linux-kernel - Re: page->_count build failure [was: mmotm 2011-07-08-12-50 uploaded]

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Date:	Mon, 11 Jul 2011 23:45:55 +0300
From:	Pekka Enberg <penberg@...nel.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Jiri Slaby <jirislaby@...il.com>, linux-kernel@...r.kernel.org,
	cl@...ux.com, Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: page->_count build failure [was: mmotm 2011-07-08-12-50 uploaded]

On Mon, Jul 11, 2011 at 11:41 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Mon, 11 Jul 2011 22:27:22 +0200
> Jiri Slaby <jirislaby@...il.com> wrote:
>
>> On 07/08/2011 09:50 PM, akpm@...ux-foundation.org wrote:
>> > The mm-of-the-moment snapshot 2011-07-08-12-50 has been uploaded to
>>
>> Hi, am I the only one encountering a build failure due to double
>> definition of _count in page struct introduced by:
>> commit fc9bb8c768abe7ae10861c3510e01a95f98d5933
>> Author: Christoph Lameter <cl@...ux.com>
>> Date:   Wed Jun 1 12:25:48 2011 -0500
>>
>>     mm: Rearrange struct page
>>
>> in the -next tree?
>>
>> $ make
>> ...
>>   CC      arch/x86/kernel/asm-offsets.s
>> In file included from include/linux/sched.h:64:0,
>>                  from arch/x86/kernel/asm-offsets.c:9:
>> include/linux/mm_types.h:74:15: error: duplicate member ____count___
>> make[1]: *** [arch/x86/kernel/asm-offsets.s] Error 1
>> make: *** [prepare0] Error 2
>> $ gcc --version
>> gcc (SUSE Linux) 4.6.0 20110607 [gcc-4_6-branch revision 174741]
>>
>
> Oh dear.  Yes, I would call that an error.  Afacit the two `_count's
> end up at the same offset in the page struct.  Perhaps that makes it
> legal and your compiler got it wrong.  Or perhaps it is illegal and all
> other compilers got it wrong.
>
> Still, we should fix it.  It's daft to have two distinct page->_count's.
>
> To fix it, we could rename one of them.  Which means that we're going
> to need to work out which references to page->_count are referring to
> which field.  Which demonstrates the problem!
>
>
> hmm, let's jiggle the offset:
>
> --- a/include/linux/mm_types.h~a
> +++ a/include/linux/mm_types.h
> @@ -71,6 +71,7 @@ struct page {
>                                         * Kernel may make use of this field even when slub
>                                         * uses the rest of the double word!
>                                         */
> +                                       int foo;
>                                        atomic_t _count;
>                                };
>                        };
> _
>
> nope, it still compiles OK.  It has to be gcc bustification I think.
> Fixed in gcc-4.6.0.

Didn't commit ea6bd8ee1a2ccdffc38b2b1fcfe941addfafaade fix the issue?
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