On Wed, Feb 11, 2026 at 06:33:20PM +0800, Cui Chao wrote:
> In some physical memory layout designs, the address space of CFMW (CXL
> Fixed Memory Window) resides between multiple segments of system memory
> belonging to the same NUMA node. In numa_cleanup_meminfo, these multiple
> segments of system memory are merged into a larger numa_memblk. When
> identifying which NUMA node the CFMW belongs to, it may be incorrectly
> assigned to the NUMA node of the merged system memory.
>
> When a CXL RAM region is created in userspace, the memory capacity of
> the newly created region is not added to the CFMW-dedicated NUMA node.
> Instead, it is accumulated into an existing NUMA node (e.g., NUMA0
> containing RAM). This makes it impossible to clearly distinguish
> between the two types of memory, which may affect memory-tiering
> applications.
>
> Example memory layout:
>
> Physical address space:
> 0x00000000 - 0x1FFFFFFF System RAM (node0)
> 0x20000000 - 0x2FFFFFFF CXL CFMW (node2)
> 0x40000000 - 0x5FFFFFFF System RAM (node0)
> 0x60000000 - 0x7FFFFFFF System RAM (node1)
>
> After numa_cleanup_meminfo, the two node0 segments are merged into one:
> 0x00000000 - 0x5FFFFFFF System RAM (node0) // CFMW is inside the range
> 0x60000000 - 0x7FFFFFFF System RAM (node1)
>
> So the CFMW (0x20000000-0x2FFFFFFF) will be incorrectly assigned to node0.
>
> To address this scenario, accurately identifying the correct NUMA node
> can be achieved by checking whether the region belongs to both
> numa_meminfo and numa_reserved_meminfo.
>
Changelog comments after this are a bit much but other than that
>
> Fixes: 779dd20cfb56 ("cxl/region: Add region creation support")
> Signed-off-by: Cui Chao <[email protected]>
> Reviewed-by: Jonathan Cameron <[email protected]>
Reviewed-by: Gregory Price <[email protected]>
