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

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

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

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