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

[Dan Williams]

On Wed, Aug 28, 2019 at 3:53 PM Dave Chinner <david@xxxxxxxxxxxxx> 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.