Re: [PATCH 8/9] rbd: fix ceph_pg_poolid_by_name()

[Alex Elder <elder@inktank.com>]

On 12/17/2012 10:49 AM, Sage Weil wrote:
> On Mon, 17 Dec 2012, Alex Elder wrote:
>> On 12/14/2012 11:17 PM, Sage Weil wrote:
>>> Most of the code uses int64_t/__s64 for the pool id, although in a few 
>>> cases we screwed up and limited it to 32 bits.  In reality, that's way 
>>> overkill anyway; we could have left it at 32 bits to begin with.
>>
>> The differing types representing the same abstraction are precisely
>> the point of making this change.  What really needs to happen is we
>> need to fix *that*; that is, decide whether a pool id is 32 or 64
>> bits, signed or not, and then make sure it's that and only that
>> throughout the code.
>>
>> In the mean time, this change is defensive, making sure there's
>> no uncertainty in what we're dealing with within rbd.  The code
>> will guarantee some future change won't inadvertently let a
>> wrong-sized pool id attempt to sneak through an interface
>> unnoticed.  It may seem like overkill but this kind of bug is
>> really hard to track down, and it's better to simply preclude
>> it from happening.
>>
>>> My first instinct would be to change the return type to long long or s64 
>>> and avoid the use magic #defines...
>>
>> I absolutely like using base types (like long long).  But where
>> those types are used to represent a true abstraction (like a
>> snapshot id, or a pool id), it is the one place I think the use
>> of typedefs and "magic #defines" is actually a real help because
>> it makes explicit you're working with something more than an (e.g.)
>> an int.  A typedef makes obviously to the reader that it's restricted
>> a bit (so, for example, it isn't meaningful to do math on it).
> 
> Completely agreed.
> 
>> And symbolic constants make it a lot easier to search through
>> code for special situations like this.
> 
> Okay with me.  Just keep in mind that most of the other code looks for a 
> negative int64_t return value (i.e., the pool id is 63 bits).

I.e., if I do this here but not elsewhere we're subject to
the same kind of "someday" problems...  In fact, it's just
a different form of mismatched type--here returning an unsigned
when elsewhere a signed value is assumed.

I still like the symbolic values, or in this case, maybe
a macro ceph_pool_id_valid() or something.  It just makes
it easier to make other changes later, because you can
easily (or maybe more precisely) search for the effects
of a proposed change.

I find the time spent searching through code is large
enough that I tend to do things in a way that facilitates
that.

Let's talk about this today and come to an agreement
about the best way to resolve this.

Thanks.

					-Alex

> The reason there is a mismatch: it used to be a 32-bit value, and at one 
> point we thought we'd do a pool per radosgw bucket and did a huge 
> conversion to 64-bit.  And missed a few places.  The whole transition was 
> ill-conceived and generally a bad idea, though; we should never have that 
> many pools.  So it's not clear it's worth the effort to spend another 
> feature bit to fix it up.
> 
> sage
> 
>> This stuff is all sort of philosophical rather than technical.
>> The code before works, and the code as I've changed it works.
>>
>> Anybody else have thoughts?
>>
>> 					-Alex
>>>
>>>
>>> On Thu, 13 Dec 2012, Alex Elder wrote:
>>>
>>>> Currently ceph_pg_poolid_by_name() returns an int, which is used to
>>>> encode a ceph pool id.  This could be a problem because a pool id
>>>> (at least in some cases) is a 64-bit value.  We have a defined pool
>>>> id value that represents "no pool," and that's a very sensible
>>>> return value here.
>>>>
>>>> This patch changes ceph_pg_poolid_by_name() to return a 64-bit
>>>> pool id value, or CEPH_NOPOOL if the named pool is not found.
>>>>
>>>> The patch also gratuitously renames the function, separating "pool"
>>>> from "id" in the name by an underscore.
>>>>
>>>> Signed-off-by: Alex Elder <elder@inktank.com>
>>>> ---
>>>>  drivers/block/rbd.c         |    6 +++---
>>>>  include/linux/ceph/osdmap.h |    2 +-
>>>>  net/ceph/osdmap.c           |   14 ++++++++------
>>>>  3 files changed, 12 insertions(+), 10 deletions(-)
>>>>
>>>> diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
>>>> index 4daa400..706824b 100644
>>>> --- a/drivers/block/rbd.c
>>>> +++ b/drivers/block/rbd.c
>>>> @@ -3642,11 +3642,11 @@ static ssize_t rbd_add(struct bus_type *bus,
>>>>  	ceph_opts = NULL;	/* rbd_dev client now owns this */
>>>>
>>>>  	/* pick the pool */
>>>> +	rc = -ENOENT;
>>>>  	osdc = &rbdc->client->osdc;
>>>> -	rc = ceph_pg_poolid_by_name(osdc->osdmap, spec->pool_name);
>>>> -	if (rc < 0)
>>>> +	spec->pool_id = ceph_pg_pool_id_by_name(osdc->osdmap, spec->pool_name);
>>>> +	if (spec->pool_id == CEPH_NOPOOL)
>>>>  		goto err_out_client;
>>>> -	spec->pool_id = (u64) rc;
>>>>
>>>>  	rbd_dev = rbd_dev_create(rbdc, spec);
>>>>  	if (!rbd_dev)
>>>> diff --git a/include/linux/ceph/osdmap.h b/include/linux/ceph/osdmap.h
>>>> index 5ea57ba..c841396 100644
>>>> --- a/include/linux/ceph/osdmap.h
>>>> +++ b/include/linux/ceph/osdmap.h
>>>> @@ -124,6 +124,6 @@ extern int ceph_calc_pg_primary(struct ceph_osdmap
>>>> *osdmap,
>>>>  				struct ceph_pg pgid);
>>>>
>>>>  extern const char *ceph_pg_pool_name_by_id(struct ceph_osdmap *map, u64
>>>> id);
>>>> -extern int ceph_pg_poolid_by_name(struct ceph_osdmap *map, const char
>>>> *name);
>>>> +extern __u64 ceph_pg_pool_id_by_name(struct ceph_osdmap *map, const
>>>> char *name);
>>>>
>>>>  #endif
>>>> diff --git a/net/ceph/osdmap.c b/net/ceph/osdmap.c
>>>> index de73214..27e904e 100644
>>>> --- a/net/ceph/osdmap.c
>>>> +++ b/net/ceph/osdmap.c
>>>> @@ -485,19 +485,21 @@ const char *ceph_pg_pool_name_by_id(struct
>>>> ceph_osdmap *map, u64 id)
>>>>  }
>>>>  EXPORT_SYMBOL(ceph_pg_pool_name_by_id);
>>>>
>>>> -int ceph_pg_poolid_by_name(struct ceph_osdmap *map, const char *name)
>>>> +__u64 ceph_pg_pool_id_by_name(struct ceph_osdmap *map, const char *name)
>>>>  {
>>>>  	struct rb_node *rbp;
>>>>
>>>>  	for (rbp = rb_first(&map->pg_pools); rbp; rbp = rb_next(rbp)) {
>>>> -		struct ceph_pg_pool_info *pi =
>>>> -			rb_entry(rbp, struct ceph_pg_pool_info, node);
>>>> +		struct ceph_pg_pool_info *pi;
>>>> +
>>>> +		pi = rb_entry(rbp, struct ceph_pg_pool_info, node);
>>>>  		if (pi->name && strcmp(pi->name, name) == 0)
>>>> -			return pi->id;
>>>> +			return (__u64) pi->id;
>>>>  	}
>>>> -	return -ENOENT;
>>>> +
>>>> +	return CEPH_NOPOOL;
>>>>  }
>>>> -EXPORT_SYMBOL(ceph_pg_poolid_by_name);
>>>> +EXPORT_SYMBOL(ceph_pg_pool_id_by_name);
>>>>
>>>>  static void __remove_pg_pool(struct rb_root *root, struct
>>>> ceph_pg_pool_info *pi)
>>>>  {
>>>> -- 
>>>> 1.7.9.5
>>>>
>>>> --
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>>>>
>>
>>