Re: [PATCH V3 06/21] dax: Add fs_dax_get() func to prepare dax for fs-dax usage

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

On Wed,  7 Jan 2026 09:33:15 -0600
John Groves <John@Groves.net> wrote:

> 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>
Hi John,

A few passing comments on this one.

Jonathan

> ---

>  #define dax_driver_register(driver) \
> diff --git a/drivers/dax/super.c b/drivers/dax/super.c
> index ba0b4cd18a77..68c45b918cff 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
> @@ -121,6 +122,59 @@ 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_FS)
> +/**
> + * 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();

Given this is an srcu_read_lock under the hood you could do similar
to the DEFINE_LOCK_GUARD_1 for the srcu (srcu.h) (though here it's a
DEFINE_LOCK_GUARD_0 given the lock itself isn't a parameter and then
use scoped_guard() here.  Might not be worth the hassle and would need
a wrapper macro to poke &dax_srcu in which means exposing that at least
a little in a header.

DEFINE_LOCK_GUARD_0(_T->idx = dax_read_lock, dax_read_lock(_T->idx), idx);
Based loosely on the irqflags.h irqsave one. 

> +	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 || !dev_dax->dev.driver) {
> +		iput(&dax_dev->inode);
> +		return -ENODEV;
> +	}
> +
> +	dax_drv = to_dax_drv(dev_dax->dev.driver);
> +	if (dax_drv->type != DAXDRV_FSDEV_TYPE) {
> +		iput(&dax_dev->inode);
> +		return -EOPNOTSUPP;
> +	}
> +
> +	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 /* DEV_DAX_FS */
> +
>  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 3fcd8562b72b..76f2a75f3144 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -53,6 +53,7 @@ struct dax_holder_operations {
>  struct dax_device *alloc_dax(void *private, const struct dax_operations *ops);
>  
>  #if IS_ENABLED(CONFIG_DEV_DAX_FS)
> +int fs_dax_get(struct dax_device *dax_dev, void *holder, const struct dax_holder_operations *hops);
I'd wrap this.  It's rather long and there isn't a huge readability benefit in keeping
it on one line.
>  struct dax_device *inode_dax(struct inode *inode);
>  #endif
>  void *dax_holder(struct dax_device *dax_dev);