Just a comment update, so I don't forget why I'm checking for
    duplicate mark region requests.
    
    <       /* ATTENTION -- this check should not be necessary.   **
    <       ** Why are regions being marked again before a clear? */
    ---
    >       /*
    >        * In the mirroring code, it is possible for a write
    >        * to complete and call rh_dec - putting the region on
    >        * the clear_region list.  However, before the actual
    >        * clear request is issued to the log (rh_update_states)
    >        * another mark happens.  So, we check for and remove
    >        * duplicates.
    >        */