m_buflen
wmark = m_buflen(m);
wmark += m_buflen(m);
KASSERT(off >= (wmark - m_buflen(m)));
d += (off - (wmark - m_buflen(m)));
KASSERT(off <= m_buflen(n));
if (__predict_false(m_buflen(n) < (off + len))) {
const size_t mlen = m_buflen(m);
if (m == nbuf->nb_mbuf0 && m_buflen(m) == mlen) {
KASSERT(foff < m_buflen(m) && foff < m_length(m));