set_super_anon in fs/super.c

[Rohan Puri]

On Thu, Oct 25, 2012 at 9:40 AM, Abhijit Pawar <abhi.c.pawar at gmail.com>wrote:

>  On 10/24/2012 01:50 PM, Rohan Puri wrote:
>
> Look inline for comments.
>
> On Tue, Oct 23, 2012 at 7:30 PM, Abhijit Chandrakant Pawar <
> abhi.c.pawar at gmail.com> wrote:
>
>>  Hi Rohan,
>>
>>
>> On Tue, 2012-10-23 at 18:47 +0530, Rohan Puri wrote:
>>
>>
>>
>> On Tue, Oct 23, 2012 at 6:33 PM, Abhijit Chandrakant Pawar <
>> abhi.c.pawar at gmail.com> wrote:
>>
>> I am working on the layered file systems. I came across a function called
>> set_super_anon.
>> This is a callback to the sget function to compare the superblock . This
>> function accepts two parameters. first is superblock * and second is void
>> *.  If you look at the definition of this function, the void* is never
>> used.
>> Many filesystem uses this function when they are mounting the superblock.
>> Some pass NULL and some pass actual data.I have looked till 2.6.31 but
>> there isnt any trace of the usage of second parameter.
>>
>> If it is never used then why its added to the function param list?  Is
>> there any historical reason during the older kernel days?
>>
>> Regards,
>> Abhijit Pawar
>>
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>>
>>
>> Hi Abhijit,
>>
>> See the issue is this function is passed as an argument to sget(), now
>> their are many other file-systems that defined their own set_super function
>> & for that they need data argument where they usually pass mount-related
>> data
>>
>> For eg. see the definition and usage of function nfs_set_super().
>>
>> So, the prototype of the sget() should contain function ptr (set_super())
>> and this function ptr should have data argument also. Now one usage can
>> imply NO USE of the data parameter, which is set_super_anon, but other
>> file-systems may require, so the sget() prototype should be generic to
>> support, both the cases.
>>
>>   Yes... thats what I thought.   many are passing data un-necessarily to
>> this function wherein they already have captured the required information
>> for their purpose in their own defined function.
>>
>  Do you mean to say, each fs's own set_super function makes a call to
> set_anon_super() with data parameter as their specific data, but
> set_anon_super makes no use of it?
>
>>
>> Wouldnt that cause stack to store the value un-necessarily? It would be
>> good if everybody passes NULL as second param.
>>
>>  Yes, each fs's set_super, if makes a call to anon_super() should pass
> NULL as the second parameter(void *data) since anon_super doesnt make use
> of this parameter, need for this parameter just arises to match the
> prototype of sget()'s function ptr agrument. Also do remember the pointer
> to this data is passed, so only a word-size of extra stack is utilized when
> a call to this function is made.
>
> I have made a patch for those filesystems and submitted to the kernel
> list.
>
>     - Rohan
>>
>>
> - Rohan
>
>
>  Cool :)

- Rohan
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