From: Ted Ts'o <tytso@mit.edu>
Subject: Re: e4defrag modifies folder's mtime.
Date: Wed, 4 Jan 2012 00:27:18 -0500
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 <20120103203443.GC30502@thunk.org>
 <20120104045318.GA17026@dastard>
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Cc: Thomas Lindroth <thomas.lindroth@gmail.com>,
	linux-ext4@vger.kernel.org
To: Dave Chinner <david@fromorbit.com>
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On Wed, Jan 04, 2012 at 03:53:18PM +1100, Dave Chinner wrote:
> 
> xfs_fsr doesn't use these donor files to avoid mtime updates - it
> uses invisible IO (i.e. O_NOATIME|O_NOCMTIME) to do that.

O_NOCMTIME doesn't exist yet.  From include/linux/fs.h:

/*
 * Don't update ctime and mtime.
 *
 * Currently a special hack for the XFS open_by_handle ioctl, but we'll
 * hopefully graduate it to a proper O_CMTIME flag supported by open(2) soon.
 */
#define FMODE_NOCMTIME		((__force fmode_t)0x800)

I'd have to check, but it wouldn't surprise me if glibc is filtering
out all fcntl flags it doesn't understand, so for now, this is an
XFS-only special...

						- Ted