linux-kernel - Re: [PATCH 2/4] ia64: allocate percpu area for cpu0 like percpu areas for other cpus

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Date:	Wed, 23 Sep 2009 11:11:02 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Christoph Lameter <cl@...ux-foundation.org>
CC:	Nick Piggin <npiggin@...e.de>, Tony Luck <tony.luck@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	linux-ia64 <linux-ia64@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] ia64: allocate percpu area for cpu0 like percpu areas
 for other cpus

Christoph Lameter wrote:
> On Tue, 22 Sep 2009, Tejun Heo wrote:
> 
>> +#ifdef	CONFIG_SMP
>> +  . = ALIGN(PERCPU_PAGE_SIZE);
>> +  __cpu0_per_cpu = .;
> 
> __per_cpu_start?
> 
>> +  . = . + PERCPU_PAGE_SIZE;	/* cpu0 per-cpu space */
>> +#endif
> 
> This is a statically sized per cpu area that is used by __get_cpu_var()
> Data is access via a cpu specific memory mapping. How does this work when
> the area grows beyond  PERCPU_PAGE_SIZE? As far as I can see: It seems
> that __get_cpu_var would then cause a memory fault?

On ia64, the first chunk is fixed at PERCPU_PAGE_SIZE.  It's something
hardwired into the page fault logic and the linker script.  Build will
fail if the static + reserved area goes over PERCPU_PAGE_SIZE and in
that case ia64 will need to update the special case page fault logic
and increase PERCPU_PAGE_SIZE.  The area reserved above is interim
per-cpu area for cpu0 which is used between head.S and proper percpu
area setup and will be ditched once initialization is complete.

Thanks.

-- 
tejun
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