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

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

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.



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