Re: [PATCH v6 8/9] dax/hmem, cxl: Defer and resolve ownership of Soft Reserved memory ranges

Koralahalli Channabasappa, Smita
2026-02-20T19:54:57Z

On 2/18/2026 10:05 AM, Dave Jiang wrote:
>
>
> On 2/9/26 11:45 PM, Smita Koralahalli wrote:
>> The current probe time ownership check for Soft Reserved memory based
>> solely on CXL window intersection is insufficient. dax_hmem probing is not
>> always guaranteed to run after CXL enumeration and region assembly, which
>> can lead to incorrect ownership decisions before the CXL stack has
>> finished publishing windows and assembling committed regions.
>>
>> Introduce deferred ownership handling for Soft Reserved ranges that
>> intersect CXL windows. When such a range is encountered during dax_hmem
>> probe, schedule deferred work and wait for the CXL stack to complete
>> enumeration and region assembly before deciding ownership.
>>
>> Evaluate ownership of Soft Reserved ranges based on CXL region
>> containment.
>>
>> - If all Soft Reserved ranges are fully contained within committed CXL
>> regions, DROP handling Soft Reserved ranges from dax_hmem and allow
>> dax_cxl to bind.
>>
>> - If any Soft Reserved range is not fully claimed by committed CXL
>> region, REGISTER the Soft Reserved ranges with dax_hmem.
>>
>> Use dax_cxl_mode to coordinate ownership decisions for Soft Reserved
>> ranges. Once, ownership resolution is complete, flush the deferred work
>> from dax_cxl before allowing dax_cxl to bind.
>>
>> This enforces a strict ownership. Either CXL fully claims the Soft
>> reserved ranges or it relinquishes it entirely.
>>
>> 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>
>> ---
>> drivers/dax/bus.c | 3 ++
>> drivers/dax/bus.h | 19 ++++++++++
>> drivers/dax/cxl.c | 1 +
>> drivers/dax/hmem/hmem.c | 78 +++++++++++++++++++++++++++++++++++++++--
>> 4 files changed, 99 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
>> index 92b88952ede1..81985bcc70f9 100644
>> --- a/drivers/dax/bus.c
>> +++ b/drivers/dax/bus.c
>> @@ -25,6 +25,9 @@ DECLARE_RWSEM(dax_region_rwsem);
>> */
>> DECLARE_RWSEM(dax_dev_rwsem);
>>
>> +enum dax_cxl_mode dax_cxl_mode = DAX_CXL_MODE_DEFER;
>> +EXPORT_SYMBOL_NS_GPL(dax_cxl_mode, "CXL");
>> +
>> static DEFINE_MUTEX(dax_hmem_lock);
>> static dax_hmem_deferred_fn hmem_deferred_fn;
>> static void *dax_hmem_data;
>> diff --git a/drivers/dax/bus.h b/drivers/dax/bus.h
>> index b58a88e8089c..82616ff52fd1 100644
>> --- a/drivers/dax/bus.h
>> +++ b/drivers/dax/bus.h
>> @@ -41,6 +41,25 @@ struct dax_device_driver {
>> void (*remove)(struct dev_dax *dev);
>> };
>>
>> +/*
>> + * enum dax_cxl_mode - State machine to determine ownership for CXL
>> + * tagged Soft Reserved memory ranges.
>> + * @DAX_CXL_MODE_DEFER: Ownership resolution pending. Set while waiting
>> + * for CXL enumeration and region assembly to complete.
>> + * @DAX_CXL_MODE_REGISTER: CXL regions do not fully cover Soft Reserved
>> + * ranges. Fall back to registering those ranges via dax_hmem.
>> + * @DAX_CXL_MODE_DROP: All Soft Reserved ranges intersecting CXL windows
>> + * are fully contained within committed CXL regions. Drop HMEM handling
>> + * and allow dax_cxl to bind.
>> + */
>> +enum dax_cxl_mode {
>> + DAX_CXL_MODE_DEFER,
>> + DAX_CXL_MODE_REGISTER,
>> + DAX_CXL_MODE_DROP,
>> +};
>> +
>> +extern enum dax_cxl_mode dax_cxl_mode;
>> +
>> typedef void (*dax_hmem_deferred_fn)(void *data);
>>
>> int dax_hmem_register_work(dax_hmem_deferred_fn fn, void *data);
>> diff --git a/drivers/dax/cxl.c b/drivers/dax/cxl.c
>> index a2136adfa186..3ab39b77843d 100644
>> --- a/drivers/dax/cxl.c
>> +++ b/drivers/dax/cxl.c
>> @@ -44,6 +44,7 @@ static struct cxl_driver cxl_dax_region_driver = {
>>
>> static void cxl_dax_region_driver_register(struct work_struct *work)
>> {
>> + dax_hmem_flush_work();
>> cxl_driver_register(&cxl_dax_region_driver);
>> }
>>
>> diff --git a/drivers/dax/hmem/hmem.c b/drivers/dax/hmem/hmem.c
>> index 1e3424358490..85854e25254b 100644
>> --- a/drivers/dax/hmem/hmem.c
>> +++ b/drivers/dax/hmem/hmem.c
>> @@ -3,6 +3,7 @@
>> #include <linux/memregion.h>
>> #include <linux/module.h>
>> #include <linux/dax.h>
>> +#include <cxl/cxl.h>
>> #include "../bus.h"
>>
>> static bool region_idle;
>> @@ -69,8 +70,18 @@ static int hmem_register_device(struct device *host, int target_nid,
>> if (IS_ENABLED(CONFIG_DEV_DAX_CXL) &&
>> region_intersects(res->start, resource_size(res), IORESOURCE_MEM,
>> IORES_DESC_CXL) != REGION_DISJOINT) {
>> - dev_dbg(host, "deferring range to CXL: %pr\n", res);
>> - return 0;
>> + switch (dax_cxl_mode) {
>> + case DAX_CXL_MODE_DEFER:
>> + dev_dbg(host, "deferring range to CXL: %pr\n", res);
>> + dax_hmem_queue_work();
>> + return 0;
>> + case DAX_CXL_MODE_REGISTER:
>> + dev_dbg(host, "registering CXL range: %pr\n", res);
>> + break;
>> + case DAX_CXL_MODE_DROP:
>> + dev_dbg(host, "dropping CXL range: %pr\n", res);
>> + return 0;
>> + }
>> }
>>
>> rc = region_intersects_soft_reserve(res->start, resource_size(res));
>> @@ -123,8 +134,70 @@ static int hmem_register_device(struct device *host, int target_nid,
>> return rc;
>> }
>>
>> +static int hmem_register_cxl_device(struct device *host, int target_nid,
>> + const struct resource *res)
>> +{
>> + if (region_intersects(res->start, resource_size(res), IORESOURCE_MEM,
>> + IORES_DESC_CXL) != REGION_DISJOINT)
>> + return hmem_register_device(host, target_nid, res);
>> +
>> + return 0;
>> +}
>> +
>> +static int soft_reserve_has_cxl_match(struct device *host, int target_nid,
>> + const struct resource *res)
>> +{
>> + if (region_intersects(res->start, resource_size(res), IORESOURCE_MEM,
>> + IORES_DESC_CXL) != REGION_DISJOINT) {
>> + if (!cxl_region_contains_soft_reserve((struct resource *)res))
>> + return 1;
>> + }
>> +
>> + return 0;
>> +}
>> +
>> +static void process_defer_work(void *data)
>> +{
>> + struct platform_device *pdev = data;
>> + int rc;
>> +
>> + /* relies on cxl_acpi and cxl_pci having had a chance to load */
>> + wait_for_device_probe();
>> +
>> + rc = walk_hmem_resources(&pdev->dev, soft_reserve_has_cxl_match);
>> +
>> + if (!rc) {
>> + dax_cxl_mode = DAX_CXL_MODE_DROP;
>> + dev_dbg(&pdev->dev, "All Soft Reserved ranges claimed by CXL\n");
>> + } else {
>> + dax_cxl_mode = DAX_CXL_MODE_REGISTER;
>> + dev_warn(&pdev->dev,
>> + "Soft Reserved not fully contained in CXL; using HMEM\n");
>> + }
>> +
>> + walk_hmem_resources(&pdev->dev, hmem_register_cxl_device);
>> +}
>> +
>> +static void kill_defer_work(void *data)
>> +{
>> + struct platform_device *pdev = data;
>> +
>> + dax_hmem_flush_work();
>> + dax_hmem_unregister_work(process_defer_work, pdev);
>> +}
>> +
>> static int dax_hmem_platform_probe(struct platform_device *pdev)
>> {
>> + int rc;
>> +
>> + rc = dax_hmem_register_work(process_defer_work, pdev);
>
> Do we need to take a reference on pdev when we queue the work?
>
> DJ

I thought it might not be required. But correct me if I'm wrong.

There is only one hmem_platform device. Also devm_add_action_or_reset()
registers kill_defer_work(), which calls flush_work() before the device
is torn down. So pdev cannot be freed while the deferred work is still
in progress. flush_work() blocks until process_defer_work() has fully
returned, and only then does device removal proceed.

But this needs a deadlock fix which Gregory pointed. If probe fails
after work is already queued, the devres cleanup calls flush_work()
which blocks on wait_for_device_probe() while still inside probe
context. I will fix in v7.

Thanks
Smita

>
>> + if (rc)
>> + return rc;
>> +
>> + rc = devm_add_action_or_reset(&pdev->dev, kill_defer_work, pdev);
>> + if (rc)
>> + return rc;
>> +
>> return walk_hmem_resources(&pdev->dev, hmem_register_device);
>> }
>>
>> @@ -174,3 +247,4 @@ MODULE_ALIAS("platform:hmem_platform*");
>> MODULE_DESCRIPTION("HMEM DAX: direct access to 'specific purpose' memory");
>> MODULE_LICENSE("GPL v2");
>> MODULE_AUTHOR("Intel Corporation");
>> +MODULE_IMPORT_NS("CXL");
>