KVM: Don't set Accessed/Dirty bits for ZERO_PAGE

Don't set Accessed/Dirty bits for a struct page with PG_reserved set,
i.e. don't set A/D bits for the ZERO_PAGE.  The ZERO_PAGE (or pages
depending on the architecture) should obviously never be written, and
similarly there's no point in marking it accessed as the page will never
be swapped out or reclaimed.  The comment in page-flags.h is quite clear
that PG_reserved pages should be managed only by their owner, and
strictly following that mandate also simplifies KVM's logic.

Fixes: 7df003c8 ("KVM: fix overflow of zero page refcount with ksm running")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220429010416.2788472-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/kvm_main.c

@@ -2850,16 +2850,28 @@ void kvm_release_pfn_dirty(kvm_pfn_t pfn)
 }
 EXPORT_SYMBOL_GPL(kvm_release_pfn_dirty);
 
+static bool kvm_is_ad_tracked_pfn(kvm_pfn_t pfn)
+{
+	if (!pfn_valid(pfn))
+		return false;
+
+	/*
+	 * Per page-flags.h, pages tagged PG_reserved "should in general not be
+	 * touched (e.g. set dirty) except by its owner".
+	 */
+	return !PageReserved(pfn_to_page(pfn));
+}
+
 void kvm_set_pfn_dirty(kvm_pfn_t pfn)
 {
-	if (!kvm_is_reserved_pfn(pfn) && !kvm_is_zone_device_pfn(pfn))
+	if (kvm_is_ad_tracked_pfn(pfn))
 		SetPageDirty(pfn_to_page(pfn));
 }
 EXPORT_SYMBOL_GPL(kvm_set_pfn_dirty);
 
 void kvm_set_pfn_accessed(kvm_pfn_t pfn)
 {
-	if (!kvm_is_reserved_pfn(pfn) && !kvm_is_zone_device_pfn(pfn))
+	if (kvm_is_ad_tracked_pfn(pfn))
 		mark_page_accessed(pfn_to_page(pfn));
 }
 EXPORT_SYMBOL_GPL(kvm_set_pfn_accessed);