From: Frank Mayhar Subject: Re: Question on fallocate/ftruncate sequence Date: Tue, 21 Jul 2009 14:29:31 -0700 Message-ID: <1248211771.20743.2.camel@bobble.smo.corp.google.com> References: <6601abe90907200936w61ebda92reae368a2b9efac66@mail.gmail.com> <4A64F37D.7020803@redhat.com> Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Cc: Curt Wohlgemuth , ext4 development To: Eric Sandeen Return-path: Received: from smtp-out.google.com ([216.239.33.17]:24670 "EHLO smtp-out.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751625AbZGUV3w (ORCPT ); Tue, 21 Jul 2009 17:29:52 -0400 In-Reply-To: <4A64F37D.7020803@redhat.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Mon, 2009-07-20 at 17:45 -0500, Eric Sandeen wrote: > 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 :) I've spent a little while today digging into this. My guess (only a guess at this point until I have a chance to prove it) is that i_disksize should be updated by fallocate() even when KEEP_SIZE is specified. It's currently not updated in that case. It's my understanding that i_disksize should be the real allocation, right? While i_size is the size that has actually been used? If so, then setting i_disksize is probably what's missing. -- Frank Mayhar Google, Inc.