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From: Vivek Goyal <vgoyal@redhat.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Chinner <david@fromorbit.com>,
	Christoph Hellwig <hch@infradead.org>,
	Miklos Szeredi <miklos@szeredi.hu>,
	linux-nvdimm <linux-nvdimm@lists.01.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	virtio-fs@redhat.com, Stefan Hajnoczi <stefanha@redhat.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH 01/19] dax: remove block device dependencies
Date: Mon, 16 Dec 2019 13:10:14 -0500
Message-ID: <20191216181014.GA30106@redhat.com> (raw)
In-Reply-To: <CAPcyv4jGEAbYSJef2zLzgg6Arozsuz7eN_vZL1iTcd1XQuNT4Q@mail.gmail.com>

On Wed, Aug 28, 2019 at 05:04:11PM -0700, Dan Williams wrote:
> On Wed, Aug 28, 2019 at 3:53 PM Dave Chinner <david@fromorbit.com> wrote:
> >
> > On Wed, Aug 28, 2019 at 01:58:43PM -0400, Vivek Goyal wrote:
> > > On Tue, Aug 27, 2019 at 11:58:09PM -0700, Christoph Hellwig wrote:
> > > > On Tue, Aug 27, 2019 at 12:38:28PM -0400, Vivek Goyal wrote:
> > > > > > 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.
> > > >
> > > > Well, then we need to find a way to get partitions for dax devices,
> > > > as we really should not expect a block device hiding behind a dax
> > > > dev.  That is just a weird legacy assumption - block device need to
> > > > layer on top of the dax device optionally.
> > > >
> > > > >
> > > > > 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.
> > > >
> > > > And where would the file system get that information from?
> > >
> > > File system knows about block device, can it just call get_start_sect()
> > > while filling iomap->addr. And this means we don't have to have
> > > parition information in dax device. Will something like following work?
> > > (Just a proof of concept patch).
> > >
> > >
> > > ---
> > >  drivers/dax/super.c |   11 +++++++++++
> > >  fs/dax.c            |    6 +++---
> > >  fs/ext4/inode.c     |    6 +++++-
> > >  include/linux/dax.h |    1 +
> > >  4 files changed, 20 insertions(+), 4 deletions(-)
> > >
> > > Index: rhvgoyal-linux/fs/ext4/inode.c
> > > ===================================================================
> > > --- rhvgoyal-linux.orig/fs/ext4/inode.c       2019-08-28 13:51:16.051937204 -0400
> > > +++ rhvgoyal-linux/fs/ext4/inode.c    2019-08-28 13:51:44.453937204 -0400
> > > @@ -3589,7 +3589,11 @@ retry:
> > >                       WARN_ON_ONCE(1);
> > >                       return -EIO;
> > >               }
> > > -             iomap->addr = (u64)map.m_pblk << blkbits;
> > > +             if (IS_DAX(inode))
> > > +                     iomap->addr = ((u64)map.m_pblk << blkbits) +
> > > +                                   (get_start_sect(iomap->bdev) * 512);
> > > +             else
> > > +                     iomap->addr = (u64)map.m_pblk << blkbits;
> >
> > I'm not a fan of returning a physical device sector address from an
> > interface where ever other user/caller expects this address to be a
> > logical block address into the block device. It creates a landmine
> > in the iomap API that callers may not be aware of and that's going
> > to cause bugs. We're trying really hard to keep special case hacks
> > like this out of the iomap infrastructure, so on those grounds alone
> > I'd suggest this is a dead end approach.
> >
> > Hence I think that if the dax device needs a physical offset from
> > the start of the block device the filesystem sits on, it should be
> > set up at dax device instantiation time and so the filesystem/bdev
> > never needs to be queried again for this information.
> >
> 
> 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.

Thanks
Vivek
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  parent reply index

Thread overview: 77+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-21 17:57 [PATCH v3 00/19][RFC] virtio-fs: Enable DAX support Vivek Goyal
2019-08-21 17:57 ` [PATCH 01/19] dax: remove block device dependencies Vivek Goyal
2019-08-26 11:51   ` Christoph Hellwig
2019-08-27 16:38     ` Vivek Goyal
2019-08-28  6:58       ` Christoph Hellwig
2019-08-28 17:58         ` Vivek Goyal
2019-08-28 22:53           ` Dave Chinner
2019-08-29  0:04             ` Dan Williams
2019-08-29  9:32               ` Christoph Hellwig
2019-12-16 18:10               ` Vivek Goyal [this message]
2020-01-07 12:51                 ` Christoph Hellwig
2020-01-07 14:22                   ` Dan Williams
2020-01-07 17:07                     ` Darrick J. Wong
2020-01-07 17:29                       ` Dan Williams
2020-01-07 18:01                         ` Vivek Goyal
2020-01-07 18:07                           ` Dan Williams
2020-01-07 18:33                             ` Vivek Goyal
2020-01-07 18:49                               ` Dan Williams
2020-01-07 19:02                                 ` Darrick J. Wong
2020-01-07 19:46                                   ` Dan Williams
2020-01-07 23:38                                     ` Dan Williams
2020-01-09 11:24                                 ` Jan Kara
2020-01-09 20:03                                   ` Dan Williams
2020-01-10 12:36                                     ` Christoph Hellwig
2020-01-14 20:31                                     ` Vivek Goyal
2020-01-14 20:39                                       ` Dan Williams
2020-01-14 21:28                                         ` Vivek Goyal
2020-01-14 22:23                                           ` Dan Williams
2020-01-15 19:56                                             ` Vivek Goyal
2020-01-15 20:17                                               ` Dan Williams
2020-01-15 21:08                                                 ` Jeff Moyer
2020-01-16 18:09                                                   ` Dan Williams
2020-01-16 18:39                                                     ` Vivek Goyal
2020-01-16 19:09                                                       ` Dan Williams
2020-01-16 19:23                                                         ` Vivek Goyal
2020-02-11 17:33                                                     ` Vivek Goyal
2020-01-15  9:03                                           ` Jan Kara
2019-08-21 17:57 ` [PATCH 02/19] dax: Pass dax_dev to dax_writeback_mapping_range() Vivek Goyal
2019-08-26 11:53   ` Christoph Hellwig
2019-08-26 20:33     ` Vivek Goyal
2019-08-26 20:58       ` Vivek Goyal
2019-08-26 21:33         ` Dan Williams
2019-08-28  6:58         ` Christoph Hellwig
2020-01-03 14:12         ` Vivek Goyal
2020-01-03 18:12           ` Dan Williams
2020-01-03 18:18             ` Dan Williams
2020-01-03 18:33               ` Vivek Goyal
2020-01-03 19:30                 ` Dan Williams
2020-01-03 18:43               ` Vivek Goyal
2019-08-27 13:45       ` Jan Kara
2019-08-21 17:57 ` [PATCH 03/19] virtio: Add get_shm_region method Vivek Goyal
2019-08-21 17:57 ` [PATCH 04/19] virtio: Implement get_shm_region for PCI transport Vivek Goyal
2019-08-26  1:43   ` [Virtio-fs] " piaojun
2019-08-26 13:06     ` Vivek Goyal
2019-08-27  9:41       ` piaojun
2019-08-27  8:34   ` Cornelia Huck
2019-08-27  8:46     ` Cornelia Huck
2019-08-27 11:53     ` Vivek Goyal
2019-08-21 17:57 ` [PATCH 05/19] virtio: Implement get_shm_region for MMIO transport Vivek Goyal
2019-08-27  8:39   ` Cornelia Huck
2019-08-27 11:54     ` Vivek Goyal
2019-08-21 17:57 ` [PATCH 06/19] fuse, dax: add fuse_conn->dax_dev field Vivek Goyal
2019-08-21 17:57 ` [PATCH 07/19] virtio_fs, dax: Set up virtio_fs dax_device Vivek Goyal
2019-08-21 17:57 ` [PATCH 08/19] fuse: Keep a list of free dax memory ranges Vivek Goyal
2019-08-21 17:57 ` [PATCH 09/19] fuse: implement FUSE_INIT map_alignment field Vivek Goyal
2019-08-21 17:57 ` [PATCH 10/19] fuse: Introduce setupmapping/removemapping commands Vivek Goyal
2019-08-21 17:57 ` [PATCH 11/19] fuse, dax: Implement dax read/write operations Vivek Goyal
2019-08-21 19:49   ` Liu Bo
2019-08-22 12:59     ` Vivek Goyal
2019-08-21 17:57 ` [PATCH 12/19] fuse, dax: add DAX mmap support Vivek Goyal
2019-08-21 17:57 ` [PATCH 13/19] fuse: Define dax address space operations Vivek Goyal
2019-08-21 17:57 ` [PATCH 14/19] fuse, dax: Take ->i_mmap_sem lock during dax page fault Vivek Goyal
2019-08-21 17:57 ` [PATCH 15/19] fuse: Maintain a list of busy elements Vivek Goyal
2019-08-21 17:57 ` [PATCH 16/19] dax: Create a range version of dax_layout_busy_page() Vivek Goyal
2019-08-21 17:57 ` [PATCH 17/19] fuse: Add logic to free up a memory range Vivek Goyal
2019-08-21 17:57 ` [PATCH 18/19] fuse: Release file in process context Vivek Goyal
2019-08-21 17:57 ` [PATCH 19/19] fuse: Take inode lock for dax inode truncation Vivek Goyal

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