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

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Message-ID: <alpine.DEB.2.00.1201301122410.28693@router.home>
Date:	Mon, 30 Jan 2012 11:22:57 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Tejun Heo <tj@...nel.org>
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

On Mon, 30 Jan 2012, Tejun Heo wrote:

> On Mon, Jan 30, 2012 at 09:15:58AM -0800, Tejun Heo wrote:
> > Percpu pointers are in a different address space and using
> > ZERO_SIZE_PTR directly will trigger sparse address space warning.
> > Also, I'm not entirely sure whether 16 is guaranteed to be unused in
> > percpu address space (maybe it is but I don't think we have anything
> > enforcing that).
>
> Another thing is that percpu address dereferencing always goes through
> rather unintuitive translation and 1. we can't (or rather currently
> don't) guarantee that fault will occur for any address 2. even if it
> does, the faulting address wouldn't be anything easily
> distinguishible.  So, unless the above shortcomings is resolved, I
> don't really see much point of using ZERO_SIZE_PTR for percpu
> allocator.

The same is true for the use of NULL pointers.

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