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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@xxxxxxxxx>
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@xxxxxxxxx>
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
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