linux-kernel - Re: HOLES_IN

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Date:	Thu, 5 Feb 2009 09:00:16 +0100
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	David Miller <davem@...emloft.net>
Cc:	mel@....ul.ie, kamezawa.hiroyu@...fujitsu.com,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	sparclinux@...r.kernel.org
Subject: Re: HOLES_IN_ZONE...

On Wed, 04 Feb 2009 22:26:51 -0800 (PST)
David Miller <davem@...emloft.net> wrote:

> I also noticed that when S390 got virtual memmap support, it acquired
> the HOLES_IN_ZONE setting as well, in this commit:
> 
> commit f4eb07c17df2e6cf9bd58bfcd9cc9e05e9489d07
> Author: Heiko Carstens <heiko.carstens@...ibm.com>
> Date:   Fri Dec 8 15:56:07 2006 +0100
> 
>     [S390] Virtual memmap for s390.
> 
> This is confusing.  Is HOLES_IN_ZONE only required when virtual mmap
> is being used?  If so, why is that?  This is a very poorly documented
> flag, and I'm saying this after pouring over every commit referencing
> it.

I should have split that into two commits back then. When writing the
vmemmap code for s390 I realized that we never guaranteed that zones
start on a MAX_ORDER boundary or have a size that is a multiple of
MAX_ORDER. So I just added HOLES_IN_ZONE.

> Later this HOLES_IN_ZONE requirement was removed on s390 by commit:
> 
> commit 9f4b0ba81f158df459fa2cfc98ab1475c090f29c
> Author: Heiko Carstens <heiko.carstens@...ibm.com>
> Date:   Sat Jan 26 14:11:02 2008 +0100
> 
>     [S390] Get rid of HOLES_IN_ZONE requirement.

This just made sure that all zones start on a MAX_ORDER boundary and
just leaves memory that doesn't fit unused. So the requirement for
HOLES_IN_ZONE went away.

Later I reduced MAX_ORDER to 9 on s390, so we don't leave large
portions of memory unused.
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