Re: [PATCH V4 8/9] dax: replace exported dax_dev_get() with non-allocating dax_dev_find()

[Richard Cheng <icheng@nvidia.com>]

On Sun, Jun 07, 2026 at 07:34:21PM +0800, John Groves wrote:
> From: John Groves <John@Groves.net>
> 
> This fix is in response to a Sashiko review, and some subsequent
> analysis.
> 
> dax_dev_get() uses iget5_locked() which creates a new inode if no
> matching one exists. This is correct for the internal caller
> (alloc_dax), but dangerous for external callers that look up devices
> from user-supplied or metadata-supplied dev_t values:
> 
> 1. A new inode is created with DAXDEV_ALIVE set but no backing driver,
>    no ops, and no IDA-allocated minor number.
> 
> 2. On teardown, dax_destroy_inode() warns because kill_dax() was never
>    called, and dax_free_inode() calls ida_free() for a minor that was
>    never ida_alloc'd -- potentially freeing the minor of a real device.
> 
> Add dax_dev_find() which uses ilookup5() for lookup-only semantics:
> it returns an existing dax_device with an elevated inode reference, or
> NULL if no device with the given dev_t exists. It never creates inodes.
> A dax_alive() check under dax_read_lock() guards against returning a
> device that is concurrently being torn down by kill_dax().
> 
> Make dax_dev_get() static again (internal to super.c for alloc_dax),
> export dax_dev_find() instead, and update the two external callers
> (famfs_inode.c, famfs.c). Also add the missing CONFIG_DAX=n stub.
> 
> About the 'fixes' tag: this removes the export of dax_dev_get(),
> which was flawed, and replaces is with dax_dev_find(). It feels like
> the fixes tag makes sense for correcting an ABI error.
> 
> Fixes: 2ae624d5a555d ("dax: export dax_dev_get()")
> 
> Reviewed-by: Dave Jiang <dave.jiang@intel.com>
> Reviewed-by: Alison Schofield <alison.schofield@intel.com>
> Signed-off-by: John Groves <john@groves.net>
> ---
>  drivers/dax/super.c | 38 ++++++++++++++++++++++++++++++++++++--
>  include/linux/dax.h |  6 +++++-
>  2 files changed, 41 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/dax/super.c b/drivers/dax/super.c
> index 96f778dcde50b..b37ae79c084bb 100644
> --- a/drivers/dax/super.c
> +++ b/drivers/dax/super.c
> @@ -557,7 +557,7 @@ static int dax_set(struct inode *inode, void *data)
>  	return 0;
>  }
>  
> -struct dax_device *dax_dev_get(dev_t devt)
> +static struct dax_device *dax_dev_get(dev_t devt)
>  {
>  	struct dax_device *dax_dev;
>  	struct inode *inode;
> @@ -580,7 +580,41 @@ struct dax_device *dax_dev_get(dev_t devt)
>  
>  	return dax_dev;
>  }
> -EXPORT_SYMBOL_GPL(dax_dev_get);
> +
> +/**
> + * dax_dev_find - look up an existing dax_device by dev_t
> + * @devt: the device number to find
> + *
> + * Returns a dax_device with an elevated inode reference, or NULL if no
> + * device with the given dev_t exists. Unlike dax_dev_get(), this never
> + * allocates a new inode — it is safe for external callers that are looking
> + * up devices from user-supplied or metadata-supplied dev_t values.
> + *
> + * Caller must put_dax() the returned device when done.
> + */
> +struct dax_device *dax_dev_find(dev_t devt)
> +{
> +	struct dax_device *dax_dev;
> +	struct inode *inode;
> +	int id;
> +
> +	inode = ilookup5(dax_superblock, hash_32(devt + DAXFS_MAGIC, 31),
> +			 dax_test, &devt);
> +	if (!inode)
> +		return NULL;
> +
> +	dax_dev = to_dax_dev(inode);
> +	id = dax_read_lock();
> +	if (!dax_alive(dax_dev)) {
> +		dax_read_unlock(id);
> +		iput(inode);
> +		return NULL;
> +	}
> +	dax_read_unlock(id);
> +
> +	return dax_dev;
> +}
> +EXPORT_SYMBOL_GPL(dax_dev_find);
>

For now I see no in-tree caller of this function, the famfs users you mention doesn't
exists yet. Cloud the dax_dev_find() addition+export go with the famfs series that
actually uses it, and this patch just do the dax_dev_get() de-export ?

--Richard
  
>  struct dax_device *alloc_dax(void *private, const struct dax_operations *ops)
>  {
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index fe6c3ded1b50f..29113eb95e72d 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -54,7 +54,7 @@ struct dax_device *alloc_dax(void *private, const struct dax_operations *ops);
>  void *dax_holder(struct dax_device *dax_dev);
>  void put_dax(struct dax_device *dax_dev);
>  void kill_dax(struct dax_device *dax_dev);
> -struct dax_device *dax_dev_get(dev_t devt);
> +struct dax_device *dax_dev_find(dev_t devt);
>  void dax_write_cache(struct dax_device *dax_dev, bool wc);
>  bool dax_write_cache_enabled(struct dax_device *dax_dev);
>  bool dax_synchronous(struct dax_device *dax_dev);
> @@ -92,6 +92,10 @@ static inline void put_dax(struct dax_device *dax_dev)
>  static inline void kill_dax(struct dax_device *dax_dev)
>  {
>  }
> +static inline struct dax_device *dax_dev_find(dev_t devt)
> +{
> +	return NULL;
> +}
>  static inline void dax_write_cache(struct dax_device *dax_dev, bool wc)
>  {
>  }
> -- 
> 2.53.0
> 
> 
>