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

[Olivier MATZ <olivier.matz@6wind.com>]

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. 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.

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?


>
> Konstantin