Re: [PATCH V7 07/19] dax: Add fs_dax_get() func to prepare dax for fs-dax usage

Dave Jiang
2026-02-19T16:07:58Z

On 1/18/26 3:32 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>
> ---
> drivers/dax/bus.c | 2 --
> drivers/dax/bus.h | 2 ++
> drivers/dax/super.c | 58 ++++++++++++++++++++++++++++++++++++++++++++-
> include/linux/dax.h | 20 ++++++++++------
> 4 files changed, 72 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
> index e79daf825b52..01402d5103ef 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..00c330ef437c 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 +124,58 @@ 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 || !dev_dax->dev.driver) {

Don't you need to hold the dev_dax->dev device lock in order to check the driver?

DJ

> + 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 /* 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 5aaaca135737..6897c5736543 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -52,9 +52,6 @@ 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_FS)
> -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);
> @@ -134,7 +131,6 @@ 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)
> {
> @@ -149,12 +145,13 @@ 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);
> @@ -168,6 +165,15 @@ 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;