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From: Andi Kleen <ak@suse.de>
Subject: Re: percpu data changes
Date: Fri, 3 Feb 2006 10:51:48 +0100
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Cc: Mikael Starvik <starvik@axis.com>, David Howells <dhowells@redhat.com>, Kyle McMartin <kyle@parisc-linux.org>, Anton Blanchard <anton@samba.org>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, Paul Mackerras <paulus@samba.org>, Martin Schwidefsky <schwidefsky@de.ibm.com>, Heiko Carstens <heiko.carstens@de.ibm.com>, Paul Mundt <lethal@linux-sh.org>, "David S. Miller" <davem@davemloft.net>, William Lee Irwin III <wli@holomorphy.com>, Andi Kleen <ak@muc.de>, Christian Zankel <chris@zankel.net>, Philippe Elie <phil.el@wanadoo.fr>, Nathan Scott <nathans@sgi.com>, Jens Axboe <axboe@suse.de>, linux-arch@vger.kernel.org, Eric Dumazet <dada1@cosmosbay.com>
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On Friday 03 February 2006 10:17, Andrew Morton wrote:
> 
> It seems that powerpc has gone and changed their implementation of percpu
> data so that there is no memory allocated for not-possible CPUs.  To save a
> bit of RAM.
> 
> This means that any code which does
> 
> 	for (i = 0; i < NR_CPUS; i++)
> 		touch(percpudata(i))
> 
> will explode on powerpc.

It also explodes since some time on x86-64.

But I added a workaround now (or rather sent one and Linus dropped it)
to point the not possible CPUs to the reference data and not free it.
With that violating that protocol is mostly harmless.

Later the plan was to point it to unmapped data to catch all users.

-Andi