Re: [PATCH V9 3/8] dax: add fsdev.c driver for fs-dax on character dax

[John Groves <John@groves.net>]

On 26/03/25 11:04AM, Ira Weiny wrote:
> John Groves wrote:
> > On 26/03/24 02:39PM, Jonathan Cameron wrote:
> > > On Tue, 24 Mar 2026 00:38:31 +0000
> > > John Groves <john@jagalactic.com> wrote:
> > > 
> > > > From: John Groves <john@groves.net>
> > > > 
> > > > The new fsdev driver provides pages/folios initialized compatibly with
> > > > fsdax - normal rather than devdax-style refcounting, and starting out
> > > > with order-0 folios.
> > > > 
> > > > When fsdev binds to a daxdev, it is usually (always?) switching from the
> > > > devdax mode (device.c), which pre-initializes compound folios according
> > > > to its alignment. Fsdev uses fsdev_clear_folio_state() to switch the
> > > > folios into a fsdax-compatible state.
> > > > 
> > > > A side effect of this is that raw mmap doesn't (can't?) work on an fsdev
> > > > dax instance. Accordingly, The fsdev driver does not provide raw mmap -
> > > > devices must be put in 'devdax' mode (drivers/dax/device.c) to get raw
> > > > mmap capability.
> > > > 
> > > > In this commit is just the framework, which remaps pages/folios compatibly
> > > > with fsdax.
> > > > 
> > > > Enabling dax changes:
> > > > 
> > > > - bus.h: add DAXDRV_FSDEV_TYPE driver type
> > > > - bus.c: allow DAXDRV_FSDEV_TYPE drivers to bind to daxdevs
> > > > - dax.h: prototype inode_dax(), which fsdev needs
> > > > 
> > > > Suggested-by: Dan Williams <dan.j.williams@intel.com>
> > > > Suggested-by: Gregory Price <gourry@gourry.net>
> > > > Signed-off-by: John Groves <john@groves.net>
> > > 
> > > I was kind of thinking you'd go with a hidden KCONFIG option with default
> > > magic to do the same build condition to you had in the Makefil, but one the
> > > user can opt in or out for is also fine.
> > > 
> > > Comments on that below. Meh, I think this is better anyway :)
> > > 
> > > Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
> > > 
> > > 
> > > 
> > > > diff --git a/drivers/dax/Kconfig b/drivers/dax/Kconfig
> > > > index d656e4c0eb84..7051b70980d5 100644
> > > > --- a/drivers/dax/Kconfig
> > > > +++ b/drivers/dax/Kconfig
> > > > @@ -61,6 +61,17 @@ config DEV_DAX_HMEM_DEVICES
> > > >  	depends on DEV_DAX_HMEM && DAX
> > > >  	def_bool y
> > > >  
> > > > +config DEV_DAX_FSDEV
> > > > +	tristate "FSDEV DAX: fs-dax compatible devdax driver"
> > > > +	depends on DEV_DAX && FS_DAX
> > > > +	help
> > > > +	  Support fs-dax access to DAX devices via a character device
> > > > +	  interface. Unlike device_dax (which pre-initializes compound folios
> > > > +	  based on device alignment), this driver leaves folios at order-0 so
> > > > +	  that fs-dax filesystems can manage folio order dynamically.
> > > > +
> > > > +	  Say M if unsure.
> > > Fine like this, but if you wanted to hide it in interests of not
> > > confusing users...
> > > 
> > > config DEV_DAX_FSDEV
> > > 	tristate
> > > 	depends on DEV_DAX && FS_DAX
> > > 	default DEV_DAX
> > 
> > I like this better. I see no reason not to default to including fsdev.
> > It does nothing other than frustrating famfs users if it's off - since
> > building it still has no effect unless you put a daxdev in famfs mode.
> > 
> > Ira, it's kinda in your hands at the moment. Do you feel like making this
> > change?
> 
> I don't mind making this change.  But we have to deal with the breakage to
> current device dax users.
> 
> https://lore.kernel.org/all/69c36921255b6_e9d8d1009b@iweiny-mobl.notmuch/
> 
> What am I missing?
> 
> Ira

OK, I can reproduce that failure with kernel 7.0.0-rc5 and 
straight ndctl v84. So it's not famfs.

I also studied the verbose logs trying to figure out if famfs
could cause it (while running a famfs kernel and ndctl), but
I don't see it.

Then I tried non-famfs kernel and ndctl and it's the same with
or without famfs kernel and famfs ndctl.

John