linux-kernel - Re: [PATCH 1/1] [virt] virtio-blk: Use ida to allocate disk index

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Date:	Thu, 16 Jun 2011 09:46:48 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Mark Wu <dwu@...hat.com>, "Michael S. Tsirkin" <mst@...hat.com>,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
	Greg Kroah-Hartman <gregkh@...e.de>
Subject: Re: [PATCH 1/1] [virt] virtio-blk: Use ida to allocate disk index

Hello,

On Thu, Jun 16, 2011 at 09:35:34AM +0930, Rusty Russell wrote:
> On Wed, 15 Jun 2011 09:06:38 +0200, Tejun Heo <tj@...nel.org> wrote:
> > It's inherited from idr which was designed to have separate
> > prepare/allocation stages so that allocation can happen inside an
> > outer spinlock.  It doesn't have too much to do with optimization.
> > It's mostly to be able to use sleepable context for memory allocation
> > while allowing atomic id[ra] allocation.
> 
> It might have made sense for a few callers, but as a general mechanism
> it stinks.  It's a lot of dancing to avoid GFP_ATOMIC allocations; we'd
> be better making idr_get_new() take a gfp_t, and have an idr_pre_alloc()
> for those who care.
> 
> *Sure* there's a chance of racing and we will need to do an atomic
> allocation.  But can anyone justify the current complexity for all
> callers?

Sure, I'm not arguing for the current interface.

> > > + * ida_simple_get - get a new id.
> > > + * @ida: the (initialized) ida.
> > > + * @min_id: the minimum id (inclusive)
> > > + * @max_id: the maximum id (inclusive)
> > > + *
> > > + * Allocates an id in the range min_id <= id <= max_id, or returns -ENOSPC.
> > > + * On allocation failure, returns -ENOMEM.  This function can sleep.
> > > + *
> > > + * Use ida_simple_remove() to get rid of an id.
> > > + */
> > > +int ida_simple_get(struct ida *ida, int min_id, int max_id)
> > 
> > Hmmm... new interface different from existing id[ra] style, but yeah
> > something like the above would have made more sense from the
> > beginning.  The only thing is that isn't (begin <= range < end) more
> > conventional form to express ranges?
> 
> Yes, but how to express an unlimited range then?  I could used unsigned
> and 0x80000000, but that seemed crude.

So, why not do this properly then?  ie. ida_get(ida, begin, end, gfp).

As for the end of range, shouldn't 0 mean the default max range?  Only
some users care about the upper limit anyway.  We can also make it
unsigned just in case and cap the value.

Thanks.

-- 
tejun
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