From: "Richardson, Bruce" <bruce.richardson@intel.com>
To: Olivier Matz <olivier.matz@6wind.com>,
Zoltan Kiss <zoltan.kiss@linaro.org>,
Zoltan Kiss <zoltan.kiss@schaman.hu>,
"dev@dpdk.org" <dev@dpdk.org>
Subject: Re: [dpdk-dev] [PATCH] mempool: adjust name string size in related data types
Date: Wed, 20 Jul 2016 14:01:59 +0000 [thread overview]
Message-ID: <59AF69C657FD0841A61C55336867B5B035AA83C3@IRSMSX103.ger.corp.intel.com> (raw)
In-Reply-To: <3f98372c-2bc7-e464-c14a-bce44417165f@6wind.com>
> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Olivier Matz
> Sent: Wednesday, July 20, 2016 2:37 PM
> To: Zoltan Kiss <zoltan.kiss@linaro.org>; Zoltan Kiss
> <zoltan.kiss@schaman.hu>; dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH] mempool: adjust name string size in
> related data types
>
> 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@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.
>
I disagree. I think we should have reasonable max names, and allow functions to return an error in case of a name being too long. However, what I think we also need to do is to guarantee a minimum maximum name length to allow apps to ensure they never hit that name-too-long error. We can guarantee that for ring/mempool etc, that the max allowed name will always be at least 20 characters, for example. That gives plenty of scope for adjusting the max as we need to, while giving others reasonable guarantees too.
/Bruce
next prev parent reply other threads:[~2016-07-20 14:02 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-07-19 14:37 [dpdk-dev] [PATCH] mempool: fix lack of free() registration Zoltan Kiss
2016-07-19 14:37 ` [dpdk-dev] [PATCH] mempool: adjust name string size in related data types Zoltan Kiss
2016-07-19 15:37 ` Olivier Matz
2016-07-19 15:59 ` Zoltan Kiss
2016-07-19 16:17 ` Olivier Matz
2016-07-20 12:41 ` Zoltan Kiss
2016-07-20 13:37 ` Olivier Matz
2016-07-20 14:01 ` Richardson, Bruce [this message]
2016-07-20 17:20 ` Zoltan Kiss
2016-07-20 17:16 ` [dpdk-dev] [PATCH v2] " Zoltan Kiss
2016-07-21 13:40 ` Olivier Matz
2016-07-21 13:47 ` Zoltan Kiss
2016-07-21 14:25 ` Olivier Matz
2016-07-21 21:16 ` Thomas Monjalon
2016-07-19 15:26 ` [dpdk-dev] [PATCH] mempool: fix lack of free() registration Olivier Matz
2016-07-19 16:17 ` Zoltan Kiss
2016-07-20 17:14 ` [dpdk-dev] [PATCH v2] mempool: fix lack of free registration Zoltan Kiss
2016-07-21 21:04 ` Thomas Monjalon
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