Re: [dpdk-dev] [dpdk-stable] [PATCH v4] mbuf: fix reset on mbuf free

[David Marchand <david.marchand@redhat.com>]

On Wed, Jan 13, 2021 at 2:28 PM Olivier Matz <olivier.matz@6wind.com> wrote:
>
> 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>
> ---
>
> v4
> * add a unit test (suggested by David)

Olivier,

Thanks, for adding it.


Ali,

You reported some performance regression, did you confirm it?
If I get no reply by monday, I'll proceed with this patch.


Thanks.
-- 
David Marchand