Date: Thu, 24 Nov 2016 20:39:00 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Fabian Frederick <fabf@skynet.be>
Cc: tytso@mit.edu, Jan Kara <jack@suse.com>, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1 linux-next] ext4: add compatibility flag check
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On Thu, Nov 24, 2016 at 08:47:41PM +0100, Fabian Frederick wrote:
> data=journal mount option should disable O_DIRECT access
> (See Documentation/filesystems/ext4.txt) but open operations
> using O_CREAT|O_RDWR|O_DIRECT|O_SYNC have no warning in return and file is being
> created. This patch adds vfs super_operations compatibility flag function
> returning -EPERM in such a case.

Why not simply check it in ->open()?  Occam's Razor and all such...