From: Ted Ts'o <tytso@mit.edu>
Subject: Re: [PATCH 13/23] ext4: Add new feature to make block group
 checksums use metadata_csum algorithm
Date: Mon, 13 Feb 2012 17:28:28 -0500
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Cc: "Darrick J. Wong" <djwong@us.ibm.com>,
	Sunil Mushran <sunil.mushran@oracle.com>,
	Martin K Petersen <martin.petersen@oracle.com>,
	Greg Freemyer <greg.freemyer@gmail.com>,
	Amir Goldstein <amir73il@gmail.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Andi Kleen <andi@firstfloor.org>,
	Mingming Cao <cmm@us.ibm.com>,
	Joel Becker <jlbec@evilplan.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-ext4@vger.kernel.org, Coly Li <colyli@gmail.com>
To: Andreas Dilger <adilger.kernel@dilger.ca>
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On Mon, Feb 13, 2012 at 02:40:26PM -0700, Andreas Dilger wrote:
> 
> If a kernel understands METADATA_CSUM, it will check this first and
> ignore whether GDT_CSUM is set or not (though it shouldn't ever be
> set at the same time).  Either of these features will cause an older
> kernel to mount the filesystem read-only, which is all that is needed.

... note that when we check for the uninit bits, this will have to be
done if GDT_CSUM || METADATA_CSUM are set.

					- Ted