linux-kernel - Re: [PATCH] percpu: fix unit_map[] verification in pcpu_setup_first

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Message-ID: <4AC0E6A3.4040500@kernel.org>
Date:	Tue, 29 Sep 2009 01:38:59 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Christoph Lameter <cl@...ux-foundation.org>
CC:	Tony Vroon <tony@...x.net>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] percpu: fix unit_map[] verification in pcpu_setup_first_chunk()

Tejun Heo wrote:
> Christoph Lameter wrote:
>> On Tue, 29 Sep 2009, Tejun Heo wrote:
>>
>>> pcpu_setup_first_chunk() incorrectly used NR_CPUS as the impossible
>>> unit number while unit number can equal and go over NR_CPUS with
>>> sparse unit map.  This triggers BUG_ON() spuriously on machines which
>>> have non-power-of-two number of cpus.  Use UINT_MAX instead.
>> Uhhh. Funky. The assumption nr_cpu_ids < NR_CPUS has been broken. Wonder
>> what other effects that will have. In particular since the slab and page
>> allocators have arrays indexed by the cpu number and those arrays are
>> dimensioned for NR_CPUS.
> 
> Heh.. that's not what's broken.  unit# can legally go over NR_CPUS or
> nr_cpu_ids as cpus can be sparsely mapped to units.  The problem is
> that
      ^
      I'm delusional at the moment.  :-)

Christoph, can you please elaborate why nr_cpu_ids < NR_CPUS is
broken?

Thanks.

-- 
tejun
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