Re: [patch] delayacct: fix iotop on x86_64

[Jeff Mahoney <jeffm@suse.com>]

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On 12/14/2010 03:16 PM, Andrew Morton wrote:
> On Tue, 14 Dec 2010 13:32:39 +0530
> Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> 
>> * Dan Carpenter <error27@gmail.com> [2010-12-14 10:02:43]:
>>
>>> We changed how the taskstats was exported to user space in:
>>> 85893120699 "delayacct: align to 8 byte boundary on 64-bit systems"
>>> This was important because it fixes a run time warning on IA64.  In
>>> theory it shouldn't have broken anything, if you just assume that user
>>> space programmers don't smoke crack all day long.
>>>
>>> But actually it breaks iotop on x86_64.
>>>
>>> Reported-by: Brian Rogers <brian@xyzw.org>
>>> Signed-off-by: Dan Carpenter <error27@gmail.com>
>>>
>>> diff --git a/kernel/taskstats.c b/kernel/taskstats.c
>>> index c8231fb..a0758de 100644
>>> --- a/kernel/taskstats.c
>>> +++ b/kernel/taskstats.c
>>> @@ -358,7 +358,19 @@ static struct taskstats *mk_reply(struct sk_buff *skb, int type, u32 pid)
>>>  	 * This causes lots of runtime warnings on systems requiring 8 byte
>>>  	 * alignment */
>>>  	u32 pids[2] = { pid, 0 };
>>> -	int pid_size = ALIGN(sizeof(pid), sizeof(long));
>>> +	int pid_size;
>>> +
>>> +	/*
>>> +	 * IA64 can't be aligned on a 4 byte boundary.  But iotop on x86_64
>>> +	 * depends on the current struct layout.  The next version of iotop
>>> +	 * will fix this so maybe we can move everything to the new code in
>>> +	 * a couple years.
>>> +	 */
>>> +#if defined(CONFIG_IA64)
>>> +	pid_size = ALIGN(sizeof(pid), sizeof(long));
>>> +#else
>>> +	pid_size = sizeof(u32);
>>> +#endif
>>
>> I would rather abstract this better
> 
> Well.  Abstracting something tends to make it permanent.  When you have
> an ugly, special-case temporary hack, there is merit to having it
> sitting there in the middle of the code staring you in the face.  It's
> very explicit and we won't forget about it.
> 
>> and I'd be apprehensive about the
>> fix if iotop was at fault to begin with, I would rather fix iotop.
>> IOW, are we fixing what iotop got wrong? Isn't it easier to backport
>> the correct behaviour in iotop. I understand we broke the ABI, but
>> user space can still live.
> 
> Nah, let's not knowingly break a userspace app.
> 
> 
> This is a versioned interface, is it not?  How is that supposed
> to work?  Should we have upped the version number when making this
> change?

Perhaps. Balbir suggested it, but it didn't make it into the final
version. Not that it would have mattered. iotop doesn't look at the
version anyway.

- -Jeff

- -- 
Jeff Mahoney
SUSE Labs
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