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

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Date:   Tue, 11 Feb 2020 12:33:31 -0500
From:   Vivek Goyal <vgoyal@...hat.com>
To:     Dan Williams <dan.j.williams@...el.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 10:09:46AM -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.

Hi Dan,

Are you planning to look into making this work?

We can easily disable partition scanning by specifying gendisk
GENHD_FL_NO_PART_SCAN flag. But what about partition additiona path,
ioctl(BLKPG_ADD_PARTITION). That does not seem to do any checks whether
block device supports in kernel partitions or not. 

So kernel partitions (hence /dev/pmemXpY) objects are created anyway and
this will conflict with all the new planned udev rules.

If you block ioctl(BLKPG_ADD_PARTITION), then user space tools like
parted and fdisk started breaking when trying to create a partition
on /dev/pmeme0. IIUC, we have to allow partition table creation on
/dev/pmem0 so that later kpartx can parse it and create dm-linear
partitions.

Thanks
Vivek