linux-kernel - Re: [PATCH 3/7] xfs: protect S_DAX transitions in XFS read path

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Date:   Tue, 26 Sep 2017 11:11:55 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...chiereds.net>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        linux-xfs@...r.kernel.org
Subject: Re: [PATCH 3/7] xfs: protect S_DAX transitions in XFS read path

On Tue, Sep 26, 2017 at 7:33 AM, Christoph Hellwig <hch@....de> wrote:
> On Tue, Sep 26, 2017 at 06:59:37AM -0700, Dan Williams wrote:
>> > I think you probably want an IOCB_DAX flag to check IS_DAX once and
>> > then stick to it, similar to what we do for direct I/O.
>>
>> I wonder if this works better with a reference count mechanism
>> per-file so that we don't need a hold a lock over the whole
>> transition. Similar to request_queue reference counting, when DAX is
>> being turned off we block new references and drain the in-flight ones.
>
> Maybe.  But that assumes we want to be stuck in a perpetual binary
> DAX on/off state on a given file.  Which makes not only for an awkward
> interface (inode or mount flag), but also might be fundamentally the
> wrong thing to do for some media where you'd happily read directly
> from it but rather buffer writes in DRAM.

I think we'll always need an explicit override available, but yes we
need to think about what the override looks like in the context of a
kernel that is able to automatically pick the right I/O policy
relative to the media type. A potential mixed policy for reads vs
writes makes sense. Where would this finer grained I/O policy
selection go other than more inode flags?