[dpdk-dev] [PATCH 2/2] Remove RTE_MBUF_REFCNT references

[Ananyev, Konstantin]

> -----Original Message-----
> From: Olivier MATZ [mailto:olivier.matz at 6wind.com]
> Sent: Wednesday, February 18, 2015 10:15 AM
> To: Richardson, Bruce; Ananyev, Konstantin
> Cc: dev at dpdk.org
> Subject: Re: [dpdk-dev] [PATCH 2/2] Remove RTE_MBUF_REFCNT references
> 
> On 02/18/2015 11:00 AM, Bruce Richardson wrote:
> > On Wed, Feb 18, 2015 at 09:48:58AM +0000, Ananyev, Konstantin wrote:
> >> Hi lads,
> >>
> >>> -----Original Message-----
> >>> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Bruce Richardson
> >>> Sent: Wednesday, February 18, 2015 9:36 AM
> >>> To: Olivier MATZ
> >>> Cc: dev at dpdk.org
> >>> Subject: Re: [dpdk-dev] [PATCH 2/2] Remove RTE_MBUF_REFCNT references
> >>>
> >>> On Wed, Feb 18, 2015 at 10:16:56AM +0100, Olivier MATZ wrote:
> >>>> Hi Sergio,
> >>>>
> >>>> On 02/16/2015 05:08 PM, Sergio Gonzalez Monroy wrote:
> >>>>> This patch removes all references to RTE_MBUF_REFCNT, setting the refcnt
> >>>>> field in the mbuf struct permanently.
> >>>>>
> >>>>> Signed-off-by: Sergio Gonzalez Monroy <sergio.gonzalez.monroy at intel.com>
> >>>>
> >>>> I think removing the refcount compile option goes in the right
> >>>> direction. However, activating the refcount will break the applications
> >>>> that reserve a private zone in mbufs. This is due to the macros
> >>>> RTE_MBUF_TO_BADDR() and RTE_MBUF_FROM_BADDR() that suppose that
> >>>> the beginning of the mbuf is 128 bytes (sizeof mbuf) before the
> >>>> data buffer.
> >>>>
> >>>
> >>> While I understand how the macros make certain assumptions, how does activating
> >>> the refcnt specifically lead to the problems you describe? Could you explain
> >>> that part in a bit more detail?
> >>>
> >>> Thanks,
> >>> /Bruce
> >>>
> >>
> >> Olivier, I also don't understand your concern here.
> >> As I can see, that patch has nothing to do with RTE_MBUF_FROM_BADDR/ RTE_MBUF_FROM_BADDR macros.
> >> They are still there, for example rte_pktmbuf_detach() still uses it to restore mbuf's buf_addr.
> >> The only principal change here, is that we don't rely more  on RTE_MBUF_FROM_BADDR to determine,
> >> Is that indirect mbuf or not.
> >> Instead we use a special falg for that purpose:
> >>
> >> -#define RTE_MBUF_INDIRECT(mb)   (RTE_MBUF_FROM_BADDR((mb)->buf_addr) != (mb))
> >> +#define RTE_MBUF_INDIRECT(mb)   (mb->ol_flags & IND_ATTACHED_MBUF)
> >>
> >> BTW, Sergio as I said before, I think it should be:
> >> #define RTE_MBUF_INDIRECT(mb)   ((mb)->ol_flags & IND_ATTACHED_MBUF)
> >>
> >> Konstantin
> >>
> >>
> >>>> For RTE_MBUF_TO_BADDR(), it's relatively easy to replace it. The
> >>>> mbuf pool could store the size of the private size like it's done
> >>>> for mbp_priv->mbuf_data_room_size. Using rte_mempool_from_obj(m)
> >>>> or m->pool, we can retrieve the mbuf pool and this value, then
> >>>> compute the buffer address.
> >
> > Agreed, that makes sense.
> >
> >>>>
> >>>> For RTE_MBUF_FROM_BADDR(), it's more complex. We could ensure that
> >>>> a backpointer to the mbuf is always located before the data buffer,
> >>>> but it looks difficult to do.
> >
> > On the other hand, with the proposed refcnt change Sergio proposes, we no
> > longer use this macro in any of the built-in mbuf handling for freeing mbufs.
> > Does this need to be solved at anything other than the application level?
> 
> It's still used in __rte_pktmbuf_prefree_seg() to retrieve the
> parent mbuf (direct) from the indirect mbuf beeing freed.
> 

Yes, if the INDIRECT flag is set.
Though I still don't understand, what is the problem with these 2 macros with that patch?
Why we need to replace it with something?
What exactly you think will be broken?

Konstantin

> 
> 
> >>>>
> >>>> Another idea would be to add a field in indirect mbufs that stores
> >>>> the pointer to the "parent" mbuf.
> >>>>
> >>>> Regards,
> >>>> Olivier
> >>>>