linux-kernel - Re: [PATCH 1/3] powerpc: Allocate per-cpu areas for node IDs for SLQB to use as per-node areas

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Date:	Mon, 21 Sep 2009 18:42:23 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Daniel Walker <dwalker@...o99.com>
Cc:	Nick Piggin <npiggin@...e.de>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Christoph Lameter <cl@...ux-foundation.org>,
	heiko.carstens@...ibm.com, sachinp@...ibm.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Tejun Heo <tj@...nel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH 1/3] powerpc: Allocate per-cpu areas for node IDs for
	SLQB to use as per-node areas

On Mon, Sep 21, 2009 at 10:17:52AM -0700, Daniel Walker wrote:
> On Mon, 2009-09-21 at 17:10 +0100, Mel Gorman wrote:
> > SLQB uses DEFINE_PER_CPU to define per-node areas. An implicit
> > assumption is made that all valid node IDs will have matching valid CPU
> > ids. In memoryless configurations, it is possible to have a node ID with
> > no CPU having the same ID. When this happens, a per-cpu are is not
> > created and the value of paca[cpu].data_offset is some random value.
> > This is later deferenced and the system crashes after accessing some
> > invalid address.
> > 
> > This patch hacks powerpc to allocate per-cpu areas for node IDs that
> > have no corresponding CPU id. This gets around the immediate problem but
> > it should be discussed if there is a requirement for a DEFINE_PER_NODE
> > and how it should be implemented.
> > 
> > Signed-off-by: Mel Gorman <mel@....ul.ie>
> > ---
> >  arch/powerpc/kernel/setup_64.c |   20 ++++++++++++++++++++
> >  1 files changed, 20 insertions(+), 0 deletions(-)
> > 
> > diff --git a/arch/powerpc/kernel/setup_64.c b/arch/powerpc/kernel/setup_64.c
> > index 1f68160..a5f52d4 100644
> > --- a/arch/powerpc/kernel/setup_64.c
> > +++ b/arch/powerpc/kernel/setup_64.c
> > @@ -588,6 +588,26 @@ void __init setup_per_cpu_areas(void)
> >  		paca[i].data_offset = ptr - __per_cpu_start;
> >  		memcpy(ptr, __per_cpu_start, __per_cpu_end - __per_cpu_start);
> >  	}
> > +#ifdef CONFIG_SLQB
> > +	/* 
> > +	 * SLQB abuses DEFINE_PER_CPU to setup a per-node area. This trick
> > +	 * assumes that ever node ID will have a CPU of that ID to match.
> > +	 * On systems with memoryless nodes, this may not hold true. Hence,
> > +	 * we take a second pass initialising a "per-cpu" area for node-ids
> > +	 * that SLQB can use
> > +	 */
> 
> Very trivial, but there's a little trailing whitespace in the first line
> of the comment (checkpatch warns on it.)

D'oh, will clean it up in the next revision if the substance of the
patch is agreed upon.

> You also spelled initializing wrong.

It's spelt correctly for British English.

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