Re: [PATCH V9 7/8] dax: Add fs_dax_get() func to prepare dax for fs-dax usage

[Dave Jiang <dave.jiang@intel.com>]

On 3/23/26 5:39 PM, John Groves wrote:
> From: John Groves <john@groves.net>
> 
> The fs_dax_get() function should be called by fs-dax file systems after
> opening a fsdev dax device. This adds holder_operations, which provides
> a memory failure callback path and effects exclusivity between callers
> of fs_dax_get().
> 
> fs_dax_get() is specific to fsdev_dax, so it checks the driver type
> (which required touching bus.[ch]). fs_dax_get() fails if fsdev_dax is
> not bound to the memory.
> 
> This function serves the same role as fs_dax_get_by_bdev(), which dax
> file systems call after opening the pmem block device.
> 
> This can't be located in fsdev.c because struct dax_device is opaque
> there.
> 
> This will be called by fs/fuse/famfs.c in a subsequent commit.
> 
> Signed-off-by: John Groves <john@groves.net>

Reviewed-by: Dave Jiang <dave.jiang@intel.com>

> ---
>  drivers/dax/bus.c   |  2 --
>  drivers/dax/bus.h   |  2 ++
>  drivers/dax/super.c | 66 ++++++++++++++++++++++++++++++++++++++++++++-
>  include/linux/dax.h | 17 +++++++++---
>  4 files changed, 80 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
> index 562e2b06f61a..8a8710a8234e 100644
> --- a/drivers/dax/bus.c
> +++ b/drivers/dax/bus.c
> @@ -39,8 +39,6 @@ static int dax_bus_uevent(const struct device *dev, struct kobj_uevent_env *env)
>  	return add_uevent_var(env, "MODALIAS=" DAX_DEVICE_MODALIAS_FMT, 0);
>  }
>  
> -#define to_dax_drv(__drv)	container_of_const(__drv, struct dax_device_driver, drv)
> -
>  static struct dax_id *__dax_match_id(const struct dax_device_driver *dax_drv,
>  		const char *dev_name)
>  {
> diff --git a/drivers/dax/bus.h b/drivers/dax/bus.h
> index 880bdf7e72d7..dc6f112ac4a4 100644
> --- a/drivers/dax/bus.h
> +++ b/drivers/dax/bus.h
> @@ -42,6 +42,8 @@ struct dax_device_driver {
>  	void (*remove)(struct dev_dax *dev);
>  };
>  
> +#define to_dax_drv(__drv) container_of_const(__drv, struct dax_device_driver, drv)
> +
>  int __dax_driver_register(struct dax_device_driver *dax_drv,
>  		struct module *module, const char *mod_name);
>  #define dax_driver_register(driver) \
> diff --git a/drivers/dax/super.c b/drivers/dax/super.c
> index ba0b4cd18a77..d4ab60c406bf 100644
> --- a/drivers/dax/super.c
> +++ b/drivers/dax/super.c
> @@ -14,6 +14,7 @@
>  #include <linux/fs.h>
>  #include <linux/cacheinfo.h>
>  #include "dax-private.h"
> +#include "bus.h"
>  
>  /**
>   * struct dax_device - anchor object for dax services
> @@ -111,6 +112,10 @@ struct dax_device *fs_dax_get_by_bdev(struct block_device *bdev, u64 *start_off,
>  }
>  EXPORT_SYMBOL_GPL(fs_dax_get_by_bdev);
>  
> +#endif /* CONFIG_BLOCK && CONFIG_FS_DAX */
> +
> +#if IS_ENABLED(CONFIG_FS_DAX)
> +
>  void fs_put_dax(struct dax_device *dax_dev, void *holder)
>  {
>  	if (dax_dev && holder &&
> @@ -119,7 +124,66 @@ void fs_put_dax(struct dax_device *dax_dev, void *holder)
>  	put_dax(dax_dev);
>  }
>  EXPORT_SYMBOL_GPL(fs_put_dax);
> -#endif /* CONFIG_BLOCK && CONFIG_FS_DAX */
> +
> +/**
> + * fs_dax_get() - get ownership of a devdax via holder/holder_ops
> + *
> + * 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.
> + *
> + * @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)
> +{
> +	struct dev_dax *dev_dax;
> +	struct dax_device_driver *dax_drv;
> +	int id;
> +
> +	id = dax_read_lock();
> +	if (!dax_dev || !dax_alive(dax_dev) || !igrab(&dax_dev->inode)) {
> +		dax_read_unlock(id);
> +		return -ENODEV;
> +	}
> +	dax_read_unlock(id);
> +
> +	/* Verify the device is bound to fsdev_dax driver */
> +	dev_dax = dax_get_private(dax_dev);
> +	if (!dev_dax) {
> +		iput(&dax_dev->inode);
> +		return -ENODEV;
> +	}
> +
> +	device_lock(&dev_dax->dev);
> +	if (!dev_dax->dev.driver) {
> +		device_unlock(&dev_dax->dev);
> +		iput(&dax_dev->inode);
> +		return -ENODEV;
> +	}
> +	dax_drv = to_dax_drv(dev_dax->dev.driver);
> +	if (dax_drv->type != DAXDRV_FSDEV_TYPE) {
> +		device_unlock(&dev_dax->dev);
> +		iput(&dax_dev->inode);
> +		return -EOPNOTSUPP;
> +	}
> +	device_unlock(&dev_dax->dev);
> +
> +	if (cmpxchg(&dax_dev->holder_data, NULL, holder)) {
> +		iput(&dax_dev->inode);
> +		return -EBUSY;
> +	}
> +
> +	dax_dev->holder_ops = hops;
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(fs_dax_get);
> +#endif /* CONFIG_FS_DAX */
>  
>  enum dax_device_flags {
>  	/* !alive + rcu grace period == no new operations / mappings */
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index b19bfe0c2fd1..bf37b9a982f3 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -130,7 +130,6 @@ int dax_add_host(struct dax_device *dax_dev, struct gendisk *disk);
>  void dax_remove_host(struct gendisk *disk);
>  struct dax_device *fs_dax_get_by_bdev(struct block_device *bdev, u64 *start_off,
>  		void *holder, const struct dax_holder_operations *ops);
> -void fs_put_dax(struct dax_device *dax_dev, void *holder);
>  #else
>  static inline int dax_add_host(struct dax_device *dax_dev, struct gendisk *disk)
>  {
> @@ -145,12 +144,13 @@ static inline struct dax_device *fs_dax_get_by_bdev(struct block_device *bdev,
>  {
>  	return NULL;
>  }
> -static inline void fs_put_dax(struct dax_device *dax_dev, void *holder)
> -{
> -}
>  #endif /* CONFIG_BLOCK && CONFIG_FS_DAX */
>  
>  #if IS_ENABLED(CONFIG_FS_DAX)
> +void fs_put_dax(struct dax_device *dax_dev, void *holder);
> +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);
>  int dax_writeback_mapping_range(struct address_space *mapping,
>  		struct dax_device *dax_dev, struct writeback_control *wbc);
>  int dax_folio_reset_order(struct folio *folio);
> @@ -164,6 +164,15 @@ dax_entry_t dax_lock_mapping_entry(struct address_space *mapping,
>  void dax_unlock_mapping_entry(struct address_space *mapping,
>  		unsigned long index, dax_entry_t cookie);
>  #else
> +static inline void fs_put_dax(struct dax_device *dax_dev, void *holder)
> +{
> +}
> +
> +static inline int fs_dax_get(struct dax_device *dax_dev, void *holder,
> +			     const struct dax_holder_operations *hops)
> +{
> +	return -EOPNOTSUPP;
> +}
>  static inline struct page *dax_layout_busy_page(struct address_space *mapping)
>  {
>  	return NULL;