linux-ext4 - Question on fallocate/ftruncate sequence

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Date:	Mon, 20 Jul 2009 09:36:33 -0700
From:	Curt Wohlgemuth <curtw@...gle.com>
To:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Question on fallocate/ftruncate sequence

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
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