linux-kernel - Re: [PATCH 01/19] dax: remove block device dependencies

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Message-ID: <CAPcyv4ggH7-QhYg+YOOWn_m25uds+-0L46=N09ap-LALeGuU_A@mail.gmail.com>
Date:   Tue, 7 Jan 2020 09:29:17 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Vivek Goyal <vgoyal@...hat.com>,
        Dave Chinner <david@...morbit.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        virtio-fs@...hat.com, Stefan Hajnoczi <stefanha@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 01/19] dax: remove block device dependencies

On Tue, Jan 7, 2020 at 9:08 AM Darrick J. Wong <darrick.wong@...cle.com> wrote:
>
> On Tue, Jan 07, 2020 at 06:22:54AM -0800, Dan Williams wrote:
> > On Tue, Jan 7, 2020 at 4:52 AM Christoph Hellwig <hch@...radead.org> wrote:
> > >
> > > On Mon, Dec 16, 2019 at 01:10:14PM -0500, Vivek Goyal wrote:
> > > > > Agree. In retrospect it was my laziness in the dax-device
> > > > > implementation to expect the block-device to be available.
> > > > >
> > > > > It looks like fs_dax_get_by_bdev() is an intercept point where a
> > > > > dax_device could be dynamically created to represent the subset range
> > > > > indicated by the block-device partition. That would open up more
> > > > > cleanup opportunities.
> > > >
> > > > Hi Dan,
> > > >
> > > > After a long time I got time to look at it again. Want to work on this
> > > > cleanup so that I can make progress with virtiofs DAX paches.
> > > >
> > > > I am not sure I understand the requirements fully. I see that right now
> > > > dax_device is created per device and all block partitions refer to it. If
> > > > we want to create one dax_device per partition, then it looks like this
> > > > will be structured more along the lines how block layer handles disk and
> > > > partitions. (One gendisk for disk and block_devices for partitions,
> > > > including partition 0). That probably means state belong to whole device
> > > > will be in common structure say dax_device_common, and per partition state
> > > > will be in dax_device and dax_device can carry a pointer to
> > > > dax_device_common.
> > > >
> > > > I am also not sure what does it mean to partition dax devices. How will
> > > > partitions be exported to user space.
> > >
> > > Dan, last time we talked you agreed that partitioned dax devices are
> > > rather pointless IIRC.  Should we just deprecate partitions on DAX
> > > devices and then remove them after a cycle or two?
> >
> > That does seem a better plan than trying to force partition support
> > where it is not needed.
>
> Question: if one /did/ have a partitioned DAX device and used kpartx to
> create dm-linear devices for each partition, will DAX still work through
> that?

The device-mapper support will continue, but it will be limited to
whole device sub-components. I.e. you could use kpartx to carve up
/dev/pmem0 and still have dax, but not partitions of /dev/pmem0.