Re: [PATCH v6 5/9] dax: Track all dax_region allocations under a global resource tree

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

On Tue, 10 Feb 2026 06:44:57 +0000
Smita Koralahalli <Smita.KoralahalliChannabasappa@amd.com> wrote:

> Introduce a global "DAX Regions" resource root and register each
> dax_region->res under it via request_resource(). Release the resource on
> dax_region teardown.
> 
> By enforcing a single global namespace for dax_region allocations, this
> ensures only one of dax_hmem or dax_cxl can successfully register a
> dax_region for a given range.
> 
> Co-developed-by: Dan Williams <dan.j.williams@intel.com>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> Signed-off-by: Smita Koralahalli <Smita.KoralahalliChannabasappa@amd.com>

One question inline about the locking.

Is intent to serialize beyond this new resource tree?  If it's just
the resource tree the write_lock(&resource_lock); in the request
and release_resource() should be sufficient.  

> ---
>  drivers/dax/bus.c | 23 ++++++++++++++++++++---
>  1 file changed, 20 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
> index fde29e0ad68b..5f387feb95f0 100644
> --- a/drivers/dax/bus.c
> +++ b/drivers/dax/bus.c
> @@ -10,6 +10,7 @@
>  #include "dax-private.h"
>  #include "bus.h"
>  
> +static struct resource dax_regions = DEFINE_RES_MEM_NAMED(0, -1, "DAX Regions");
>  static DEFINE_MUTEX(dax_bus_lock);
>  
>  /*
> @@ -625,6 +626,8 @@ static void dax_region_unregister(void *region)
>  {
>  	struct dax_region *dax_region = region;
>  
> +	scoped_guard(rwsem_write, &dax_region_rwsem)
> +		release_resource(&dax_region->res);

Do we need the locking? resource stuff all runs under the global
resource_lock so if aim is just to serialize adds and removes that should
be enough. Maybe there is a justification in that being an internal
implementation detail.



>  	sysfs_remove_groups(&dax_region->dev->kobj,
>  			dax_region_attribute_groups);
>  	dax_region_put(dax_region);
> @@ -635,6 +638,7 @@ struct dax_region *alloc_dax_region(struct device *parent, int region_id,
>  		unsigned long flags)
>  {
>  	struct dax_region *dax_region;
> +	int rc;
>  
>  	/*
>  	 * The DAX core assumes that it can store its private data in
> @@ -667,14 +671,27 @@ struct dax_region *alloc_dax_region(struct device *parent, int region_id,
>  		.flags = IORESOURCE_MEM | flags,
>  	};
>  
> -	if (sysfs_create_groups(&parent->kobj, dax_region_attribute_groups)) {
> -		kfree(dax_region);
> -		return NULL;
> +	scoped_guard(rwsem_write, &dax_region_rwsem)
> +		rc = request_resource(&dax_regions, &dax_region->res);
> +	if (rc) {
> +		dev_dbg(parent, "dax_region resource conflict for %pR\n",
> +			&dax_region->res);
> +		goto err_res;
>  	}
>  
> +	if (sysfs_create_groups(&parent->kobj, dax_region_attribute_groups))
> +		goto err_sysfs;
> +
>  	if (devm_add_action_or_reset(parent, dax_region_unregister, dax_region))
>  		return NULL;
>  	return dax_region;
> +
> +err_sysfs:
> +	scoped_guard(rwsem_write, &dax_region_rwsem)
> +		release_resource(&dax_region->res);
> +err_res:
> +	kfree(dax_region);
> +	return NULL;
>  }
>  EXPORT_SYMBOL_GPL(alloc_dax_region);
>