Re: [dpdk-dev] [PATCH 2/2] mbuf: reset nb_segs of chained packet

[Olivier MATZ <olivier.matz@6wind.com>]

Hi Ilya,

On Thu, Nov 16, 2017 at 11:15:18PM +0400, Ilya Matveychikov wrote:
> 
> > On Nov 16, 2017, at 9:01 PM, Stephen Hemminger <stephen@networkplumber.org> wrote:
> > 
> > On Thu, 16 Nov 2017 18:05:35 +0400
> > Ilya Matveychikov <matvejchikov@gmail.com> wrote:
> > 
> >> Fixes: 139debc42dc0 ("mbuf: move chaining from ip_frag library")
> >> Cc: simon.kagstrom@netinsight.net
> >> 
> >> Signed-off-by: Ilya V. Matveychikov <matvejchikov@gmail.com>
> >> ---
> >> lib/librte_mbuf/rte_mbuf.h | 5 ++++-
> >> 1 file changed, 4 insertions(+), 1 deletion(-)
> >> 
> >> diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> >> index ce8a05ddf..2126dc94b 100644
> >> --- a/lib/librte_mbuf/rte_mbuf.h
> >> +++ b/lib/librte_mbuf/rte_mbuf.h
> >> @@ -1828,9 +1828,12 @@ static inline int rte_pktmbuf_chain(struct rte_mbuf *head, struct rte_mbuf *tail
> >> 	head->nb_segs += tail->nb_segs;
> >> 	head->pkt_len += tail->pkt_len;
> >> 
> >> -	/* pkt_len is only set in the head */
> >> +	/* nb_segs and pkt_len are only set in the head */
> >> +	tail->nb_segs = 1;
> >> 	tail->pkt_len = tail->data_len;
> >> 
> >> +	__rte_mbuf_sanity_check(head, 1);
> >> +
> >> 	return 0;
> >> }
> > 
> > My understanding is that nb_segs and pkt_len are only valid
> > in head. For other packets in the chain nb_segs and pkt_len
> > can be anything.
> 
> So why not to keep them in consistency with multi-seg logic?
> I mean that pkt_len/nb_segs for the head always have meaning but for
> the rest of chain pkt_len is the same as data_len and nb_segs := 1
> 

Stephen is right: like most mbuf fields, nb_segs and pkt_len are only
valid for the first mbuf of the chain.

What would be the advantage of changing this?
In addition, I think it would require to do the same change that in many
places, like drivers that build multi-seg mbufs.

If you are fixing an issue, please describe it in the commit log.

Olivier