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

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Date:	Thu, 19 Aug 2010 14:48:29 +0200
From:	Bernd Petrovitsch <bernd@...prog.at>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	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 Don, 2010-08-19 at 14:23 +0200, Andi Kleen wrote:
> Namhyung Kim <namhyung@...il.com> writes:
> 
> > When I compiled allyesconfig'ed kernel with C=1, I got 1519 lines of
> > following message:
> >
> >  include/linux/mm.h:599:16: warning: potentially expensive pointer subtraction
> >
> > which is around 10% of total warnings. this was caused by page_to_pfn() macro
> > so I think it's worth to remove it by calculating pointer subtraction
> > manually.
> 
> 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.

The code above is IMHO actually (the line with "return" in)
----  snip  ----
static __always_inline void *lowmem_page_address(struct page *page)
{
	return __va(PFN_PHYS(page_to_pfn(page)));
}
----  snip  ----
If so, the warning seems valid as sizeof(struct page) is probably not
(always) a power of 2. On a native build on x86_64 it is 56 bytes
hereover.
Hmm ....

	Bernd
-- 
mobile: +43 664 4416156              http://www.sysprog.at/
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