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

[Olivier Matz <olivier.matz@6wind.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
>