Re: [RFC v2 01/17] mempool: fix phys contig check if populate default skipped

[Andrew Rybchenko <arybchenko@solarflare.com>]

On 02/01/2018 12:30 PM, santosh wrote:
> On Thursday 01 February 2018 02:48 PM, Andrew Rybchenko wrote:
>> On 02/01/2018 12:09 PM, santosh wrote:
>>> On Thursday 01 February 2018 12:24 PM, Andrew Rybchenko wrote:
>>>> On 02/01/2018 08:05 AM, santosh wrote:
>>>>> On Wednesday 31 January 2018 10:15 PM, Olivier Matz wrote:
>>>>>> On Tue, Jan 23, 2018 at 01:15:56PM +0000, Andrew Rybchenko wrote:
>>>>>>> There is not specified dependency between rte_mempool_populate_default()
>>>>>>> and rte_mempool_populate_iova(). So, the second should not rely on the
>>>>>>> fact that the first adds capability flags to the mempool flags.
>>>>>>>
>>>>>>> Fixes: 65cf769f5e6a ("mempool: detect physical contiguous objects")
>>>>>>> Cc: stable@dpdk.org
>>>>>>>
>>>>>>> Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
>>>>>> Looks good to me. I agree it's strange that the mp->flags are
>>>>>> updated with capabilities only in rte_mempool_populate_default().
>>>>>> I see that this behavior is removed later in the patchset since the
>>>>>> get_capa() is removed!
>>>>>>
>>>>>> However maybe this single patch could go in 18.02.
>>>>>> +Santosh +Jerin since it's mostly about Octeon.
>>>>> rte_mempool_xmem_size should return correct size if MEMPOOL_F_CAPA_BLK_ALIGNED_OBJECTS flag
>>>>> is set in 'mp->flags'. Thats why _ops_get_capabilities() called in _populate_default() but not
>>>>> at _populate_iova().
>>>>> I think, this 'alone' patch may break octeontx mempool.
>>>> The patch does not touch rte_mempool_populate_default().
>>>> _ops_get_capabilities() is still called there before
>>>> rte_mempool_xmem_size(). The theoretical problem which
>>>> the patch tries to fix is the case when
>>>> rte_mempool_populate_default() is not called at all. I.e. application
>>>> calls _ops_get_capabilities() to get flags, then, together with
>>>> mp->flags, calls rte_mempool_xmem_size() directly, allocates
>>>> calculated amount of memory and calls _populate_iova().
>>>>
>>> In that case, Application does like below:
>>>
>>>      /* Get mempool capabilities */
>>>      mp_flags = 0;
>>>      ret = rte_mempool_ops_get_capabilities(mp, &mp_flags);
>>>      if ((ret < 0) && (ret != -ENOTSUP))
>>>          return ret;
>>>
>>>      /* update mempool capabilities */
>>>      mp->flags |= mp_flags;
>> Above line is not mandatory. "mp->flags | mp_flags" could be simply
>> passed to  rte_mempool_xmem_size() below.
>>
> That depends and again upto application requirement, if app further down
> wants to refer mp->flags for _align/_contig then better update to mp->flags.
>
> But that wasn't the point of discussion, I'm trying to understand that
> w/o this patch, whats could be the application level problem?

The problem that it is fragile. If application does not use
rte_mempool_populate_default() it has to care about addition
of mempool capability flags into mempool flags. If it is not done,
rte_mempool_populate_iova/virt/iova_tab() functions will work
incorrectly since F_CAPA_PHYS_CONTIG and
F_CAPA_BLK_ALIGNED_OBJECTS are missing.

The idea of the patch is to make it a bit more robust. I have no
idea how it can break something. If capability flags are already
there - no problem. If no, just make sure that we locally have them.