linux-kernel - Re: [PATCH 1/3] percpu: use ZERO_SIZE_PTR / ZERO_OR_NULL

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Date:	Mon, 30 Jan 2012 10:02:24 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Dmitry Antipov <dmitry.antipov@...aro.org>,
	Rusty Russell <rusty@...tcorp.com.au>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, patches@...aro.org,
	linaro-dev@...ts.linaro.org
Subject: Re: [PATCH 1/3] percpu: use ZERO_SIZE_PTR / ZERO_OR_NULL_PTR

Hello,

On Mon, Jan 30, 2012 at 11:58:52AM -0600, Christoph Lameter wrote:
> > No, NULL is never gonna be a valid return from any allocator including
> > percpu.  Percpu allocator doesn't and will never do so.
> 
> How do you prevent the percpu allocator from returning NULL? I thought the
> per cpu offsets can wrap around?

I thought it didn't.  I rememer thinking about this and determining
that NULL can't be allocated for dynamic addresses.  Maybe I'm
imagining things.  Anyways, if it can return NULL for valid
allocation, it is a bug and should be fixed.

> > I'm saying we don't have this for ZERO_SIZE_PTR in any meaningful way
> > at this point.  If somebody wants to implement it properly, please
> > feel free to, but simply applying ZERO_SIZE_PTR without other changes
> > doesn't make any sense.
> 
> We have no clean notion of how a percpu pointer needs to be handled. Both
> ways of handling things have drawbacks.

We don't have returned addr >= PAGE_SIZE guarantee yet but I'm fairly
sure that's the only acceptable direction if we want any improvement
in this area.

Thanks.

-- 
tejun
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