linux-kernel - Re: HOLES_IN

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Message-ID: <20090205104138.GF26878@csn.ul.ie>
Date:	Thu, 5 Feb 2009 10:41:38 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	David Miller <davem@...emloft.net>
Cc:	kamezawa.hiroyu@...fujitsu.com, heiko.carstens@...ibm.com,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	sparclinux@...r.kernel.org
Subject: Re: HOLES_IN_ZONE...

On Thu, Feb 05, 2009 at 02:14:29AM -0800, David Miller wrote:
> From: Mel Gorman <mel@....ul.ie>
> Date: Thu, 5 Feb 2009 10:10:23 +0000
> 
> > On Wed, Feb 04, 2009 at 10:26:51PM -0800, David Miller wrote:
> > > As a suggestion, it would have been a lot more pleasant if the code
> > > validated this requirement (in the !HOLES_IN_ZONE case) at boot time
> > > instead of after 2 hours of stress testing :-(
> > > 
> > 
> > Nice maybe, but we'd take a hit on pfn_valid_within() which goes from
> > being compiled-away on architectures that don't need it to being
> > a read of a shared cacheline and a branch.
> 
> I said at boot time Mel, not in the fast paths. 

pfn_valid_within() is not used in any critical paths, but it's used in
the page free path so it would be a hit there.

> Meaning, we'd
> check it after all the available ranges get registered and
> tossed into the free page lists.
> 

It's not a free-list problem, it's walkers of PFN ranges. Anyway, it's
not important to the current problem.

> Anyways, it appears we have a fix already, read the rest of
> the thread.
> 

Yes. Thanks Kamezawa Hiroyu for the fix.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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