set_super_anon in fs/super.c

[Rohan Puri]

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.

- Rohan
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