Re: [PATCH] cxl/test: Map mock device nodes to an online node

On Thu, Jul 23, 2026 at 11:06:30AM -0700, Davidlohr Bueso wrote:
> cxl_mem_init() assigns alternating NUMA nodes to the mock memdevs
> to model multi-node topologies. On a single-node host node 1 has
> no pgdat, and the first node-aware allocation in an odd device's
> probe faults on a NULL zonelist:
> 
>   Oops: general protection fault, probably for non-canonical address 0xdffffc00000006e3
>   KASAN: probably user-memory-access in range [0x0000000000003718-0x000000000000371f]
>   Workqueue: async async_run_entry_fn
>   RIP: __alloc_frozen_pages_noprof+0x288
>    new_slab
>    ___slab_alloc
>    __kmalloc_node_track_caller_noprof
>    devm_kmalloc
>    cxl_mock_mem_probe
> 
> Map the requested node to an online one, as dax_hmem already does
> for its target node.
> 
> Fixes: cf1f6877b088 ("cxl/memdev: Add numa_node attribute")
> Fixes: e41c8452b9b2 ("tools/testing/cxl: Add a single-port host-bridge regression config")
> Fixes: c9435dbee119 ("tools/testing/cxl: Add an RCH topology")
> Signed-off-by: Davidlohr Bueso <[email protected]>
> ---
> Found while regression testing unrelated work.
> 
>  tools/testing/cxl/test/cxl.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/testing/cxl/test/cxl.c b/tools/testing/cxl/test/cxl.c
> index ef92dd35e030..5156516a82d2 100644
> --- a/tools/testing/cxl/test/cxl.c
> +++ b/tools/testing/cxl/test/cxl.c
> @@ -1748,7 +1748,7 @@ static int cxl_mem_init(void)
>  		if (!pdev)
>  			goto err_mem;
>  		pdev->dev.parent = &dport->dev;
> -		set_dev_node(&pdev->dev, i % 2);
> +		set_dev_node(&pdev->dev, numa_map_to_online_node(i % 2));
>  
>  		rc = cxl_mock_platform_device_add(pdev, &cxl_mem[i]);
>  		if (rc)
> @@ -1763,7 +1763,7 @@ static int cxl_mem_init(void)
>  		if (!pdev)
>  			goto err_single;
>  		pdev->dev.parent = &dport->dev;
> -		set_dev_node(&pdev->dev, i % 2);
> +		set_dev_node(&pdev->dev, numa_map_to_online_node(i % 2));
>  
>  		rc = cxl_mock_platform_device_add(pdev, &cxl_mem_single[i]);
>  		if (rc)
> @@ -1779,7 +1779,7 @@ static int cxl_mem_init(void)
>  		if (!pdev)
>  			goto err_rcd;
>  		pdev->dev.parent = &rch->dev;
> -		set_dev_node(&pdev->dev, i % 2);
> +		set_dev_node(&pdev->dev, numa_map_to_online_node(i % 2));
>  
>  		rc = cxl_mock_platform_device_add(pdev, &cxl_rcd[i]);
>  		if (rc)
> -- 
> 2.39.5
>

Hi Davidlohr,

I was hitting the same repeated crash while running `modprobe cxl_test` on a single-node virtme-ng guest.

For reference, the guest had only NUMA node 0 available and online:

# cat /sys/devices/system/node/possible 0  # cat /sys/devices/system/node/online 0

The crash followed the same path:

__alloc_frozen_pages_noprof  new_slab  ___slab_alloc  __kmalloc_node_track_caller_noprof  devm_kmalloc  cxl_mock_mem_probe

I found this patch while looking through the linux-cxl archives and applied the same `numa_map_to_online_node()` fix to my current cxl/next tree.

The code has changed since this patch was posted. The original patch updates three sites in `cxl_mem_init()`, while current cxl/next has those corresponding sites under `cxl_type3_mem_init()`, along with a separate `cxl_type2_mem_init()` path.

After applying the equivalent changes to the three `set_dev_node()` calls in `cxl_type3_mem_init()`, the repeated crash is resolved and `modprobe cxl_test` succeeds.

I also noticed that `cxl_type2_mem_init()` now has:


set_dev_node(&pdev->dev, i % 2);


`NR_CXL_TYPE2_ACCEL` is currently 1, so this does not trigger the offline-node case today, but perhaps it would be worth using `numa_map_to_online_node()` there as well for consistency and future expansion.

Tested-by: Shaikh Kamaluddin <[email protected]>

Thanks,
Shaikh