Re: [dpdk-dev] [PATCH RFC] mbuf/ip_frag: Move mbuf chaining to common code

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

Hi lads,

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Olivier MATZ
> Sent: Monday, September 07, 2015 8:33 AM
> To: Simon Kagstrom; dev@dpdk.org; Zhang, Helin; Gonzalez Monroy, Sergio; Burakov, Anatoly
> Subject: Re: [dpdk-dev] [PATCH RFC] mbuf/ip_frag: Move mbuf chaining to common code
> 
> Hi Simon,
> 
> I think it's a good idea. Please see some minor comments below.
> 
> On 08/31/2015 02:41 PM, Simon Kagstrom wrote:
> > Chaining/segmenting mbufs can be useful in many places, so make it
> > global.
> >
> > Signed-off-by: Simon Kagstrom <simon.kagstrom@netinsight.net>
> > Signed-off-by: Johan Faltstrom <johan.faltstrom@netinsight.net>
> > ---
> > NOTE! Only compile-tested.
> >
> > We were looking for packet segmenting functionality in the MBUF API but
> > didn't find it. This patch moves the implementation, apart from the
> > things which look ip_frag-specific.
> >
> >  lib/librte_ip_frag/ip_frag_common.h      | 23 -----------------------
> >  lib/librte_ip_frag/rte_ipv4_reassembly.c |  7 +++++--
> >  lib/librte_ip_frag/rte_ipv6_reassembly.c |  7 +++++--
> >  lib/librte_mbuf/rte_mbuf.h               | 23 +++++++++++++++++++++++
> >  4 files changed, 33 insertions(+), 27 deletions(-)
> >
> > diff --git a/lib/librte_ip_frag/ip_frag_common.h b/lib/librte_ip_frag/ip_frag_common.h
> > index 6b2acee..cde6ed4 100644
> 
> > [...]
> 
> > diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> > index 8c2db1b..ef47256 100644
> > --- a/lib/librte_mbuf/rte_mbuf.h
> > +++ b/lib/librte_mbuf/rte_mbuf.h
> > @@ -1801,6 +1801,29 @@ static inline int rte_pktmbuf_is_contiguous(const struct rte_mbuf *m)
> >  }
> >
> >  /**
> > + * Chain an mbuf to another, thereby creating a segmented packet.
> > + *
> > + * @param head the head of the mbuf chain (the first packet)
> > + * @param tail the mbuf to put last in the chain
> > + */
> > +static inline void rte_pktmbuf_chain(struct rte_mbuf *head, struct rte_mbuf *tail)
> > +{
> > +	struct rte_mbuf *cur_tail;
> > +
> 
> Here, we could check if the pkt_len of tail mbuf is 0. If
> it's the case, we can just free it and return. It would avoid
> to have an empty segment inside the mbuf chain, which can be
> annoying.
> 
> if (unlikely(rte_pktmbuf_pkt_len(tail) == 0)) {
> 	rte_pktmbuf_free(tail);
> 	return;
> }

Wonder why do we need to do that?
Probably head mbuf is out of space and want to expand it using pktmbuf_chain()?
So in that case seems logical:
1) allocate new mbuf (it's pkt_len will be 0)
b) call pktmbuf_chain()

Konstantin

> 
> > +	/* Chain 'tail' onto the old tail */
> > +	cur_tail = rte_pktmbuf_lastseg(head);
> > +	cur_tail->next = tail;
> > +
> > +	/* accumulate number of segments and total length. */
> > +	head->nb_segs = (uint8_t)(head->nb_segs + tail->nb_segs);
> 
> I'm wondering if we shouldn't check the overflow here. In
> this case we would need to have a return value in case of
> failure.
> 
> > +	head->pkt_len += tail->pkt_len;
> > +
> > +	/* reset pkt_len and nb_segs for chained fragment. */
> > +	tail->pkt_len = tail->data_len;
> > +	tail->nb_segs = 1;
> 
> I don't think it's required to reset this fields in the tail mbuf.
> In any case, they will be reset again.
> 
> > +}
> > +
> > +/**
> >   * Dump an mbuf structure to the console.
> >   *
> >   * Dump all fields for the given packet mbuf and all its associated
> >
> 
> 
> Regards,
> Olivier