Reg file system hash function

[SATHISH.J]

Hi,

In the vfs layer when we see the lookup_dentry() function code we see that
a part of the code checks whether low level filesystem wants to use its
own hash. the part odf the code that calls the filesystem dependant
hashing is  "error = base->d_op->d_hash->(base,&this);". Why should it
callfilesystem dependant hashing. What is the main purpose of hashing
here.
Please help me with these details. 

Thanks in advance,
Regards,
sathish.j