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

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Message-ID: <20190827163828.GA6859@redhat.com>
Date:   Tue, 27 Aug 2019 12:38:28 -0400
From:   Vivek Goyal <vgoyal@...hat.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvdimm@...ts.01.org, virtio-fs@...hat.com, miklos@...redi.hu,
        stefanha@...hat.com, dgilbert@...hat.com,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH 01/19] dax: remove block device dependencies

On Mon, Aug 26, 2019 at 04:51:52AM -0700, Christoph Hellwig wrote:
> On Wed, Aug 21, 2019 at 01:57:02PM -0400, Vivek Goyal 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.
> 
> I think this should be split into two patches.

Hi Christoph,

Ok, will split in two patches. In fact, I think will completley drop
the second change right now as I think we might not be hitting that
path yet.

> For bdev_dax_pgoff
> I'd much rather have the partition offset if there is on in the daxdev
> somehow so that we can get rid of the block device entirely.

IIUC, there is one block_device per partition while there is only one
dax_device for the whole disk. So we can't directly move bdev logical
offset into dax_device.

We probably could put this in "iomap" and leave it to filesystems to
report offset into dax_dev in iomap that way dax generic code does not
have to deal with it. But that probably will be a bigger change.

Did I misunderstand your suggestion.

> 
> Similarly for dax_range_is_aligned I'd rather have a pure dax way
> to offload zeroing rather than this bdev hack.

Following commig introduced the change to write zeros through block
device path.

commit 4b0228fa1d753f77fe0e6cf4c41398ec77dfbd2a
Author: Vishal Verma <vishal.l.verma@...el.com>
Date:   Thu Apr 21 15:13:46 2016 -0400

 dax: for truncate/hole-punch, do zeroing through the driver if possible

IIUC, they are doing it so that they can clear gendisk->badblocks list.

So even if there is pure dax way to do it, there will have to some
involvment of block layer to clear gendisk->badblocks list.

I am not sure I fully understand your suggestion. But I am hoping its
not a must for these changes to make a progress. For now, I will drop
change to dax_range_is_aligned().

Thanks
Vivek