anchor.diff:                   nil
                    block-id
                    address))
  -        (t (panic "Page " frame " for address " address " has non-writeback type "
  +        (t (panic "Frame " frame " for address " address " has non-writeback type "
                     (physical-page-frame-type frame)))))))
   
   (defun snapshot-write-back-pages ()
  
positive.code:          ;; Allow access to the page again.
           (let* ((pte (or (get-pte-for-address address nil)
                           (panic "No PTE for CoW address?")))
                  (frame (ash (pte-physical-address (sys.int::memref-unsigned-byte-64 pte 0)) -12)))
             ;; Update PTE bits. Clear CoW bit, make writable.
             (setf (sys.int::memref-unsigned-byte-64 pte 0)
                   (make-pte frame
  
negative.code:   (without-interrupts
      (let* ((frame *snapshot-pending-writeback-pages*)
             (address (physical-page-virtual-address frame))
             (block-id (physical-page-frame-block-id frame)))
        ;; Remove frame from list.
        (setf *snapshot-pending-writeback-pages* (physical-page-frame-next frame))
  