Mbuf memory handling

[John Baldwin]

On Wednesday, February 06, 2013 10:20:50 am Jacques Fourie wrote:
> On Wed, Feb 6, 2013 at 3:36 PM, John Baldwin <jhb at freebsd.org> wrote:
> 
> > On Wednesday, February 06, 2013 4:50:39 am Lino Sanfilippo wrote:
> > >
> > > Hi all,
> > >
> > > I want to implement a device driver for a NIC which stores received data
> > into chunks within
> > > a page (>=4k) in host memory. One page shall be used for multiple
> > packets and freed
> > > after all mbufs linked to that page have been processed. So I would like
> > to know what is the recommended way
> > > to handle this in FreeBSD? Any hints are very appreciated.
> >
> > I think you can get what you want by allocating M_JUMBOP mbuf clusters for
> > your receive buffers.  When you want to split out a packet, allocate a new
> > packet header mbuf and use m_split() to let it take over the rest of the 4k
> > buffer and pass the original mbuf up to if_input() as the new packet.  The
> > new mbufs you attach to the cluster via m_split() will all hold a reference
> > on the backing cluster and it won't be freed until all the mbufs are freed.
> >
> > The resulting mbufs will not be writeable (M_WRITABLE() will evaluate to
> 0), right? I don't know if this will be an issue in this particular
> application.

No, they only propagate an existing M_RDONLY flag:

	n->m_flags |= m->m_flags & M_RDONLY;

If the first mbuf is writable the splits remain writable from my reading
of the code.  OTOH, I think in this case read-only buffers passed up to
the stack are probably fine since they are already contiguous so any
pullup should be a NOP, etc.

-- 
John Baldwin