KVM: guest_memfd: limit overzealous WARN

Because kvm_gmem_get_pfn() is called from the page fault path without
any of the slots_lock, filemap lock or mmu_lock taken, it is
possible for it to race with kvm_gmem_unbind().  This is not a
problem, as any PTE that is installed temporarily will be zapped
before the guest has the occasion to run.

However, it is not possible to have a complete unbind+bind
racing with the page fault, because deleting the memslot
will call synchronize_srcu_expedited() and wait for the
page fault to be resolved.  Thus, we can still warn if
the file is there and is not the one we expect.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index c27828b0d42d4aa083bfdbe8e66d009518793848..fd32288d0fbc492f83a326d6c39b0cb41169f7c5 100644 (file)
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -499,7 +499,8 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
 
        gmem = file->private_data;
 
-       if (WARN_ON_ONCE(xa_load(&gmem->bindings, index) != slot)) {
+       if (xa_load(&gmem->bindings, index) != slot) {
+               WARN_ON_ONCE(xa_load(&gmem->bindings, index));
                r = -EIO;
                goto out_fput;
        }