linux-kernel - Re: [PATCH] Fix get

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Date:	Tue, 06 Mar 2007 09:58:32 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	David Howells <dhowells@...hat.com>
CC:	torvalds@...l.org, akpm@...l.org, benh@...nel.crashing.org,
	linux-kernel@...r.kernel.org, johannes@...solutions.net
Subject: Re: [PATCH] Fix get_order()

David Howells wrote:
> From: David Howells <dhowells@...hat.com>
> 
> Fix get_order() to use ilog2() properly.
> 
> Signed-Off-By: David Howells <dhowells@...hat.com>
> ---
> 
>  include/asm-generic/page.h |   14 +++++++++++---
>  include/linux/log2.h       |   20 ++++++++++++++++++--
>  2 files changed, 29 insertions(+), 5 deletions(-)
> 
> diff --git a/include/asm-generic/page.h b/include/asm-generic/page.h
> index b55052c..c37571d 100644
> --- a/include/asm-generic/page.h
> +++ b/include/asm-generic/page.h
> @@ -17,10 +17,18 @@ static inline __attribute__((const))
>  int __get_order(unsigned long size, int page_shift)
>  {
>  #if BITS_PER_LONG == 32 && defined(ARCH_HAS_ILOG2_U32)
> -	int order = __ilog2_u32(size) - page_shift;
> +	int order;
> +	if (size <= 1)
> +		order = 0;
> +	else
> +		order = __ilog2_u32(size - 1) + 1 - page_shift;
>  	return order >= 0 ? order : 0;

This seems a lot more complex than it should be.

Assuming page_shift is usually constant, it would seem to make more 
sense to do:

	if (size <= (1UL << page_shift))
		return 0;
	else
		return __ilog2_u32(size-1)+1-page_shift;

... instead of having *two* conditionals...

	-hpa
-
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