> From: dev [mailto:dev-boun...@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 <m...@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: sta...@dpdk.org
> > >
> > > Signed-off-by: Olivier Matz <olivier.m...@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
> >
