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

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Date:   Tue, 7 Jan 2020 13:01:01 -0500
From:   Vivek Goyal <vgoyal@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        Christoph Hellwig <hch@...radead.org>,
        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 07, 2020 at 09:29:17AM -0800, Dan Williams wrote:
> 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.

So we can't use fdisk/parted to partition /dev/pmem0. Given /dev/pmem0
is a block device, I thought tools will expect it to be partitioned.
Sometimes I create those partitions and use /dev/pmem0. So what's
the replacement for this. People often have tools/scripts which might
want to partition the device and these will start failing. 

IOW, I do not understand that why being able to partition /dev/pmem0
(which is a block device from user space point of view), is pointless.

Thanks
Vivek