Mbuf memory handling

[Jacques Fourie]

On Wed, Feb 6, 2013 at 6:37 PM, John Baldwin <jhb at freebsd.org> wrote:

> 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.
>
> I agree that read-only buffers may be ok in this case but would like to
point out that the M_WRITABLE() macro will evaluate to 0 if the refcount on
the cluster is >1, even if the M_RDONLY flag is not set. So the various
parts of the networking code that uses M_WRITABLE() to decide if the mbuf
is writeable will treat the mbuf as read-only.

> --
> John Baldwin
>