Hi Dan,
On 3/19/2026 4:07 PM, Dan Williams wrote:
Koralahalli Channabasappa, Smita wrote:
[..]
I agree with Jonathan's comments in Patch 6, using __WORK_INITIALIZER or
initializing work in dax_hmem_init() and gating flush on pdev will fix
the WARN — I will add both for v8. But I think the WARN is likely
indicating an ordering issue here..
Yes, Jonathan is right, static initialization is also my expecation.
On initial boot, the Makefile ordering ensures dax_hmem_init() runs
before cxl_dax_region_init(), so both work items land on system_long_wq
in the right order and dax_hmem's deferred work is queued before
dax_cxl's driver registration work.
There is nothing that guarantees that 2 work items in system_long_wq run
in submission order. Unlikely that matters given the explicit flushing.
On module reload which Alison is trying here I dont think, modules are
loaded by Makefile order. I think dax_cxl's workqueue is calling
dax_hmem_flush_work() before dax_hmem probe has had a chance to queue
its work, so flush_work() flushes nothing and dax_cxl registers its
driver without waiting.
Module load order does not matter after initial probe completion.
...and dax_hmem is guaranteed to always load before dax_cxl due to the
symbol dependency of dax_hmem_flush_work().
__WORK_INITIALIZER fixes the WARN, but doesn't fix the race I guess if
we are hitting that here..
[ 34.673051] initcall dax_hmem_init+0x0/0xff0 [dax_hmem] returned 0
after 2225 usecs
[ 34.676011] calling cxl_dax_region_init+0x0/0xff0 [dax_cxl] @ 1059
These two lines indicate cxl_dax started after dax_hmem_init() returns
but I dont think that guarantees dax_hmem_platform_probe() has actually
run..
I dont know if wait_for_device_probe() in cxl_dax_region_driver_register
might help..
Thanks
Smita
Actually, thinking about this more..
dax_hmem_initial_probe lives in device.c (built-in) so it survives
module reload. On reload it's still true from the first boot. This means
hmem_register_device() skips the deferral path entirely..
Yes, that is the expectation.
The problem is this bypasses the cxl_region_contains_resource() check
that the deferred work normally does. On first boot,
process_defer_work() walks each range and decides per-range: if CXL
covers it, skip. If not, register with HMEM. On reload, that check never
happens — whoever registers first via alloc_dax_region() wins,
regardless of whether CXL actually covers the range.
Yes, I think you have hit on a real issue. There is no point in having
dax_hmem auto-attach on driver reload. If userspace unloads the driver
it gets to keep the pieces. So that means something like this:
diff --git a/drivers/dax/hmem/hmem.c b/drivers/dax/hmem/hmem.c
index 15e462589b92..7478bc78a698 100644
--- a/drivers/dax/hmem/hmem.c
+++ b/drivers/dax/hmem/hmem.c
@@ -112,10 +112,12 @@ static int hmem_register_device(struct device *host, int
target_nid,
region_intersects(res->start, resource_size(res), IORESOURCE_MEM,
IORES_DESC_CXL) != REGION_DISJOINT) {
if (!dax_hmem_initial_probe) {
- dev_dbg(host, "deferring range to CXL: %pr\n", res);
+ dev_dbg(host, "await CXL initial probe: %pr\n", res);
queue_work(system_long_wq, &dax_hmem_work.work);
return 0;
}
+ dev_dbg(host, "deferring range to CXL: %pr\n", res);
+ return 0;
}
One issue with the reload fix - At boot, hmem_register_cxl_device()
calls hmem_register_device() to register ranges that aren't claimed by
CXL. But hmem_register_device() now always returns 0 for those ranges at
boot.
I was thinking factoring out registration logic into
__hmem_register_device() and have hmem_register_cxl_device() call that
directly, bypassing the CXL gating. Something like:
+static int __hmem_register_device(...)
+{
+ /* Remaining in hmem_register_device after the CXL check */
+}
static int hmem_register_device(..)
{
if (IS_ENABLED(CONFIG_DEV_DAX_CXL) && .. {
+ if (!dax_hmem_initial_probe_done) {
+ queue_work(system_long_wq, &dax_hmem_work);
+ return 0;
+ }
+ return 0;
+ }
+ return __hmem_register_device(host, target_nid, res);
}
+static int hmem_register_cxl_device(...)
+{
...
+ return __hmem_register_device(host, target_nid, res);
+}
+static void process_defer_work(...)
+{
+ ...
- dax_hmem_initial_probe = true;
- walk_hmem_resources(&pdev->dev, hmem_register_cxl_device);
+ if (!dax_hmem_initial_probe) {
+ dax_hmem_initial_probe = true;
+ walk_hmem_resources(.., hmem_register_cxl_device);
+ }
..
+}
Tracing it
At boot:
probe -> walk(hmem_register_device)
CXL range, !dax_hmem_initial_probe -> queue_work, return 0
non-CXL ranges -> __hmem_register_device -> registers
process_defer_work:
!dax_hmem_initial_probe
dax_hmem_initial_probe = true
walk(hmem_register_cxl_device)
CXL covers -> return 0
CXL doesn't cover -> __hmem_register_device()
no CXL check again, straight to registration..
On reload:
probe -> walk(hmem_register_device)
CXL range, dax_hmem_initial_probe = true, "your return 0" -> skips
non-CXL ranges -> __hmem_register_device -> registers
process_defer_work:
dax_hmem_initial_probe = true -> skip the walk entirely..
Or do you think this can be simplified better and the above approach has
some caveats?
Thanks
Smita
rc = region_intersects_soft_reserve(res->start, resource_size(res));
---
...because if userspace wants to reload the dax_hmem driver, then it
needs to pick what happens with the CXL intersection. Userspace can
always unload cxl_acpi to force everything back to dax_hmem.
Now, you might say, "but this means that if the initial probe results in
a partial result of some regions in dax_hmem and others in dax_cxl, that
state can not be recovered outside of a reboot". I think that is ok.
This mechanism is automatic best-effort workaround for bugs / missing
capabilities in the CXL driver. Module reload fidelity is out of scope.
So if dax_cxl registers first on reload, it could claim a range that CXL
doesn't actually cover, and dax_hmem would lose a range it should own..
With the above change, dax_cxl always wins in the "reload" scenario iff
cxl_acpi is loaded. Otherwise dax_hmem owns all the Soft Reserved.
I dont know if Im thinking through this right..
You definitely identified the need for that fixup above.
