From: Eric Sandeen <sandeen@redhat.com>
Subject: Re: Question on fallocate/ftruncate sequence
Date: Mon, 20 Jul 2009 17:45:17 -0500
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Cc: ext4 development <linux-ext4@vger.kernel.org>
To: Curt Wohlgemuth <curtw@google.com>
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Curt Wohlgemuth wrote:
> 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

Yep, I think you've found a bug, I will look into this soon unless
someone beats me to it :)

-Eric