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From: Dave Chinner <david@fromorbit.com>
To: Vivek Goyal <vgoyal@redhat.com>
Cc: virtio-fs@redhat.com, linux-nvdimm@lists.01.org,
	miklos@szeredi.hu, linux-kernel@vger.kernel.org,
	dgilbert@redhat.com, Christoph Hellwig <hch@infradead.org>,
	stefanha@redhat.com, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 01/19] dax: remove block device dependencies
Date: Thu, 29 Aug 2019 08:53:22 +1000
Message-ID: <20190828225322.GA7777@dread.disaster.area> (raw)
In-Reply-To: <20190828175843.GB912@redhat.com>

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.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com
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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 [this message]
2019-08-29  0:04             ` Dan Williams
2019-08-29  9:32               ` Christoph Hellwig
2019-12-16 18:10               ` Vivek Goyal
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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