);
@@ -462,11 +449,12 @@ static struct mempolicy *kvm_gmem_get_policy(struct vm_area_struct *vma,
 	*pgoff = vma->vm_pgoff + ((addr - vma->vm_start) >> PAGE_SHIFT);
 
 	/*
-	 * Note!  Directly return whatever the lookup returns, do NOT return
-	 * the current task's policy as is done when looking up the policy for
-	 * a specific folio.  Kernel ABI for get_mempolicy() is to return
-	 * MPOL_DEFAULT when there is no defined policy, not whatever the
-	 * default policy resolves to.
+	 * Return the memory policy for this index, or NULL if none is set.
+	 *
+	 * Returning NULL is important for the .get_policy kernel ABI:
+	 * it indicates that no explicit policy has been set via mbind() for
+	 * this memory. The caller can then replace NULL with the default
+	 * memory policy instead of current's memory policy.
 	 */
 	return mpol_shared_policy_lookup(&GMEM_I(inode)->policy, *pgoff);
 }
-- 
2.43.0