Re: [PATCH 0/4] quota: add new quotactl Q_XGETQUOTA2

[Eric Sandeen <sandeen@sandeen.net>]

Hi Jan -

On 1/15/16 3:35 AM, Jan Kara wrote:
> On Wed 13-01-16 16:40:58, Eric Sandeen wrote:
>> On 1/11/16 10:28 AM, Jan Kara wrote:

...

>>> Actually, what I want from you is just an interface which is usable for VFS
>>> quotas as well since I'd like to avoid adding GETQUOTA2 quotactl shortly
>>> after XGETQUOTA2 :).
>>
>> Actually, that's exactly what I thought would *need* to happen ... we already
>> have this weird 15-year-old split-brain quota interface, so if xfs and ext4
>> both need the same functionality, then we'd probably add both GETQUOTA2 and
>> XGETQUOTA2.  If we were doing this all from scratch, sure, but adding a new
>> handles-both-quota-types interface when every other operation is already split
>> between the two almost seems to make matters worse.
> 
> Well, currently GETQUOTA and XGETQUOTA (and all the other quotactls) are
> actually translated so they work regardless of the underlying filesystem.
> So the only difference between XFS and VFS quotactls is in the formatting
> of input/output structures. So from kernel POV it seems somewhat pointless
> to add two calls doing the same thing and differing just in the formatting
> of output - especially when we want the call to be extensible.
> 
> I agree that having a unified call means having a new structure for passing
> dquot info between kernel and userspace. So just for adding that one small
> feature you want it seems like an overkill. But when thinking about new
> extensible getquota quotactl it IMHO makes sense to unify the VFS/XFS split
> brain. Thoughts?

My first lazy/hacky thought is "how terrible would it be to overload the
quotactl syscall return value with quota ID for Q_GETQUOTA2 calls?"

For a purpose-built interface of "find the next ID" that wouldn't require any
structure or interface changes...

We could name it Q_GETNEXTQUOTA / Q_XGETNEXTQUOTA to make it explicit about
the purpose, and document that return behavior.  Done & done.  ;)

A new grand unified extensible quota call sounds like a great idea, I just
hate to gate this work on designing a brand-new interface.

-Eric