Re: [PATCH v7 07/42] KVM: guest_memfd: Only prepare folios for private pages

[Suzuki K Poulose <suzuki.poulose@arm.com>]

On 02/06/2026 09:55, Suzuki K Poulose wrote:
> On 23/05/2026 01:17, Ackerley Tng via B4 Relay wrote:
>> From: Ackerley Tng <ackerleytng@google.com>
>>
>> All-shared guest_memfd used to be only supported for non-CoCo VMs where
>> preparation doesn't apply. INIT_SHARED is about to be supported for
>> non-CoCo VMs in a later patch in this series.
> 
> nit: s/non-CoCo/CoCo ?
> 
>>
>> In addition, KVM_SET_MEMORY_ATTRIBUTES2 is about to be supported in
>> guest_memfd in a later patch in this series.
>>
>> This means that the kvm fault handler may now call kvm_gmem_get_pfn() 
>> on a
>> shared folio for a CoCo VM where preparation applies.
>>
>> Add a check to make sure that preparation is only performed for private
>> folios.
>>
>> Preparation will be undone on freeing (see kvm_gmem_free_folio()) and on
>> conversion to shared.
>>
>> Signed-off-by: Michael Roth <michael.roth@amd.com>
> 
> nit: Missing Co-Developed-by: ?
> 
>> Reviewed-by: Fuad Tabba <tabba@google.com>
>> Signed-off-by: Ackerley Tng <ackerleytng@google.com>
>> ---
>>   virt/kvm/guest_memfd.c | 9 ++++++---
>>   1 file changed, 6 insertions(+), 3 deletions(-)
>>
>> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
>> index 78e5435967341..adf57a3a1f5dd 100644
>> --- a/virt/kvm/guest_memfd.c
>> +++ b/virt/kvm/guest_memfd.c
>> @@ -894,6 +894,7 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct 
>> kvm_memory_slot *slot,
>>                int *max_order)
>>   {
>>       pgoff_t index = kvm_gmem_get_index(slot, gfn);
>> +    struct inode *inode;
>>       struct folio *folio;
>>       int r = 0;
>> @@ -901,7 +902,8 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct 
>> kvm_memory_slot *slot,
>>       if (!file)
>>           return -EFAULT;
>> -    filemap_invalidate_lock_shared(file_inode(file)->i_mapping);
>> +    inode = file_inode(file);
>> +    filemap_invalidate_lock_shared(inode->i_mapping);
>>       folio = __kvm_gmem_get_pfn(file, slot, index, pfn, max_order);
>>       if (IS_ERR(folio)) {
>> @@ -914,7 +916,8 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct 
>> kvm_memory_slot *slot,
>>           folio_mark_uptodate(folio);
>>       }
>> -    r = kvm_gmem_prepare_folio(kvm, slot, gfn, folio);
>> +    if (kvm_gmem_is_private_mem(inode, index))
> 
> Don't we need to make sure the entire folio is private ? Not just the 
> page at the index ?
>      if (kvm_gmem_range_is_private(, index, folio_nr_pages(folio)) ?

Or rather, we should go through the individual pages and apply the
prepare for ones that are private ?

Suzuki

> 
> Suzuki
> 
>> +        r = kvm_gmem_prepare_folio(kvm, slot, gfn, folio);
>>       folio_unlock(folio);
>> @@ -924,7 +927,7 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct 
>> kvm_memory_slot *slot,
>>           folio_put(folio);
>>   out:
>> -    filemap_invalidate_unlock_shared(file_inode(file)->i_mapping);
>> +    filemap_invalidate_unlock_shared(inode->i_mapping);
>>       return r;
>>   }
>>   EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_gmem_get_pfn);
>>
>