Name
follow_pfn — 
     look up PFN at a user virtual address
 
Synopsis
| int follow_pfn ( | struct vm_area_struct * vma, | 
|  | unsigned long address, | 
|  | unsigned long * pfn ); | 
 
Arguments
- vma
- 
     memory mapping
     
- address
- 
     user virtual address
     
- pfn
- 
     location to store found PFN
     
Description
   Only IO mappings and raw PFN mappings are allowed.
   
   Returns zero and the pfn at pfn on success, -ve otherwise.