m_ext_ref
#define MEXT_ISEMBEDDED(m) ((m)->m_ext_ref == (m))
(n)->m_ext_ref = (o)->m_ext_ref; \
KASSERT(MEXT_ISEMBEDDED(m->m_ext_ref));
KASSERT((m->m_ext_ref->m_flags & M_EXT) != 0);
(m->m_ext_ref->m_flags & M_EXT_CLUSTER));
m->m_ext_ref = m;
m_ext_free(m->m_ext_ref);
m->m_ext_ref = m;
m->m_ext_ref = m; /* default */
#define m_ext m_ext_ref->m_ext_storage
(m)->m_ext_ref = (m); \