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

["Koralahalli Channabasappa, Smita" <skoralah@amd.com>]

Hi Jonathan,

On 3/9/2026 7:37 AM, Jonathan Cameron wrote:
> 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.

Yeah the wrapping is unnecessary I will drop it.

Thanks
Smita

> 
> 
> 
>>   	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);
>>   
>