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References: <20201104170007.8026-1-olivier.matz@6wind.com>
<20201218125238.13074-1-olivier.matz@6wind.com>
<98CBD80474FA8B44BF855DF32C47DC35C614E2@smartserver.smartshare.dk>
In-Reply-To: <98CBD80474FA8B44BF855DF32C47DC35C614E2@smartserver.smartshare.dk>
From: Ajit Khaparde <ajitkhaparde@gmail.com>
Date: Fri, 18 Dec 2020 15:33:41 -0800
Message-ID: <CAD6mc5YTFp64f5+05J4PcZg2KpMjg6LWLzVd+8vpShJssgOrcg@mail.gmail.com>
To: =?UTF-8?Q?Morten_Br=C3=B8rup?= <mb@smartsharesystems.com>
Cc: Olivier Matz <olivier.matz@6wind.com>, dev@dpdk.org,
andrew.rybchenko@oktetlabs.ru,
konstantin.ananyev@intel.com, stable@dpdk.org
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Subject: Re: [dpdk-dev] [PATCH v2] mbuf: fix reset on mbuf free
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On Fri, Dec 18, 2020 at 5:18 AM Morten Br=C3=B8rup <mb@smartsharesystems.co=
m> wrote:
>
> > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Olivier Matz
> >
> > m->nb_seg must be reset on mbuf free whatever the value of m->next,
> > because it can happen that m->nb_seg is !=3D 1. For instance in this
> > case:
> >
> > m1 =3D rte_pktmbuf_alloc(mp);
> > rte_pktmbuf_append(m1, 500);
> > m2 =3D rte_pktmbuf_alloc(mp);
> > rte_pktmbuf_append(m2, 500);
> > rte_pktmbuf_chain(m1, m2);
> > m0 =3D 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=3Dm1, nb_seg=3D3
> > - m1: next=3Dm2, nb_seg=3D2
> > - m2: next=3DNULL, nb_seg=3D1
> >
> > Then split this chain between m1 and m2, it would result in 2 packets:
> > - first packet
> > - m0: next=3Dm1, nb_seg=3D3
> > - m1: next=3Dm2, nb_seg=3D2
>
> I think you mean:
>
> - m0: next=3Dm1, nb_seg=3D2 //< nb_seg corrected
> - m1: next=3DNULL, nb_seg=3D2 //< next corrected
>
> > - second packet
> > - m2: next=3DNULL, nb_seg=3D1
> >
> > Freeing the first packet will not restore nb_seg=3D1 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>
>
> The code looks good, so:
> Acked-by: Morten Br=C3=B8rup <mb@smartsharesystems.com>
Acked-by: Ajit Khaparde <ajit.khaparde@broadcom.com>