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From: Olivier Matz <olivier.matz@6wind.com>
To: "Ananyev, Konstantin" <konstantin.ananyev@intel.com>
Cc: "dev@dpdk.org" <dev@dpdk.org>, "stable@dpdk.org" <stable@dpdk.org>
Subject: Re: [dpdk-stable] [PATCH] mbuf: fix reset on mbuf free
Date: Thu, 5 Nov 2020 08:46:26 +0100
Message-ID: <20201105074626.GL1898@platinum> (raw)
In-Reply-To: <BYAPR11MB33012A3B5DE52F038B3C70A89AEE0@BYAPR11MB3301.namprd11.prod.outlook.com>

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

> 
> 
> >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  7:46 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-04 17:00 Olivier Matz
2020-11-05  0:15 ` Ananyev, Konstantin
2020-11-05  7:46   ` Olivier Matz [this message]
2020-11-05  8:33     ` [dpdk-stable] [dpdk-dev] " Morten Brørup
2020-11-05  9:03       ` 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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