[dpdk-dev] [PATCH] doc: add known issue with mbuf segment

[Morten Brørup]

> From: Thomas Monjalon [mailto:thomas at monjalon.net]
> Sent: Friday, 6 August 2021 16.21
> 
> 05/08/2021 08:08, Morten Brørup:
> > > From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Thomas
> Monjalon
> > > Sent: Wednesday, 4 August 2021 15.30
> > >
> > > A bug with segmented packets has been discovered but the agreement
> > > to apply the fix is not concluded at the time of DPDK 21.08
> release.
> > > This bug seems to be in DPDK for many years and should be fixed in
> > > 21.11.
> > >
> > > Suggested-by: Olivier Matz <olivier.matz at 6wind.com>
> > > Signed-off-by: Thomas Monjalon <thomas at monjalon.net>
> > > ---
> > >  doc/guides/rel_notes/release_21_08.rst | 12 ++++++++++++
> > >  1 file changed, 12 insertions(+)
> > >
> > > diff --git a/doc/guides/rel_notes/release_21_08.rst
> > > b/doc/guides/rel_notes/release_21_08.rst
> > > index d7559ec6bf..14e32585b8 100644
> > > --- a/doc/guides/rel_notes/release_21_08.rst
> > > +++ b/doc/guides/rel_notes/release_21_08.rst
> > > @@ -251,6 +251,18 @@ Known Issues
> > >     Also, make sure to start the actual text at the margin.
> > >     =======================================================
> > >
> > > +* **Last mbuf segment not implicitly reset.**
> > > +
> > > +  It is expected that free mbufs have their field ``nb_seg`` set
> to 1,
> > > +  so that when it is allocated, the user does not need to set its
> > > value.
> > > +  The mbuf free functions are responsible of resetting this field
> to 1
> > > +  before returning the mbuf to the pool.
> > > +
> > > +  When a multi-segment mbuf is freed, the field ``nb_seg`` is not
> > > reset
> > > +  to 1 for the last segment of the chain. On next allocation of
> this
> > > segment,
> > > +  if the field is not explicitly reset by the user,
> > > +  an invalid mbuf can be created, and can cause an undefined
> behavior.
> > > +
> > >
> >
> > I have a couple of suggestions:
> >
> > 1. Add that this bug was present in previous DPDK releases for many
> years. If possible, mention which release introduced the bug.
> >
> > We don't want users to choose to stay on older release, thinking that
> it does not have this bug.
> >
> > Also, if a user experiences unexplainable problems with segmented
> packets an older DPDK release, he might get lucky and find these
> release notes, which could reveal the root cause of the problems he is
> experiencing.
> >
> > 2. Add a link to the patch that fixes the bug. It will let users
> affected by the bug easily fix it (possibly at the expense of degraded
> performance).
> >
> > With those two additions,
> > Acked-by: Morten Brørup <mb at smartsharesystems.com>
> 
> Applied with the suggested additions:
> 
> +  This issue has a root cause in DPDK 17.05, meaning it is 4 years
> old.
> +  A fix is available and discussed but not merged in DPDK 21.08:
> +  https://patches.dpdk.org/patch/86458/
> 

Perfect! Thanks.

-Morten