during truncate ext3 calls journal_forget() for freed blocks, but before
these blocks go to the transaction and jbd reserves space in log for them
(->t_outstanding_credits).  also, journal_forget() removes these blocks
from the transaction, but doesn't correct log space reservation.  for
example, removal of 500MB file reserves 136 blocks, but only 10 blocks go
to the log.  a commit is expensive and correct reservation allows us to
avoid needless commits.  here is the patch.  tested on UP.
Signed-off-by: Alex Tomas <alex@clusterfs.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>