Re: [PATCH] taskstats: Use better ifdef for alignment

Jeff Mahoney
2010-12-30T05:26:46Z

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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@suse.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@davemloft.net>
>> Signed-off-by: Jeff Mahoney <jeffm@suse.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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