linux-kernel - Re: [RFC][PATCH] introduce ptr

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Date:	Thu, 19 Aug 2010 14:58:28 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Namhyung Kim <namhyung@...il.com>,
	Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] introduce ptr_diff()

On Thu, Aug 19, 2010 at 06:40:17AM -0600, Matthew Wilcox wrote:
> On Thu, Aug 19, 2010 at 02:23:09PM +0200, Andi Kleen wrote:
> > IMHO it would be better to simply disable the warning in sparse instead
> > of uglying the code just to work around sparse bogosity. It doesnt' seem
> > to make much sense. A subtraction followed by a shift is not expensive.
> 
> What makes you think it's a shift?  struct page isn't necessarily a
> power of two in size.  The original poster said "allyesconfig" which is
> going to add in KMEMCHECK and WANT_PAGE_DEBUG_FLAGS.  I think that makes
> it 76 bytes on x86-32, so sparse is right to warn.

These can be still implemented cheaply. Small constants generally are
The only thing that would be really expensive is division by unknown number.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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