linux-kernel - Re: [PATCH] x86, percpu: revert commit fe8e0c25

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Date:	Tue, 26 Oct 2010 09:49:31 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Ingo Molnar <mingo@...e.hu>, torvalds@...l.org,
	Alexander van Heukelum <heukelum@...tmail.fm>,
	David Howells <dhowells@...hat.com>, akpm@...ux-foundation.org,
	linux-am33-list@...hat.com, linux-kernel@...r.kernel.org,
	Akira Takeuchi <takeuchi.akr@...panasonic.com>,
	Mark Salter <msalter@...hat.com>
Subject: Re: [PATCH] x86, percpu: revert commit fe8e0c25

On Tue, Oct 26, 2010 at 9:34 AM, Tejun Heo <tj@...nel.org> wrote:
> Hello,
>
> On 10/26/2010 03:25 PM, Ingo Molnar wrote:
>>
>> * Tejun Heo <tj@...nel.org> wrote:
>>
>>> Commit fe8e0c25 (x86, 32-bit: Align percpu area and irq stacks to THREAD_SIZE)
>>> aligned PERCPU section to THREAD_SIZE which can be larger than PAGE_SIZE,
>>> introduced DEFINE_PER_CPU_MULTIPAGE_ALIGNED() and used it to make irq stacks
>>> aligned to THREAD_SIZE on x86_32.
>>>
>>> This won't work.  The PERCPU output section is used as the template to prepare the
>>> percpu area and the actual percpu area is _alwasy_ aligned to PAGE_SIZE whether
>>> the source area is aligned to larger size or not.
>>
>> The problem is, this will reintroduce a nasty boot crash which commit fe8e0c25
>> fixed. Do you say that fe8e0c25 didnt have the alignment effect?
>
> AFAICS, not in a way which is correct.  The patch probably made the
> following two differences.
>
> * The stack in the template area is THREAD_SIZE aligned.  If something
>  was dereferencing it before percpu init, this could have helped.
>  IIRC, x86 early init code does use the template area.
>
> * The percpu address would be THREAD_SIZE aligned while the translated
>  kernel address for each cpu wouldn't be.  For masking stack pointer
>  to find out task struct, I don't think aligning the percpu address
>  would have been helpful.
>
> It's simply broken and needs to be reverted.  If the patch somehow
> fixed boot crash, yeah, we probably want to put a fix for it first
> tho.
>
> Thanks.

Probably the best fix is to go back to allocating the stacks with
get_free_pages(), and only keep the pointers in percpu memory.

--
Brian Gerst
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