linux-kernel - Re: [this_cpu_xx V7 1/8] this_cpu

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Date:	Thu, 17 Dec 2009 09:23:40 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Christoph Lameter <cl@...ux-foundation.org>
CC:	linux-kernel@...r.kernel.org, Mel Gorman <mel@....ul.ie>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Subject: Re: [this_cpu_xx V7 1/8] this_cpu_ops: page allocator conversion

Hello,

On 12/16/2009 11:55 PM, Christoph Lameter wrote:
>>> The boot pageset initialization was moved into __build_all_zonelists(). We
>>> could move the zone->pageset initialization there too?
>>
>> Maybe that is a bit less scary. (at least for me :-) The reason why
> 
> Wel sorry moving the ->pageset assign to __build_all_zonelists does not
> work since __build_all_zonelists does not scan through
> zones. zone_pcp_init is called reliable first for all zones. I think just
> leave it at is.

Ah, well, alright.

>> I'm a bit worried is that different architectures handle percpu
>> pointers differently before setup_per_cpu_areas().  x86 sets up the
>> offsets and stuff such that cpu0 can access the original percpu
>> section in the kernel image.  ia64 sets up everything properly way
>> before setup_per_cpu_areas() and in some archs percpu pointers are
>> completely invalid before setup_per_cpu_areas().  So, percpu pointer
>> being handled in generic code which is being called before percpu
>> setup is a bit worrying.
> 
> True but we are not dereferencing a per cpu pointer here. It is simply the
> assignment of the unreferenced native per cpu address generated by the
> linker. This address is unaffected by allocator bootstrap.

Yeap, the code is fine.  Probably I'm just being a bit paranoid.  If
you don't mind, can you please add a comment pointing out that the
percpu pointer isn't dereferenced before later initialization is done
which happens after percpu initialization?

> The assignment of the final per cpu areas (dynamically allocated) occurs
> after all the other memory allocators have been bootstrapped.

Great, thanks for the explanation.

-- 
tejun
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