linux-kernel - Re: [PATCH v2 12/30] dax: remove block device dependencies

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Message-ID: <20190516100737.GT29507@stefanha-x1.localdomain>
Date:   Thu, 16 May 2019 11:07:37 +0100
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Vivek Goyal <vgoyal@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Steven Whitehouse <swhiteho@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>
Subject: Re: [PATCH v2 12/30] dax: remove block device dependencies

On Wed, May 15, 2019 at 05:21:51PM -0700, Dan Williams wrote:
> On Wed, May 15, 2019 at 12:28 PM Vivek Goyal <vgoyal@...hat.com> wrote:
> >
> > From: Stefan Hajnoczi <stefanha@...hat.com>
> >
> > Although struct dax_device itself is not tied to a block device, some
> > DAX code assumes there is a block device.  Make block devices optional
> > by allowing bdev to be NULL in commonly used DAX APIs.
> >
> > When there is no block device:
> >  * Skip the partition offset calculation in bdev_dax_pgoff()
> >  * Skip the blkdev_issue_zeroout() optimization
> >
> > Note that more block device assumptions remain but I haven't reach those
> > code paths yet.
> >
> 
> Is there a generic object that non-block-based filesystems reference
> for physical storage as a bdev stand-in? I assume "sector_t" is still
> the common type for addressing filesystem capacity?
> 
> It just seems to me that we should stop pretending that the
> filesystem-dax facility requires block devices and try to move this
> functionality to generically use a dax device across all interfaces.

virtio-fs uses a PCI BAR called the DAX Window to access data.  This
object is internal to the virtio_fs.ko driver, not really a generic
object that DAX code can reference.

But does the DAX code need to reference any object at all?  It seems
like block device users just want callbacks for the partition offset
calculation and blkdev_issue_zeroout().

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