Re: [PATCH v2] Security: smack: replace kzalloc with kmem_cache for inode_smack

[Casey Schaufler]

On 10/26/2014 11:54 PM, Rohit wrote:
> On Sun, 26 Oct 2014 17:41:37 -0700
> Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote:
>
>> On 10/17/2014 10:37 AM, Casey Schaufler wrote:
>>> On 10/17/2014 9:34 AM, PINTU KUMAR wrote:
>>>> Hi,
>>>>
>>>>
>>>>> ________________________________
>>>>> From: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
>>>>> To: Rohit <rohit.kr@xxxxxxxxxxx> 
>>>>> Cc: akpm@xxxxxxxxxxxxxxxxxxxx; james.l.morris@xxxxxxxxxx;
>>>>> serge@xxxxxxxxxx; linux-security-module@xxxxxxxxxxxxxxx;
>>>>> linux-kernel@xxxxxxxxxxxxxxx; cpgs@xxxxxxxxxxx;
>>>>> pintu.k@xxxxxxxxxxx; vishnu.ps@xxxxxxxxxxx;
>>>>> iqbal.ams@xxxxxxxxxxx; ed.savinay@xxxxxxxxxxx; me.rohit@xxxxxxxx;
>>>>> pintu_agarwal@xxxxxxxxx; Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
>>>>> Sent: Friday, 17 October 2014 8:08 PM Subject: Re: [PATCH v2]
>>>>> Security: smack: replace kzalloc with kmem_cache for inode_smack
>>>>>
>>>>>
>>>>> On 10/17/2014 4:42 AM, Rohit wrote:
>>>>>> On Thu, 16 Oct 2014 09:24:01 -0700
>>>>>> Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote:
>>>>>>
>>>>>>> On 10/15/2014 5:10 AM, Rohit wrote:
>>>>>>>> The patch use kmem_cache to allocate/free inode_smack since
>>>>>>>> they are alloced in high volumes making it a perfect case for
>>>>>>>> kmem_cache.
>>>>>>>>
>>>>>>>> As per analysis, 24 bytes of memory is wasted per allocation
>>>>>>>> due to internal fragmentation. With kmem_cache, this can be
>>>>>>>> avoided.
>>>>>>> What impact does this have on performance? I am much more
>>>>>>> concerned with speed than with small amount of memory.
>>>>>>>
>>>>>> I think there should not be any performance problem as such.
>>>>>> However, please let me know how to check the performance in this
>>>>>> case.
>>>>> Any inode intensive benchmark would suffice. Even the classic
>>>>> kernel build would do.
>>>>>
>>>>>> As far as i know, kzalloc first finds the kmalloc_index
>>>>>> corresponding to the size to get the kmem_cache_object and then
>>>>>> calls kmem_cache_alloc with the kmalloc_index(kmem_cache
>>>>>> object). Here, we create kmem_cache object specific for
>>>>>> inode_smack and directly calls kmem_cache_alloc() which should
>>>>>> give better performance as compared to kzalloc.
>>>>> That would be my guess as well, but performance is tricky.
>>>>> Sometimes things that "obviously" make performance better make it
>>>>> worse. There can be unanticipated side effects.
>>>>>
>>>>>
>>>>>> Please let me know your comments.
>>>>> If you can run any sort of test that demonstrates this change
>>>>> does not have performance impact, I'm fine with it. Smack is being
>>>>> used in small devices, and both memory use and performance are
>>>>> critical to the success of these devices. Of the two, performance
>>>>> is currently more of an issue.
>>>>>
>>>> SMACK is used heavily in Tizen. We verified these changes for one
>>>> of Tizen project. During boot time we observed that this object is
>>>> used heavily, as identified by kmalloc-accounting. After replacing
>>>> this we did not observe any difference in boot time. Also there
>>>> was no side-effects seen so far. If you know of any other tests,
>>>> please let us know. We will also try to gather some performance
>>>> stats and present here.
>>> We need to be somewhat more precise than "did not observe any
>>> difference in boot time". The ideal benchmark would perform lots
>>> of changes to the filesystem without doing lots of IO. One process
>>> that matches that profile fairly well is a kernel make. I would be
>>> satisfied with something as crude as using time(1) on a small (5?)
>>> number of clean kernel makes each with and without the patch on the
>>> running kernel. At the level of accuracy you usually get from
>>> time(1) you won't find trivial differences, but if the change is a
>>> big problem (or a big win) we'll know.
>> I have not seen anything indicating that the requested performance
>> measurements have been done. I have no intention of accepting this
>> without assurance that performance has not been damaged. I request
>> that no one else carry this forward, either. The performance impact
>> of security facilities comes under too much scrutiny to ignore it.
>>
>>> ... 
> Sorry for the delay as I was on holiday for last week.
> Will verify the performance impact as per your suggestion.
> We verified it only on Tizen based ARM board, so building kernel on it
> is not possible.
> I found http://elinux.org/images/0/06/Buzov-SMACK.pdf (slides - 35-37)
> for performance verification of smack. It checks performance of file
> creation and copy in tmpfs. 
> Please let me know whether the procedure mentioned in the above
> mentioned slide is fine, else please suggest some other way to check
> performance on the target board.

The technique outlined by Buzov should provide adequate evidence.

>
>
> Regards,
> Rohit
>
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