From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [RFC PATCH 0/7] dax, ext4: Synchronous page faults
Date: Sun, 13 Aug 2017 02:25:56 -0700
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Cc: Christoph Hellwig <hch@infradead.org>, Jan Kara <jack@suse.cz>,
        linux-nvdimm <linux-nvdimm@lists.01.org>,
        Dave Chinner <david@fromorbit.com>, linux-xfs@vger.kernel.org,
        Andy Lutomirski <luto@kernel.org>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        "linux-ext4@vger.kernel.org" <linux-ext4@vger.kernel.org>
To: Dan Williams <dan.j.williams@intel.com>
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On Sat, Aug 12, 2017 at 07:44:14PM -0700, Dan Williams wrote:
> How about MAP_SYNC == (MAP_SHARED|MAP_PRIVATE)? On older kernels that
> should get -EINVAL, and on new kernels it means SYNC+SHARED.

Cute trick, but I'd hate to waster it just for our little flag.

How about:

#define __MAP_VALIDATE		MAP_SHARED|MAP_PRIVATE
#define MAP_SYNC		0x??? | __MAP_VALIDATE

so that we can reuse that trick for any new flag?