[PATCH v3 4/4] KVM: guest_memfd: Stop returning struct page from PFN lookup
Fuad Tabba
fuad.tabba at linux.dev
Fri Aug 21 01:08:36 PDT 2026
On Fri, 21 Aug 2026 at 00:32, Ackerley Tng <ackerleytng at google.com> wrote:
>
> From: Sean Christopherson <seanjc at google.com>
>
> KVM currently expects guest_memfd PFN lookups to return a refcounted struct
> page, which callers hold across fault handling.
>
> Drop the page's refcount before returning from kvm_gmem_get_pfn() to
> prepare for the in-place conversions series.
>
> CoCo shared-to-private conversion handling must inspect folio refcounts to
> ensure exclusive ownership by guest_memfd. A concurrent guest page fault
> taking a temporary reference on the folio causes conversions to fail due to
> an elevated refcount.
>
> While this refcount is also taken on host userspace page faults, that
> refcount is taken on behalf of the host userspace page tables. This
> refcount will be dropped when conversions unmaps the page. Either way, once
> there's an mmap() or userspace mapping, the pages are open to way more
> refcounts, transient or not. This patch focuses on just dropping refcounts
> before handing KVM a page.
>
> 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.
>
> Since the page refcounts are dropped, don't return the struct page pointer.
>
> Not returning the struct page from the guest_memfd PFN lookup moves KVM
> closer toward supporting memory backends that are not backed by struct
> page.
>
> Here are some notes on the cleanup in the callers of kvm_gmem_get_pfn():
>
> kvm_release_faultin_page() in ARM's gmem_abort() originally also serves to
> set the page dirty and accessed under some conditions. The dirty and
> accessed flags don't matter for guest_memfd anyway, so it is safe to just
> drop the call to kvm_release_faultin_page().
>
> For ARM's kvm_translate_vncr(), the local page pointer must be initialized
> to NULL so that the shared cleanup path that releases faulted-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 at intel.com>
> Closes: https://lore.kernel.org/all/anZ4W9o5pTWIEgMY@yzhao56-desk.sh.intel.com/
> Signed-off-by: Sean Christopherson <seanjc at google.com>
> Co-developed-by: Yan Zhao <yan.y.zhao at intel.com>
> Signed-off-by: Yan Zhao <yan.y.zhao at intel.com>
> Reviewed-by: Suzuki K Poulose <suzuki.poulose at arm.com>
> Reviewed-by: Michael Roth <michael.roth at amd.com>
> Tested-by: Michael Roth <michael.roth at amd.com>
> Tested-by: Yan Zhao <yan.y.zhao at intel.com>
> Co-developed-by: Ackerley Tng <ackerleytng at google.com>
> Signed-off-by: Ackerley Tng <ackerleytng at google.com>
For the arm64 part: tested under QEMU: guest boots with anonymous
memory and guest_memfd, plus the guest_memfd_test selftest.
Reviewed-by: Fuad Tabba <fuad.tabba at linux.dev>
Tested-by: Fuad Tabba < fuad.tabba at linux.dev>
Cheers,
/fuad
> ---
> 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, 10 insertions(+), 23 deletions(-)
>
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 6c941aaa10c63..d5aa197d2cbfd 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1613,7 +1613,6 @@ 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 kvm *kvm = s2fd->vcpu->kvm;
> void *memcache = NULL;
> kvm_pfn_t pfn;
> @@ -1641,7 +1640,7 @@ 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);
> @@ -1681,7 +1680,6 @@ static int gmem_abort(const struct kvm_s2_fault_desc *s2fd)
> }
>
> out_unlock:
> - kvm_release_faultin_page(kvm, page, !!ret, prot & KVM_PGTABLE_PROT_W);
> kvm_fault_unlock(kvm);
>
> if ((prot & KVM_PGTABLE_PROT_W) && !ret)
> 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 0375ef709ee2c..0c91c904573cc 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;
> }
>
> --
> 2.55.0.766.g2966f0265a-goog
>
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