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

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

> -----Original Message-----
> From: Olivier MATZ [mailto:olivier.matz@6wind.com]
> Sent: Wednesday, February 18, 2015 10:34 AM
> To: Richardson, Bruce
> Cc: Ananyev, Konstantin; dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH 2/2] Remove RTE_MBUF_REFCNT references
> 
> Hi,
> 
> On 02/18/2015 11:22 AM, Bruce Richardson wrote:
> > On Wed, Feb 18, 2015 at 11:14:42AM +0100, Olivier MATZ wrote:
> >> 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@dpdk.org] On Behalf Of Bruce Richardson
> >>>>> Sent: Wednesday, February 18, 2015 9:36 AM
> >>>>> To: Olivier MATZ
> >>>>> Cc: dev@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@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, my bad.
> > How was this managed before, since refcnt field seems to be necessary in order
> > to effectively manage indirect mbufs? Is this just the case that this is something
> > that never worked and that needs to be solved, or is it something that was
> > working that this patch will now break?
> 
> This is something that never worked before: refcounts are not compatible
> with reserving private data in mbufs. This patch does not change the
> issue, it is still there.
> 
> Before the patch, an application that wanted to reserve a private
> data could disable refcounts at compile-time.
> After the patch, the solution is just to avoid using refcounts.

I'd say avoid using mbuf_attach/detach.
refcnt itself has nothing to do with that.
I finally understood what you  are talking about ...
About private data - I suppose it is a matter of another patch.
I still think it would be better to reserve private data space before mbuf, not after
(at mbuf pool initialisation time). 
Then *BADDR* macros could be unaffected.
Konstantin

> 
> Regards,
> Olivier
>