.truncate function in filesystems

[Eli Carter]

All,

I'm rather at a loss to understand what .truncate is supposed to do when 
it _extends_ a file (2.5.68+ kernel).  Looking through the examples in 
the tree, I don't see where they deal with that case.  Could someone 
give me a clue as to how .truncate is supposed to...tell the kernel what 
blocks it is to use when it goes to read from the extended portion of 
the file?

So far, I have googled, grep'ed, looked at lwn.net, started in on 
linux.bkbits.net, emailed linux-fsdevel and kernel-newbies...  and I'm 
still not getting anywhere.  Even a basic explanation of how filesystems 
  deal/interact with buffer heads and inodes would probably help at this 
point.

Any clues or pointers or _anything_ would be very much appreciated. :/

Eli
--------------------. "If it ain't broke now,
Eli Carter           \                  it will be soon." -- crypto-gram
eli.carter(a)inet.com `-------------------------------------------------