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

[Ira Weiny <ira.weiny@intel.com>]

John Groves 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>
> ---

[snip]

> diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
> index e4bd5c9f006c..562e2b06f61a 100644
> --- a/drivers/dax/bus.c
> +++ b/drivers/dax/bus.c
> @@ -81,6 +81,10 @@ static int dax_match_type(const struct dax_device_driver *dax_drv, struct device
>  	    !IS_ENABLED(CONFIG_DEV_DAX_KMEM))
>  		return 1;
>  
> +	/* fsdev driver can also bind to device-type dax devices */
> +	if (dax_drv->type == DAXDRV_FSDEV_TYPE && type == DAXDRV_DEVICE_TYPE)
> +		return 1;
> +

In building up a topic branch for this I notice that this breaks the
device-dax test.

1/1 ndctl:dax / device-dax FAIL             0.19s   (exit status 250 or signal 122 SIGinvalid)
...
Ok:                 0
Expected Fail:      0
Fail:               1
Unexpected Pass:    0
Skipped:            0
Timeout:            0

Have you run this series with all the ndctl cxl selftests?

How exactly is this supposed to ensure the fsdev driver does not bind to
a regular dax device?

This can be fixed by reloading the fsdev module thus pushing that driver
later in the driver list...  So we need to come up with a more reliable
method here.

I'm holding off pushing a topic branch for the time being.

Ira

>  	return 0;
>  }
>  

[snip]