linux-kernel - Re: [PATCH] taskstats: Use better ifdef for alignment

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Date:	Thu, 30 Dec 2010 00:26:34 -0500
From:	Jeff Mahoney <jeffm@...e.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Dan Carpenter <error27@...il.com>, balbir@...ux.vnet.ibm.com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] taskstats: Use better ifdef for alignment

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On 12/29/2010 07:14 PM, Andrew Morton wrote:
> On Wed, 29 Dec 2010 19:12:08 -0500 Jeff Mahoney <jeffm@...e.com> wrote:
> 
>>  Commit 4be2c95d added a null field to align the taskstats structure but
>>  the discussion centered around ia64. The issue exists on other platforms
>>  with inefficient unaligned access and adding them piecemeal would be
>>  an unmaintainable mess.
>>
>>  This patch uses Dave Miller's suggestion of using a combination of
>>  CONFIG_64BIT && !CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS to determine
>>  whether alignment is needed.
>>
>>  Note that this will cause breakage on those platforms with applications
>>  like iotop which had hard-coded offsets into the packet to access the
>>  taskstats structure.
> 
> That seems a very good reason to not apply the patch.
> 
> Tell us more, please...

I don't want to rehash the same discussion, but any argument that
applied to ia64 applies any other 64-bit arch other than powerpc and x86_64.

- -Jeff

>>  The message seen on systems without the alignment fixes looks like:
>>  kernel unaligned access to 0xe000023879dca9bc, ip=0xa000000100133d10
>>
>>  The addresses may vary but resolve to locations inside __delayacct_add_tsk.
>>
>> Reported-by: David S. Miller <davem@...emloft.net>
>> Signed-off-by: Jeff Mahoney <jeffm@...e.com>
>> ---
>>  kernel/taskstats.c |    2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> --- a/kernel/taskstats.c
>> +++ b/kernel/taskstats.c
>> @@ -349,7 +349,7 @@ static int parse(struct nlattr *na, stru
>>  	return ret;
>>  }
>>  
>> -#ifdef CONFIG_IA64
>> +#if defined(CONFIG_64BIT) && !defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)
>>  #define TASKSTATS_NEEDS_PADDING 1
>>  #endif
>>  


- -- 
Jeff Mahoney
SUSE Labs
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