In message <200004260201.MAA00580@avalon.reed.wattle.id.au>Darren Reed writes
>Is there any reason to allocate a new mbuf by calling something like
>m_pullup() when we can just shift the data in the current mbuf ?
>Is there a case where the patch below won't work ?

Do we still do structure-sharing on external mbufs/mbuf clusters?

If so, loopback or if_faith or some others could deliver packets to
ip_input() which are still shared. In which case you better be darn
careful not to clobber the contents.

I once implemented a non-copying sendfile()/splice(). There were a
surprising number of corner cases cases where mbuf pages (whcih were
really UBC-ish pages wrapped in external mbufs) were being modified by
the networking stack.  Modify data to do a pullup or append on an mbuf
cluster, and ka-pow!, the same memory writes start showing up in your
file buffer cache.  *Not* fun.