[dpdk-dev] [PATCH] mempool: adjust name string size in related data types

Olivier Matz olivier.matz at 6wind.com
Wed Jul 20 15:37:26 CEST 2016


Hi,

On 07/20/2016 02:41 PM, Zoltan Kiss wrote:
> 
> 
> On 19/07/16 17:17, Olivier Matz wrote:
>> Hi Zoltan,
>>
>> On 07/19/2016 05:59 PM, Zoltan Kiss wrote:
>>>
>>>
>>> On 19/07/16 16:37, Olivier Matz wrote:
>>>> Hi Zoltan,
>>>>
>>>> On 07/19/2016 04:37 PM, Zoltan Kiss wrote:
>>>>> A recent fix brought up an issue about the size of the 'name' fields:
>>>>>
>>>>> 85cf0079 mem: avoid memzone/mempool/ring name truncation
>>>>>
>>>>> These relations should be observed:
>>>>>
>>>>> RTE_RING_NAMESIZE <= RTE_MEMZONE_NAMESIZE - strlen(RTE_RING_MZ_PREFIX)
>>>>> RTE_MEMPOOL_NAMESIZE <= RTE_RING_NAMESIZE -
>>>>> strlen(RTE_MEMPOOL_MZ_PREFIX)
>>>>>
>>>>> Setting all of them to 32 hides this restriction from the application.
>>>>> This patch increases the memzone string size to accomodate for these
>>>>> prefixes, and the same happens with the ring name string. The ABI
>>>>> needs to
>>>>> be broken to fix this API issue, this way doesn't break applications
>>>>> previously not failing due to the truncating bug now fixed.
>>>>>
>>>>> Signed-off-by: Zoltan Kiss <zoltan.kiss at schaman.hu>
>>>>
>>>> I agree it is a problem for an application because it cannot know what
>>>> is the maximum name length. On the other hand, breaking the ABI for
>>>> this
>>>> looks a bit overkill. Maybe we could reduce RTE_MEMPOOL_NAMESIZE and
>>>> RTE_RING_NAMESIZE instead of increasing RTE_MEMZONE_NAMESIZE? That way,
>>>> we could keep the ABI as is.
>>>
>>> But that would break the ABI too, wouldn't it? Unless you keep the array
>>> the same size (32 bytes) by using RTE_MEMZONE_NAMESIZE.
>>
>> Yes, that was the idea.
>>
>>> And even then, the API breaks anyway. There are applications - I have at
>>> least some - which use all 32 bytes to store the name. Decrease that
>>> would cause headache to change the naming scheme, because it's a 30
>>> character long id, and chopping the last few chars would cause name
>>> collisions and annoying bugs.
>>
>> Before my patch (85cf0079), long names were silently truncated when
>> mempool created its ring and/or memzones. Now, it returns an error.
> 
> With 16.04 an application could operate as expected if the first 26
> character were unique. Your patch revealed the problem that characters
> after these were left out of the name. Now applications fail where this
> never been a bug because their naming scheme guarantees the uniqueness
> on the first 26 chars (or makes it very unlikely)
> Where the first 26 is not unique, it failed earlier too, because at
> memzone creation it checks for duplicate names.

Yes, I understand that there is a behavior change for applications using
names larger than 26 between 16.04 and 16.07. I also understand that
there is no way for an application to know what is the maximum usable
size for a mempool or a ring.


>> I'm not getting why changing the struct to something like below would
>> break the API, since it would already return an error today.
>>
>>    #define RTE_MEMPOOL_NAMESIZE \
> 
> Wait, this would mean applications need to recompile to use the smaller
> value. AFAIK that's an ABI break too, right? At the moment I don't see a
> way to fix this without breaking the ABI

With this modification, if you don't recompile the application, its
behavior will still be the same as today -> it will return ENAMETOOLONG.
If you recompile it, the application will be aware of the maximum
length. To me, it seems to be a acceptable compromise for this release.

The patch you're proposing also changes the ABI of librte_ring and
librte_eal, which cannot be accepted for the release.


> 
>>        (RTE_MEMZONE_NAMESIZE - sizeof(pool_prefix) - sizeof(ring prefix))
>>    struct rte_mempool {
>>        union {
>>              char name[RTE_MEMPOOL_NAMESIZE];
>>              char pad[32];
>>        };
>>        ...
>>    }
>>
>> Anyway, it may not be the proper solution since it supposes that a
>> mempool includes a ring based on a memzone, which is not always true now
>> with mempool handlers.
> 
> Oh, as we dug deeper it gets better!
> Indeed, we don't necessarily have this ring + memzone pair underneath,
> but the user is not aware of that, and I think we should keep it that
> way. It should only care that the string passed shouldn't be bigger than
> a certain amount.

Yes. What I'm just saying here is that it's not a good solution to write
in the #define that "a mempool is based on a ring + a memzone", because
if some someone adds a new mempool handler replacing the ring, and also
creating a memzone prefixed by something larger than "rg_", we will have
to break the ABI again.


> Also, even though we don't necessarily have the ring, we still reserve
> memzone's in rte_mempool_populate_default(). And their name has a 3
> letter prefix, and a "_%d" postfix, where the %d could be as much as
> RTE_MAX_MEMZONE in worst case (2560 by default) So actually:
> 
> RTE_MEMPOOL_NAMESIZE <= RTE_MEMZONE_NAMESIZE -
> strlen(RTE_MEMPOOL_MZ_PREFIX) - strlen("_2560")
> 
> 
> As a side note, there is another bug around here: rte_ring_create()
> doesn't check for name duplications. However the user of the library can
> lookup based on the name with rte_ring_lookup(), and it will return the
> first ring with that name

The name uniqueness is checked by rte_memzone_reserve().


>>>> It would even be better to get rid of this static char[] for the
>>>> structure names and replace it by an allocated const char *. I didn't
>>>> check it's feasible for memzones. What do you think?
>>>
>>> It would work too, but I don't think it would help a lot. We would still
>>> need max sizes for the names. Storing them somewhere else won't help us
>>> in this problem.
>>
>> Why should we have a maximum length for the names?
> 
> What happens if an application loads DPDK and create a mempool with a
> name string 2 million characters long? Maybe nothing we should worry
> about, but in general I think unlimited length function parameters are
> problematic at the very least. The length should be passed at least
> (which also creates a max due to the size of the param). But I think it
> would be just easier to have these maximums set, observing the above
> constrains.

I think have a maximum name length brings more problems than not having
it, especially ABI problems.


Regards,
Olivier


More information about the dev mailing list