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

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Message-ID: <CAPcyv4irezimk8m4hysrd0rst_f0Rr+iiNxeFesqbxQnWYA2Xw@mail.gmail.com>
Date:   Thu, 16 Jan 2020 11:09:00 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Vivek Goyal <vgoyal@...hat.com>
Cc:     Jeff Moyer <jmoyer@...hat.com>, Jan Kara <jack@...e.cz>,
        "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 Thu, Jan 16, 2020 at 10:39 AM Vivek Goyal <vgoyal@...hat.com> wrote:
>
> On Thu, Jan 16, 2020 at 10:09:46AM -0800, Dan Williams wrote:
> > On Wed, Jan 15, 2020 at 1:08 PM Jeff Moyer <jmoyer@...hat.com> wrote:
> > >
> > > Hi, Dan,
> > >
> > > Dan Williams <dan.j.williams@...el.com> writes:
> > >
> > > > I'm going to take a look at how hard it would be to develop a kpartx
> > > > fallback in udev. If that can live across the driver transition then
> > > > maybe this can be a non-event for end users that already have that
> > > > udev update deployed.
> > >
> > > I just wanted to remind you that label-less dimms still exist, and are
> > > still being shipped.  For those devices, the only way to subdivide the
> > > storage is via partitioning.
> >
> > True, but if kpartx + udev can make this transparent then I don't
> > think users lose any functionality. They just gain a device-mapper
> > dependency.
>
> So udev rules will trigger when a /dev/pmemX device shows up and run
> kpartx which in turn will create dm-linear devices and device nodes
> will show up in /dev/mapper/pmemXpY.
>
> IOW, /dev/pmemXpY device nodes will be gone. So if any of the scripts or
> systemd unit files are depenent on /dev/pmemXpY, these will still be
> broken out of the box and will have to be modified to use device nodes
> in /dev/mapper/ directory instead. Do I understand it right, Or I missed
> the idea completely.

No, I'd write the udev rule to create links from /dev/pmemXpY to the
/dev/mapper device, and that rule would be gated by a new pmem device
attribute to trigger when kpartx needs to run vs the kernel native
partitions.