typedef struct _MI_PER_SESSION_PROTOS
{
     ULONG u1;
     PMMADDRESS_NODE LeftChild;
     PMMADDRESS_NODE RightChild;
     union
     {
          ULONG SessionId;
          ULONG StartingVpn;
          PSUBSECTION Subsection;
     };
     ULONG EndingVpn;
     PMMPTE SubsectionBase;
     ULONG u2;
} MI_PER_SESSION_PROTOS, *PMI_PER_SESSION_PROTOS;

Windows Vista Kernel Structures