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

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Message-ID: <87obtkjx3d.fsf@rustcorp.com.au>
Date:	Tue, 31 Jan 2012 15:18:54 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>
Cc:	Dmitry Antipov <dmitry.antipov@...aro.org>, 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 10:16:39 -0800, Tejun Heo <tj@...nel.org> wrote:
> On Mon, Jan 30, 2012 at 12:12:18PM -0600, Christoph Lameter wrote:
> > > 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 dont see anything that would hinder an arbitrary value to be returned.
> > NULL is also used for the failure case. Definitely a bug.
> 
> Given the address translation we do and kernel image layout, I don't
> think this can happen on x86.  It may theoretically possible on other
> archs tho.  Anyways, yeah, this one needs improving.

I tried setting the lower bit on all percpu ptrs, but since non-dynamic
percpu vars could have odd alignments, that fails in general.

> > > 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.
> > 
> > The ZERO_SIZE_PTR patch would not make the situation that much worse.
> 
> I'm not objecting to marking zero-sized allocations per-se.  I'm
> saying the patch is pointless at this point.  It doesn't contribute
> anything while giving the illusion of better error checking than we
> actually do.  Let's do it when it can actually work.

Disagree: This patch works.  It allows zero-size per-cpu allocs, like
the other allocators.  Nor does it fail in practice.

We should do better, but the perfect is the enemy of the good.

Cheers,
Rusty.
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