Date: Wed, 6 Feb 2013 11:37:35 -0500 From: John Baldwin <jhb@freebsd.org> To: Jacques Fourie <jacques.fourie@gmail.com> Cc: Hackers freeBSD <freebsd-hackers@freebsd.org>, Axel Fischer <afischer@marvell.com>, Lino Sanfilippo <lsanfil@marvell.com>, Markus Althoff <malthoff@marvell.com> Subject: Re: Mbuf memory handling Message-ID: <201302061137.35651.jhb@freebsd.org> In-Reply-To: <CALX0vxADMfWe1-_mOYnx0C-9cRxf-ETv6wOPn=%2B34NATNUbbKA@mail.gmail.com> References: <175CCF5F49938B4D99B2E3EF7F558EBE1C73F401F3@SC-VEXCH4.marvell.com> <201302060836.55404.jhb@freebsd.org> <CALX0vxADMfWe1-_mOYnx0C-9cRxf-ETv6wOPn=%2B34NATNUbbKA@mail.gmail.com>
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On Wednesday, February 06, 2013 10:20:50 am Jacques Fourie wrote: > On Wed, Feb 6, 2013 at 3:36 PM, John Baldwin <jhb@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
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