On Sat, 2012-03-24 at 16:27 +0100, Christoph Hellwig wrote:
> >         has_discard = !fallocate(s->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
> 
> There is no point in using FALLOC_FL_KEEP_SIZE together with
> FALLOC_FL_PUNCH_HOLE.

It's *required*. From the man page [0], "The FALLOC_FL_PUNCH_HOLE flag
must be ORed with FALLOC_FL_KEEP_SIZE in mode;"

[0] http://man7.org/linux/man-pages/man2/fallocate.2.html

-- 
Richard