6934977: (bf) MappedByteBuffer.load can SIGBUS if file is truncated

[Alan Bateman]

If the file underlying a mapped byte buffer is truncated then it's 
possible that invoking the MappedByteBuffer's load method will SIGBUS. 
The reason is that the load touches each page in a native method and the 
VM's handler cannot deal with the fault. A second and related issue 
(6799037) is that same native code is reading an int from each page 
without taking alignment into consideration or the possibility that it 
might read beyond the end of the mapping. Yet another issue, is that 
isLoaded fails with an IOException for the case that the mapping doesn't 
start on a multiple of the page size. The load method has a similar 
problem but it doesn't throw IOException because it is ignoring the 
return value from madvise(3C).

The webrev with the proposed changes to fix these issues is here:
  http://cr.openjdk.java.net/~alanb/6934977/webrev

The main change is that load is changed to touch the pages via Unsafe 
and so uses the existing handler.

-Alan.