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From: Morten Brørup <mb@smartsharesystems.com>
To: "Olivier Matz" <olivier.matz@6wind.com>,
	"Ananyev, Konstantin" <konstantin.ananyev@intel.com>
Cc: <dev@dpdk.org>, <stable@dpdk.org>
Subject: Re: [dpdk-stable] [dpdk-dev] [PATCH] mbuf: fix reset on mbuf free
Date: Thu, 5 Nov 2020 09:33:58 +0100
Message-ID: <98CBD80474FA8B44BF855DF32C47DC35C613EB@smartserver.smartshare.dk> (raw)
In-Reply-To: <20201105074626.GL1898@platinum>

> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Olivier Matz
> Sent: Thursday, November 5, 2020 8:46 AM
> 
> On Thu, Nov 05, 2020 at 12:15:49AM +0000, Ananyev, Konstantin wrote:
> >
> > Hi Olivier,
> >
> > > 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
> > >
> > > Freeing this mbuf chain will not restore nb_seg=1 in the second
> > > segment.
> >
> > Hmm, not sure why is that?
> > You are talking about freeing m1, right?
> > rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
> > {
> > 	...
> > 	if (m->next != NULL) {
> >                         m->next = NULL;
> >                         m->nb_segs = 1;
> >                 }
> >
> > m1->next != NULL, so it will enter the if() block,
> > and will reset both next and nb_segs.
> > What I am missing here?
> > Thinking in more generic way, that change:
> >  -		if (m->next != NULL) {
> >  -			m->next = NULL;
> >  -			m->nb_segs = 1;
> >  -		}
> >  +		m->next = NULL;
> >  +		m->nb_segs = 1;
> 
> Ah, sorry. I oversimplified the example and now it does not
> show the issue...
> 
> The full example also adds a split() to break the mbuf chain
> between m1 and m2. The kind of thing that would be done for
> software TCP segmentation.
> 
> After this operation, we have 2 mbuf chain:
>  - m0 with 2 segments, the last one has next=NULL but nb_seg=2
>  - new_m with 1 segment
> 
> Freeing m0 will not restore nb_seg=1 in the second segment.
> 
> > Assumes that it is ok to have an mbuf with
> > nb_seg > 1 and next == NULL.
> > Which seems wrong to me.
> 
> I don't think it is wrong: nb_seg is just ignored when not in the first
> segment, and there is nothing saying it should be set to 1. Typically,
> rte_pktmbuf_chain() does not change it, and I guess it's the same for
> many similar functions in applications.
> 
> Olivier

Acked-by: Morten Brørup <mb@smartsharesystems.com>

And while you are at it, please consider extending the description of the two mbuf fields with their invariants:
1. nb_segs is only valid for the first segment of a multi-segment packet.
2. next is NULL for non-segmented packets.

> 
> >
> >
> > >This 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>
> > > ---
> > >  lib/librte_mbuf/rte_mbuf.c |  6 ++----
> > >  lib/librte_mbuf/rte_mbuf.h | 12 ++++--------
> > >  2 files changed, 6 insertions(+), 12 deletions(-)
> > >
> > > diff --git a/lib/librte_mbuf/rte_mbuf.c
> b/lib/librte_mbuf/rte_mbuf.c
> > > index 8a456e5e64..e632071c23 100644
> > > --- a/lib/librte_mbuf/rte_mbuf.c
> > > +++ b/lib/librte_mbuf/rte_mbuf.c
> > > @@ -129,10 +129,8 @@ rte_pktmbuf_free_pinned_extmem(void *addr,
> void *opaque)
> > >
> > >  	rte_mbuf_ext_refcnt_set(m->shinfo, 1);
> > >  	m->ol_flags = EXT_ATTACHED_MBUF;
> > > -	if (m->next != NULL) {
> > > -		m->next = NULL;
> > > -		m->nb_segs = 1;
> > > -	}
> > > +	m->next = NULL;
> > > +	m->nb_segs = 1;
> > >  	rte_mbuf_raw_free(m);
> > >  }
> > >
> > > diff --git a/lib/librte_mbuf/rte_mbuf.h
> b/lib/librte_mbuf/rte_mbuf.h
> > > index a1414ed7cd..ef5800c8ef 100644
> > > --- a/lib/librte_mbuf/rte_mbuf.h
> > > +++ b/lib/librte_mbuf/rte_mbuf.h
> > > @@ -1329,10 +1329,8 @@ rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
> > >  				return NULL;
> > >  		}
> > >
> > > -		if (m->next != NULL) {
> > > -			m->next = NULL;
> > > -			m->nb_segs = 1;
> > > -		}
> > > +		m->next = NULL;
> > > +		m->nb_segs = 1;
> > >
> > >  		return m;
> > >
> > > @@ -1346,10 +1344,8 @@ rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
> > >  				return NULL;
> > >  		}
> > >
> > > -		if (m->next != NULL) {
> > > -			m->next = NULL;
> > > -			m->nb_segs = 1;
> > > -		}
> > > +		m->next = NULL;
> > > +		m->nb_segs = 1;
> > >  		rte_mbuf_refcnt_set(m, 1);
> > >
> > >  		return m;
> > > --
> > > 2.25.1
> >


  reply	other threads:[~2020-11-05  8:34 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-04 17:00 [dpdk-stable] " Olivier Matz
2020-11-05  0:15 ` Ananyev, Konstantin
2020-11-05  7:46   ` Olivier Matz
2020-11-05  8:33     ` Morten Brørup [this message]
2020-11-05  9:03       ` [dpdk-stable] [dpdk-dev] " Olivier Matz
2020-11-05  9:09     ` Andrew Rybchenko
2020-11-08  7:25 ` Ali Alnubani
2020-12-18 12:52 ` [dpdk-stable] [PATCH v2] " Olivier Matz
2020-12-18 13:18   ` [dpdk-stable] [dpdk-dev] " Morten Brørup
2020-12-18 23:33     ` Ajit Khaparde
2021-01-06 13:33 ` [dpdk-stable] [PATCH v3] " Olivier Matz
2021-01-10  9:28   ` Ali Alnubani
2021-01-11 13:14   ` Ananyev, Konstantin
2021-01-13 13:27 ` [dpdk-stable] [PATCH v4] " Olivier Matz
2021-01-15 13:59   ` David Marchand
2021-01-15 18:39     ` Ali Alnubani
2021-01-18 17:52       ` Ali Alnubani
2021-01-19  8:32         ` Olivier Matz
2021-01-19  8:53           ` Morten Brørup
2021-01-19 12:00             ` Ferruh Yigit
2021-01-19 12:27               ` [dpdk-stable] [dpdk-dev] " Morten Brørup
2021-01-19 14:03                 ` Ferruh Yigit
2021-01-19 14:21                   ` Morten Brørup
2021-01-21  9:15                     ` Ferruh Yigit
2021-01-19 14:04           ` [dpdk-stable] " Slava Ovsiienko
2021-07-24  8:47             ` [dpdk-stable] [dpdk-dev] " Thomas Monjalon
2021-07-30 12:36               ` Olivier Matz
2021-07-30 14:35                 ` Morten Brørup
2021-07-30 14:54                   ` Thomas Monjalon
2021-07-30 15:14                     ` Olivier Matz
2021-07-30 15:23                       ` Morten Brørup
2021-01-21  9:19       ` [dpdk-stable] " Ferruh Yigit
2021-01-21  9:29         ` [dpdk-stable] [dpdk-dev] " Morten Brørup
2021-01-21 16:35           ` [dpdk-stable] [dpdklab] " Lincoln Lavoie
2021-01-23  8:57             ` [dpdk-stable] [dpdk-dev] [dpdklab] " Morten Brørup
2021-01-25 17:00               ` Brandon Lo
2021-01-25 18:42             ` [dpdk-stable] [dpdklab] RE: [dpdk-dev] " Ferruh Yigit
2021-06-15 13:56   ` [dpdk-stable] " Morten Brørup

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