linux-kernel - Re: get_order() broken !

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Message-ID: <2793.1173202996@redhat.com>
Date:	Tue, 06 Mar 2007 17:43:16 +0000
From:	David Howells <dhowells@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: get_order() broken ! 

Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> Yeah, I think the recent get_order() changes to change it to use ilog2() 
> are just crap. I actually looked at it when it went in, but I assumed that 
> it had been tested. I don't think it has.

It had, it's just that the testing totally failed to show up the problem; that
and I think my userspace testing went wrong because I made the wrong
assumptions about what the answer should be.

However, having discussed it with Ben, HPA and David Woodhouse and others I've
come up with a patch (sent separately) that should fix it by using:

	ilog2((n) - 1) + 1 

as the basis for the calculation as this should round up the result.  I've
encapsulated this as a function called ilog2_up() and fixed the docs to make
it cleared how ilog2() and co actually work.

David
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