linux-ext4 - Re: [RFC PATCH 0/7] dax, ext4: Synchronous page faults

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Message-ID: <CAA9_cmd=fVP5n=Z=QThZEbCnWptNv2Amv6W7ujGG+O7VtXPs6g@mail.gmail.com>
Date:   Sun, 13 Aug 2017 10:08:41 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-xfs@...r.kernel.org, Jan Kara <jack@...e.cz>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Dave Chinner <david@...morbit.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: [RFC PATCH 0/7] dax, ext4: Synchronous page faults

On Sun, Aug 13, 2017 at 2:25 AM, Christoph Hellwig <hch@...radead.org> wrote:
> 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?

Yes, even better. I think that effectively kicks the need for mmap3()
down the road a bit.