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

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

On Thu, Nov 05, 2020 at 09:33:58AM +0100, Morten Brørup wrote:
> > 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.

Good point, will add it in v2.

> 
> > 
> > >
> > >
> > > >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
> > >
>