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

[Jan Kara <jack@suse.cz>]

On Fri 15-01-16 11:31:00, Eric Sandeen wrote:
> 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?"

As you said, that won't quite work...

> 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.

OK, ok. I like Dave's proposal for quotactl2(). So let's leave the unification
for later and implent Q_GETNEXTQUOTA and Q_XGETNEXTQUOTA with the
functionality of your original Q_XGETQUOTA2. Having separate call to get
next ID would save us one new quotactl but OTOH we would need two syscalls
(and quota structure lookups) to report one structure and there are
potentially *lots* of them.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR