Re: [dpdk-dev] [dpdk-stable] [PATCH v4] mbuf: fix reset on mbuf free

["Morten Brørup" <mb@smartsharesystems.com>]

> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Olivier Matz
> Sent: Friday, 30 July 2021 17.15
> 
> Hi,
> 
> On Fri, Jul 30, 2021 at 04:54:05PM +0200, Thomas Monjalon wrote:
> > 30/07/2021 16:35, Morten Brørup:
> > > > From: Olivier Matz [mailto:olivier.matz@6wind.com]
> > > > Sent: Friday, 30 July 2021 14.37
> > > >
> > > > Hi Thomas,
> > > >
> > > > On Sat, Jul 24, 2021 at 10:47:34AM +0200, Thomas Monjalon wrote:
> > > > > What's the follow-up for this patch?
> > > >
> > > > Unfortunatly, I still don't have the time to work on this topic
> yet.
> > > >
> > > > In my initial tests, in our lab, I didn't notice any performance
> > > > regression, but Ali has seen an impact (0.5M PPS, but I don't
> know how
> > > > much in percent).
> > > >
> > > >
> > > > > 19/01/2021 15:04, Slava Ovsiienko:
> > > > > > Hi, All
> > > > > >
> > > > > > Could we postpose this patch at least to rc2? We would like
> to
> > > > conduct more investigations?
> > > > > >
> > > > > > With best regards, Slava
> > > > > >
> > > > > > From: Olivier Matz <olivier.matz@6wind.com>
> > > > > > > On Mon, Jan 18, 2021 at 05:52:32PM +0000, Ali Alnubani
> wrote:
> > > > > > > > Hi,
> > > > > > > > (Sorry had to resend this to some recipients due to mail
> server
> > > > problems).
> > > > > > > >
> > > > > > > > Just confirming that I can still reproduce the regression
> with
> > > > single core and
> > > > > > > 64B frames on other servers.
> > > > > > >
> > > > > > > Many thanks for the feedback. Can you please detail what is
> the
> > > > amount of
> > > > > > > performance loss in percent, and confirm the test case? (I
> > > > suppose it is
> > > > > > > testpmd io forward).
> > > > > > >
> > > > > > > Unfortunatly, I won't be able to spend a lot of time on
> this soon
> > > > (sorry for
> > > > > > > that). So I see at least these 2 options:
> > > > > > >
> > > > > > > - postpone the patch again, until I can find more time to
> analyze
> > > > > > >   and optimize
> > > > > > > - apply the patch if the performance loss is acceptable
> compared
> > > > to
> > > > > > >   the added value of fixing a bug
> > > > > > >
> > > > > [...]
> > > >
> > > > Statu quo...
> > > >
> > > > Olivier
> > > >
> > >
> > > The decision should be simple:
> > >
> > > Does the DPDK project support segmented packets?
> > > If yes, then apply the patch to fix the bug!
> > >
> > > If anyone seriously cares about the regression it introduces,
> optimization patches are welcome later. We shouldn't wait for it.
> >
> > You're right, but the regression is flagged to a 4-years old patch,
> > that's why I don't consider it as urgent.
> >
> > > If the patch is not applied, the documentation must be updated to
> mention that we are releasing DPDK with a known bug: that segmented
> packets are handled incorrectly in the scenario described in this
> patch.
> >
> > Yes, would be good to document the known issue,
> > no matter how old it is.
> 
> The problem description could be something like this:
> 
>   It is expected that free mbufs have their field m->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 m->nb_seg field 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.
> 

And it needs to be put somewhere very prominent if we expect the users to read it.

Would adding an RTE_VERIFY() - instead of fixing the bug - cause a regression? If not, then any affected user will know what went wrong and where. This would still be an improvement, if the bugfix patch cannot be applied.

> 
> > > Generally, there could be some performance to gain by not
> supporting segmented packets at all, as a compile time option. But that
> is a different discussion.
> > >
> > >
> > > -Morten
> >
> >
> >