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Date:   Mon, 7 Aug 2017 17:24:08 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Christoph Hellwig <hch@...radead.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Dave Chinner <david@...morbit.com>, linux-xfs@...r.kernel.org,
        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 Tue, Aug 1, 2017 at 4:26 AM, Jan Kara <jack@...e.cz> wrote:
> On Tue 01-08-17 04:02:41, Christoph Hellwig wrote:
>> On Fri, Jul 28, 2017 at 11:38:21AM +0200, Jan Kara wrote:
>> > Well, you are right I can make the implementation work with struct file
>> > flag as well - let's call it O_DAXDSYNC. However there are filesystem
>> > operations where you may need to answer question: Is there any fd with
>> > O_DAXDSYNC open against this inode (for operations that change file offset
>> > -> block mapping)? And in that case inode flag is straightforward while
>> > file flag is a bit awkward (you need to implement counter of fd's with that
>> > flag in the inode).
>>
>> We can still keep and inode flag as the internal implementation
>> detail.  As mentioned earlier the right flag to control behavior
>> of a mapping is an mmap flag.  And the initial naive implementation
>> would simply mark the inode as sync once the first MAP_SYNC open happens
>> on it.  We could then move to more precise tracking if/when needed.
>
> OK, makes sense and I like the MAP_SYNC proposal. I'll change it in my
> implementation.

Does sys_mmap() reject unknown flag values today? I'm either not
looking in the right place or it's missing and we'll need some
interface/mechanism to check if MAP_SYNC is honored.

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