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

[Bruce Richardson <bruce.richardson@intel.com>]

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?

/Bruce

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