KVM: MMU: track the refcount when unmap the page

It will trigger a WARN_ON if the page has been freed but it is still
used in mmu, it can help us to detect mm bug early

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index a5d6ef7..685a485 100644 (file)
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -556,6 +556,14 @@ static int mmu_spte_clear_track_bits(u64 *sptep)
                return 0;
 
        pfn = spte_to_pfn(old_spte);
+
+       /*
+        * KVM does not hold the refcount of the page used by
+        * kvm mmu, before reclaiming the page, we should
+        * unmap it from mmu first.
+        */
+       WARN_ON(!kvm_is_mmio_pfn(pfn) && !page_count(pfn_to_page(pfn)));
+
        if (!shadow_accessed_mask || old_spte & shadow_accessed_mask)
                kvm_set_pfn_accessed(pfn);
        if (!shadow_dirty_mask || (old_spte & shadow_dirty_mask))