Re: [PATCH v2 4/4] KVM: guest_memfd: Stop returning struct page from PFN lookup

[Michael Roth]

On Tue, Aug 18, 2026 at 09:15:55AM +0000, Ackerley Tng wrote:
> From: Sean Christopherson <seanjc@xxxxxxxxxx>
> 
> KVM currently expects guest_memfd PFN lookups to return a refcounted
> struct page, which callers hold across fault handling.
> 
> Holding a page reference across fault handling is problematic for
> guest_memfd. In-place memory conversions between confidential
> computing shared and private states inspect folio refcounts to ensure
> exclusive ownership by guest_memfd. A concurrent guest page fault
> taking a reference on the folio causes conversions to fail due to an
> elevated refcount.
> 
> guest_memfd already notifies KVM of page invalidations, so callers
> within KVM only need to respect the MMU invalidation protocol to safely
> rely on guest_memfd for page presence.
> 
> Furthermore, removing struct page from the guest_memfd PFN lookup moves
> KVM closer toward supporting memory backends that are not backed by
> struct page.
> 
> Drop the folio reference immediately before returning from the
> guest_memfd PFN lookup, and stop returning the struct page pointer.
> 
> For ARM, initialize the local page pointer to NULL so that the shared
> cleanup path that releases fault-in pages safely no-ops for guest_memfd.
> 
> For x86, no additional changes are required in the MMU fault path
> because the page fault tracking structure is zero-initialized at the
> start of page fault handling, ensuring the refcounted page pointer is
> already NULL.
> 
> Reported-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> Closes: https://lore.kernel.org/all/anZ4W9o5pTWIEgMY@xxxxxxxxxxxxxxxxxxxxxxxxx/
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Co-developed-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> Co-developed-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
> Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>

Other than what Suzuki pointed out:

Reviewed-by: Michael Roth <michael.roth@xxxxxxx>
Tested-by: Michael Roth <michael.roth@xxxxxxx>

> ---
>  arch/arm64/kvm/mmu.c     | 4 ++--
>  arch/arm64/kvm/nested.c  | 4 ++--
>  arch/x86/kvm/mmu/mmu.c   | 2 +-
>  arch/x86/kvm/svm/sev.c   | 8 ++------
>  include/linux/kvm_host.h | 6 ++----
>  virt/kvm/guest_memfd.c   | 9 ++-------
>  6 files changed, 11 insertions(+), 22 deletions(-)
> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 6c941aaa10c63..e5d637a5ec558 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1613,7 +1613,7 @@ static int gmem_abort(const struct kvm_s2_fault_desc *s2fd)
>  	enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_R;
>  	struct kvm_pgtable *pgt = s2fd->vcpu->arch.hw_mmu->pgt;
>  	unsigned long mmu_seq;
> -	struct page *page;
> +	struct page *page = NULL;
>  	struct kvm *kvm = s2fd->vcpu->kvm;
>  	void *memcache = NULL;
>  	kvm_pfn_t pfn;
> @@ -1641,7 +1641,7 @@ static int gmem_abort(const struct kvm_s2_fault_desc *s2fd)
>  	/* Pairs with the smp_wmb() in kvm_mmu_invalidate_end(). */
>  	smp_rmb();
>  
> -	ret = kvm_gmem_get_pfn(kvm, s2fd->memslot, gfn, &pfn, &page, NULL);
> +	ret = kvm_gmem_get_pfn(kvm, s2fd->memslot, gfn, &pfn, NULL);
>  	if (ret) {
>  		kvm_prepare_memory_fault_exit(s2fd->vcpu, s2fd->fault_ipa, PAGE_SIZE,
>  					      write_fault, exec_fault, false);
> diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
> index fb54f6dad995c..43523bb17621a 100644
> --- a/arch/arm64/kvm/nested.c
> +++ b/arch/arm64/kvm/nested.c
> @@ -1360,7 +1360,7 @@ static int kvm_translate_vncr(struct kvm_vcpu *vcpu, bool *is_gmem)
>  	bool write_fault, writable;
>  	unsigned long mmu_seq;
>  	struct vncr_tlb *vt;
> -	struct page *page;
> +	struct page *page = NULL;
>  	u64 va, pfn, gfn;
>  	int ret;
>  
> @@ -1411,7 +1411,7 @@ static int kvm_translate_vncr(struct kvm_vcpu *vcpu, bool *is_gmem)
>  		if (is_error_noslot_pfn(pfn) || (write_fault && !writable))
>  			return -EFAULT;
>  	} else {
> -		ret = kvm_gmem_get_pfn(vcpu->kvm, memslot, gfn, &pfn, &page, NULL);
> +		ret = kvm_gmem_get_pfn(vcpu->kvm, memslot, gfn, &pfn, NULL);
>  		if (ret) {
>  			kvm_prepare_memory_fault_exit(vcpu, vt->wr.pa, PAGE_SIZE,
>  					      write_fault, false, false);
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index c519e8e8d646f..129d403308051 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4604,7 +4604,7 @@ static int kvm_mmu_faultin_pfn_gmem(struct kvm_vcpu *vcpu,
>  	}
>  
>  	r = kvm_gmem_get_pfn(vcpu->kvm, fault->slot, fault->gfn, &fault->pfn,
> -			     &fault->refcounted_page, &max_order);
> +			     &max_order);
>  	if (r) {
>  		kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
>  		return r;
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index d3d620bc04dce..32db979daaaf0 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -4016,7 +4016,6 @@ static void __sev_snp_reload_vmsa(struct kvm_vcpu *vcpu, gpa_t gpa)
>  	struct kvm *kvm = vcpu->kvm;
>  	gfn_t gfn = gpa_to_gfn(gpa);
>  	unsigned long mmu_seq;
> -	struct page *page;
>  	kvm_pfn_t pfn;
>  
>  	lockdep_assert_held(&svm->sev_es.snp_vmsa_mutex);
> @@ -4060,9 +4059,8 @@ static void __sev_snp_reload_vmsa(struct kvm_vcpu *vcpu, gpa_t gpa)
>  	 * The new VMSA will be private memory guest memory, so retrieve the
>  	 * PFN from the gmem backend.
>  	 */
> -	if (kvm_gmem_get_pfn(vcpu->kvm, slot, gfn, &pfn, &page, NULL))
> +	if (kvm_gmem_get_pfn(vcpu->kvm, slot, gfn, &pfn, NULL))
>  		return;
> -	kvm_release_page_clean(page);
>  
>  	read_lock(&kvm->mmu_lock);
>  	/*
> @@ -5003,7 +5001,6 @@ void sev_handle_rmp_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code)
>  	struct kvm *kvm = vcpu->kvm;
>  	int order, rmp_level, ret;
>  	unsigned long mmu_seq;
> -	struct page *page;
>  	bool assigned;
>  	kvm_pfn_t pfn;
>  	gfn_t gfn;
> @@ -5033,13 +5030,12 @@ void sev_handle_rmp_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code)
>  	mmu_seq = kvm->mmu_invalidate_seq;
>  	smp_rmb();
>  
> -	ret = kvm_gmem_get_pfn(kvm, slot, gfn, &pfn, &page, &order);
> +	ret = kvm_gmem_get_pfn(kvm, slot, gfn, &pfn, &order);
>  	if (ret) {
>  		pr_warn_ratelimited("SEV: Unexpected RMP fault, no backing page for private GPA 0x%llx\n",
>  				    gpa);
>  		return;
>  	}
> -	kvm_release_page_unused(page);
>  
>  	ret = snp_lookup_rmpentry(pfn, &assigned, &rmp_level);
>  	if (ret || !assigned) {
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 03bfc92864b6e..502465119ca0c 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -2586,13 +2586,11 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
>  
>  #ifdef CONFIG_KVM_GUEST_MEMFD
>  int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
> -		     gfn_t gfn, kvm_pfn_t *pfn, struct page **page,
> -		     int *max_order);
> +		     gfn_t gfn, kvm_pfn_t *pfn, int *max_order);
>  #else
>  static inline int kvm_gmem_get_pfn(struct kvm *kvm,
>  				   struct kvm_memory_slot *slot, gfn_t gfn,
> -				   kvm_pfn_t *pfn, struct page **page,
> -				   int *max_order)
> +				   kvm_pfn_t *pfn, int *max_order)
>  {
>  	KVM_BUG_ON(1, kvm);
>  	return -EIO;
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index b596486d184ca..589762140c3ef 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -751,8 +751,7 @@ static struct folio *__kvm_gmem_get_pfn(struct file *file,
>  }
>  
>  int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
> -		     gfn_t gfn, kvm_pfn_t *pfn, struct page **page,
> -		     int *max_order)
> +		     gfn_t gfn, kvm_pfn_t *pfn, int *max_order)
>  {
>  	pgoff_t index = kvm_gmem_get_index(slot, gfn);
>  	struct folio *folio;
> @@ -780,11 +779,7 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
>  #endif
>  
>  	folio_unlock(folio);
> -
> -	if (!r)
> -		*page = folio_file_page(folio, index);
> -	else
> -		folio_put(folio);
> +	folio_put(folio);
>  
>  	return r;
>  }
> 
> -- 
> 2.55.0.699.gb54405d56f-goog
>