block_invalidatepage — invalidate part or all of a buffer-backed page
| void fsfuncblock_invalidatepage ( | struct page * page, | 
| unsigned long offset ); | 
   block_invalidatepage is called when all or part of the page has become
   invalidated by a truncate operation.
   
   block_invalidatepage does not have to release all buffers, but it must
   ensure that no dirty buffer is left outside offset and that no I/O
   is underway against any of the blocks which are outside the truncation
   point.  Because the caller is about to free (and possibly reuse) those
   blocks on-disk.