Re: [dpdk-dev] [PATCH v3 1/5] mbuf: fix clone support when application uses private mbuf data

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

Hi Olivier,

> -----Original Message-----
> From: Olivier MATZ [mailto:olivier.matz@6wind.com]
> Sent: Wednesday, April 08, 2015 10:44 AM
> To: Ananyev, Konstantin; dev@dpdk.org
> Cc: zoltan.kiss@linaro.org; Richardson, Bruce
> Subject: Re: [PATCH v3 1/5] mbuf: fix clone support when application uses private mbuf data
> 
> Hi Konstantin,
> 
> On 04/07/2015 07:17 PM, Ananyev, Konstantin wrote:
> >> Just to be sure we're on the same line:
> >>
> >> - before the patch series
> >>
> >>     - private area was working before that patch series if clones were not
> >>       used. To use a private are, the user had to provide another
> >>       function derived from pktmbuf_init() to change m->buf_addr and
> >>       m->buf_len.
> >>     - using both private area + clones was broken
> >>
> >> - after the patch series
> >>
> >>     - private area is working with or without clone. But yo use it,
> >>       the user still has to provide another function to change
> >>       m->buf_addr, m->buf_len *and m->priv_size*.
> >>
> >> The series just fixes the fact that "clones + priv" was not working.
> >> It does not address the problem that providing a new pktmbuf_init()
> >> function is required to use privata area. To fix this, I think it
> >> could require a API evolution that should be part of another series.
> >
> > I don't think we need new pktmbuf_init().
> > We just need to update it, so both pktmbuf_init() and detach() setup
> > buf_addr, buf_len (and priv_size) to exactly the same values.
> > If they don't do that, it means that you can't use attach/detach with
> > mempools created with pktmbuf_init() any more.
> >
> > BTW, another thing that I just realised:
> > examples/ipv4_multicast and examples/ip_fragmentation/ -
> > both create a pool of mbufs with elem_size < 2K and don't populate mempool's private area -
> > so mbp_priv->mbuf_data_room_size == 0, for them.
> >
> > So that code in detach():
> >
> >   +	mbp_priv = rte_mempool_get_priv(mp);
> >   +	m->priv_size = mp->elt_size - RTE_PKTMBUF_HEADROOM -
> >   +		mbp_priv->mbuf_data_room_size -
> >   +		sizeof(struct rte_mbuf);
> >
> >
> > Would break both these samples.
> > I suppose we need to handle situation when mp->elt_size < RTE_PKTMBUF_HEADROOM + sizeof(struct rte_mbuf),
> > (and probably also when mbuf_data_room_size == 0) correctly.
> 
> Indeed. I think a mbuf pool (even with buf_len == 0 like in
> ip_fragmentation example) should have a pool with a private area and
> should call rte_pktmbuf_pool_init() to populate it. So
> rte_pktmbuf_pool_init() has to be fixed first to use elt_size
> and support the buf_len < RTE_PKTMBUF_HEADROOM, then we can
> update frag/multicast examples.
> 
> Unfortunately, we don't know the size of the mbuf private area
> in rte_pktmbuf_pool_init() if the opaque arg (data_room_size)
> is 0, which is the default. I think it should be replaced by a structure
> containing data_room_size and mbuf_priv_size, but it would break
> applications that are setting data_room_size.

Yes, same thoughts here.

> I don't see any good
> solution to do that while keeping a backward compatibility for
> rte_pktmbuf_pool_init(), but as the current API is not ideal,
> I think it's worth changing it and add something in the release
> note.

If no one else has a better alternative than that, then I suppose it is good enough. 

> 
> We may also want to introduce a new helper as discussed previously:
> 
> struct rte_mempool *
> rte_pktmbuf_pool_create(const char *name, unsigned n, unsigned elt_size,
> 	unsigned cache_size, size_t mbuf_priv_size,
> 	rte_mempool_obj_ctor_t *obj_init, void *obj_init_arg,
> 	int socket_id, unsigned flags)
> 
> Any comment?

Looks good to me.
Should we also introduce rte_pktmbuf_pool_xmem_create()? 
Konstantin

> 
> 
> >
> > Konstantin