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 10:17:52 -0700
From:	Daniel Walker <dwalker@...o99.com>
To:	Mel Gorman <mel@....ul.ie>
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, 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.) You also spelled initializing
wrong.

Daniel

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