Question on fallocate/ftruncate sequence

[Curt Wohlgemuth <curtw@google.com>]

We've recently seen some interesting behavior with ftruncate()
following a fallocate() call on ext4, and would like to know if this
is intended or not.

The sequence used from user space:

fd = open()
fallocate(fd, FALLOC_FL_KEEP_SIZE, 8MB)
write(fd, buf, 64KB)
ftruncate(fd, 64KB)
close(fd)

Since inode_setattr() only does something if the input size is not the
same as inode->i_size, the ftruncate() call above does nothing; no
blocks from the fallocate() are freed up.

Yes, removing the KEEP_SIZE flag gets the behavior I'm expecting, but
KEEP_SIZE is quite convenient in recovering from errors.

I would have thought that ftruncate() would alter i_disksize even if
this value is different from i_size.

Any comments?  I looked at other Linux file systems, and none that I
saw that support fallocate() have this issue.

Thanks,
Curt