If we have a look at the register_filesystem() function defined in 
fs/filesystems.c, we see that if a filesystem with a same name has 
already been registered then the find_filesystem() function will return 
NON-NULL otherwise it will return NULL.

Hence, register_filesystem() should return EEXIST instead of EBUSY. 
Returning EBUSY is misleading (unless of course I'm missing something 
obvious) to the caller of register_filesystem().

Thanks,

Hareesh Nagarajan