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Date: Mon, 24 Nov 2014 07:21:01 -0800
From: Christoph Hellwig <hch@infradead.org>
To: "Theodore Ts'o" <tytso@mit.edu>
Cc: linux-fsdevel@vger.kernel.org,
        Ext4 Developers List <linux-ext4@vger.kernel.org>, xfs@oss.sgi.com,
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Subject: Re: [PATCH 1/4] fs: split update_time() into update_time() and
 write_time()
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On Fri, Nov 21, 2014 at 02:59:21PM -0500, Theodore Ts'o wrote:
> We needed to preserve update_time() because btrfs wants to have a
> special btrfs_root_readonly() check; otherwise we could drop the
> update_time() inode operation entirely.

Can't btrfs just set the immutable flag on every inode that is read
when the root has the BTRFS_ROOT_SUBVOL_RDONLY flag?  That would
cut down the places that need this check to the ioctl path so that
we prevent users from clearling the immutable flag.