[dpdk-dev] [PATCH v4 1/2] mbuf: support attaching external buffer to mbuf

[Andrew Rybchenko]

On 04/25/2018 02:34 AM, Yongseok Koh wrote:
> On Tue, Apr 24, 2018 at 09:15:38PM +0200, Olivier Matz wrote:
>> On Tue, Apr 24, 2018 at 09:21:00PM +0300, Andrew Rybchenko wrote:
>>> On 04/24/2018 07:02 PM, Olivier Matz wrote:
>>>>>> +	m->ol_flags |= EXT_ATTACHED_MBUF;
>>>>>> +	m->shinfo = shinfo;
>>>>>> +
>>>>>> +	rte_mbuf_ext_refcnt_set(shinfo, 1);
>>>>> Why is assignment used here? Cannot we attach extbuf already attached to
>>>>> other mbuf?
>>>> In rte_pktmbuf_attach(), this is true. That's not illogical to
>>>> keep the same approach here. Maybe an assert could be added?
> Like I described in the doc, intention is to attach external buffer by
> _attach_extbuf() for the first time and _attach() is just for additional mbuf
> attachment. Will add an assert.

Not sure that I understand. How should the second chunk with shared shinfo
of the huge buffer be attached to a new mbuf?

>>> Other option is to have shinfo per small buf plus reference counter
>>> per huge buf (which is decremented when small buf reference counter
>>> becomes zero and free callback is executed). I guess it is assumed above.
>>> My fear is that it is too much reference counters:
>>>   1. mbuf reference counter
>>>   2. small buf reference counter
>>>   3. huge buf reference counter
>>> May be it is possible use (1) for (2) as well?
>> I would prefer to have only 2 reference counters, one in the mbuf
>> and one in the shinfo.
> Good discussion. It should be a design decision by user.
>
> In my use-case, it would be a good idea to make all the mbufs in a same chunk
> point to the same shared info in the head of the chunk and reset the refcnt of
> shinfo to the total number of slices in the chunk.
>
> +--+----+----+--------------+----+--------------+---+- - -
> |global |head|mbuf1 data    |head|mbuf2 data    |   |
> | shinfo|room|              |room|              |   |
> +--+----+----+--------------+----+--------------+---+- - -

I don't understand how it can be achieved using proposed API.

Andrew.