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

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Message-ID: <CAPcyv4hKb1PshbjLxyWz2fdj=dK2fi2qgJLFaT9pVnmaOoWV6g@mail.gmail.com>
Date:   Tue, 26 Sep 2017 06:59:37 -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 Mon, Sep 25, 2017 at 11:32 PM, Christoph Hellwig <hch@....de> wrote:
> We can't just take locking one level up, as we need differnet locking
> for different kinds of I/O.
>
> 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.