set_super_anon in fs/super.c

[Abhijit Pawar]

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 <mailto: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 <mailto: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

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