Re: [dpdk-dev] [PATCH v5] mbuf: fix reset on mbuf free

[Ali Alnubani <alialnu@oss.nvidia.com>]

> -----Original Message-----
> From: Olivier Matz <olivier.matz@6wind.com>
> Sent: Thursday, September 30, 2021 12:37 AM
> To: dev@dpdk.org
> Cc: ajit.khaparde@broadcom.com; ajitkhaparde@gmail.com; Ali Alnubani
> <alialnu@nvidia.com>; andrew.rybchenko@oktetlabs.ru;
> konstantin.ananyev@intel.com; mb@smartsharesystems.com;
> stable@dpdk.org; Slava Ovsiienko <viacheslavo@nvidia.com>
> Subject: [PATCH v5] mbuf: fix reset on mbuf free
> 
> m->nb_seg must be reset on mbuf free whatever the value of m->next,
> because it can happen that m->nb_seg is != 1. For instance in this
> case:
> 
>   m1 = rte_pktmbuf_alloc(mp);
>   rte_pktmbuf_append(m1, 500);
>   m2 = rte_pktmbuf_alloc(mp);
>   rte_pktmbuf_append(m2, 500);
>   rte_pktmbuf_chain(m1, m2);
>   m0 = rte_pktmbuf_alloc(mp);
>   rte_pktmbuf_append(m0, 500);
>   rte_pktmbuf_chain(m0, m1);
> 
> As rte_pktmbuf_chain() does not reset nb_seg in the initial m1 segment (this
> is not required), after this code the mbuf chain have 3 segments:
>   - m0: next=m1, nb_seg=3
>   - m1: next=m2, nb_seg=2
>   - m2: next=NULL, nb_seg=1
> 
> Then split this chain between m1 and m2, it would result in 2 packets:
>   - first packet
>     - m0: next=m1, nb_seg=2
>     - m1: next=NULL, nb_seg=2
>   - second packet
>     - m2: next=NULL, nb_seg=1
> 
> Freeing the first packet will not restore nb_seg=1 in the second segment.
> This is an issue because it is expected that mbufs stored in pool have their
> nb_seg field set to 1.
> 
> Fixes: 8f094a9ac5d7 ("mbuf: set mbuf fields while in pool")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
> Acked-by: Morten Brørup <mb@smartsharesystems.com>
> Acked-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
> Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
> ---

Tested-by: Ali Alnubani <alialnu@nvidia.com>