* The key words are stored in @key on success.
*
* For shared mappings (when @fshared), the key is:
+ *
* ( inode->i_sequence, page->index, offset_within_page )
+ *
* [ also see get_inode_sequence_number() ]
*
* For private mappings (or when !@fshared), the key is:
+ *
* ( current->mm, address, 0 )
*
* This allows (cross process, where applicable) identification of the futex