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

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> On Thu, Nov 05, 2020 at 11:26:51AM +0300, Andrew Rybchenko wrote:
> > On 11/5/20 10:46 AM, Olivier Matz wrote:
> > > 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.
> > >
> >
> > If so, may be the right solution is to care about nb_segs
> > when next is set to NULL on split? Any place when next is set
> > to NULL. Just to keep the optimization in a more generic place.


> The problem with that approach is that there are already several
> existing split() or trim() implementations in different DPDK-based
> applications. For instance, we have some in 6WINDGate. If we force
> applications to set nb_seg to 1 when resetting next, it has to be
> documented because it is not straightforward. 

I think it is better to go that way.
From my perspective it seems natural to reset nb_seg at same time
we reset next, otherwise inconsistency will occur. 

> I think the approach from
> this patch is safer.

It might be easier from perspective that changes in less places are required,
Though I think that this patch will introduce some performance drop.
As now each mbuf_prefree_seg() will cause update of 2 cache lines unconditionally.
 
> By the way, for 21.11, if we are able to do some optimizations and have
> both pool (index?) and next in the first cache line, we may reconsider
> the fact that next and nb_segs are already set for new allocated mbufs,
> because it is not straightforward either.

My suggestion - let's put future optimization discussion aside for now,
and concentrate on that particular patch. 

> 
> > > 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
> > >>
> >