Re: [RFC V2 02/18] dev_dax_iomap: Add fs_dax_get() func to prepare dax for fs-dax usage

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

On Thu,  3 Jul 2025 13:50:16 -0500
John Groves <John@Groves.net> wrote:

> This function should be called by fs-dax file systems after opening the
> devdax device. This adds holder_operations, which effects exclusivity
> between callers of fs_dax_get().
> 
> This function serves the same role as fs_dax_get_by_bdev(), which dax
> file systems call after opening the pmem block device.
> 
> This also adds the CONFIG_DEV_DAX_IOMAP Kconfig parameter
> 
> Signed-off-by: John Groves <john@groves.net>
Trivial stuff inline.


> ---
>  drivers/dax/Kconfig |  6 ++++++
>  drivers/dax/super.c | 30 ++++++++++++++++++++++++++++++
>  include/linux/dax.h |  5 +++++
>  3 files changed, 41 insertions(+)
> 
> diff --git a/drivers/dax/Kconfig b/drivers/dax/Kconfig
> index d656e4c0eb84..ad19fa966b8b 100644
> --- a/drivers/dax/Kconfig
> +++ b/drivers/dax/Kconfig
> @@ -78,4 +78,10 @@ config DEV_DAX_KMEM
>  
>  	  Say N if unsure.
>  
> +config DEV_DAX_IOMAP
> +       depends on DEV_DAX && DAX
> +       def_bool y
> +       help
> +         Support iomap mapping of devdax devices (for FS-DAX file
> +         systems that reside on character /dev/dax devices)
>  endif
> diff --git a/drivers/dax/super.c b/drivers/dax/super.c
> index e16d1d40d773..48bab9b5f341 100644
> --- a/drivers/dax/super.c
> +++ b/drivers/dax/super.c
> @@ -122,6 +122,36 @@ void fs_put_dax(struct dax_device *dax_dev, void *holder)
>  EXPORT_SYMBOL_GPL(fs_put_dax);
>  #endif /* CONFIG_BLOCK && CONFIG_FS_DAX */
>  
> +#if IS_ENABLED(CONFIG_DEV_DAX_IOMAP)
> +/**
> + * fs_dax_get()

Trivial but from what I recall kernel-doc isn't going to like this.
Needs a short description.

> + *
> + * fs-dax file systems call this function to prepare to use a devdax device for
> + * fsdax. This is like fs_dax_get_by_bdev(), but the caller already has struct
> + * dev_dax (and there  * is no bdev). The holder makes this exclusive.

there is no *bdev?  So * in wrong place.

> + *
> + * @dax_dev: dev to be prepared for fs-dax usage
> + * @holder: filesystem or mapped device inside the dax_device
> + * @hops: operations for the inner holder
> + *
> + * Returns: 0 on success, <0 on failure
> + */
> +int fs_dax_get(struct dax_device *dax_dev, void *holder,
> +	const struct dax_holder_operations *hops)
> +{
> +	if (!dax_dev || !dax_alive(dax_dev) || !igrab(&dax_dev->inode))
> +		return -ENODEV;
> +
> +	if (cmpxchg(&dax_dev->holder_data, NULL, holder))
> +		return -EBUSY;
> +
> +	dax_dev->holder_ops = hops;
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(fs_dax_get);
> +#endif /* DEV_DAX_IOMAP */
> +
>  enum dax_device_flags {
>  	/* !alive + rcu grace period == no new operations / mappings */
>  	DAXDEV_ALIVE,
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index df41a0017b31..86bf5922f1b0 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -51,6 +51,11 @@ struct dax_holder_operations {
>  
>  #if IS_ENABLED(CONFIG_DAX)
>  struct dax_device *alloc_dax(void *private, const struct dax_operations *ops);
> +
> +#if IS_ENABLED(CONFIG_DEV_DAX_IOMAP)
> +int fs_dax_get(struct dax_device *dax_dev, void *holder, const struct dax_holder_operations *hops);
> +struct dax_device *inode_dax(struct inode *inode);
> +#endif
>  void *dax_holder(struct dax_device *dax_dev);
>  void put_dax(struct dax_device *dax_dev);
>  void kill_dax(struct dax_device *dax_dev);