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

Jonathan Cameron
2026-03-09T14:37:51Z

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);
>