KVM: Put the extra pfn reference when reusing a pfn in the gpc cache

Put the struct page reference to pfn acquired by hva_to_pfn() when the
old and new pfns for a gfn=>pfn cache match.  The cache already has a
reference via the old/current pfn, and will only put one reference when
the cache is done with the pfn.

Fixes: 982ed0de ("KVM: Reinstate gfn_to_pfn_cache with invalidation support")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220429210025.3293691-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/pfncache.c

@@ -206,6 +206,14 @@ int kvm_gfn_to_pfn_cache_refresh(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
 
 		if (gpc->usage & KVM_HOST_USES_PFN) {
 			if (new_pfn == old_pfn) {
+				/*
+				 * Reuse the existing pfn and khva, but put the
+				 * reference acquired hva_to_pfn_retry(); the
+				 * cache still holds a reference to the pfn
+				 * from the previous refresh.
+				 */
+				gpc_release_pfn_and_khva(kvm, new_pfn, NULL);
+
 				new_khva = old_khva;
 				old_pfn = KVM_PFN_ERR_FAULT;
 				old_khva = NULL;