> But some things won't like a zero-lenght mbuf at the head of the chain
> (beside being hihgly inefficient for small packets).

oops, it explains why i couldn't reproduce the panic in vlan.
(m_copyback in my local tree has some optimiztions. :-)

> > besides, i don't think it's a good idea to assume that
> > the result of m_copyback is a single mbuf.
> 
> Of course :) So we should set m_len to min(MHLEN, rx->status) before calling
> m_copyback(). Also, it seems m_copyback() will never allocate a mbuf cluster,
> so it won't be efficient for large packets. I wonder if using m_copyback()
> here was a good idea at all ...

i really dislike m_copyback's "extend" behaviour. :-)

YAMAMOTO Takashi