linux-kernel - Re: [PATCH v2 1/1] mm: numa_memblks: Identify the accurate NUMA ID of CFMW

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Message-ID: <9132054c-3017-4af0-84e0-e4359b0794a6@phytium.com.cn>
Date: Thu, 15 Jan 2026 17:43:02 +0800
From: Cui Chao <cuichao1753@...tium.com.cn>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Jonathan Cameron <Jonathan.Cameron@...wei.com>,
 Mike Rapoport <rppt@...nel.org>, Wang Yinfeng <wangyinfeng@...tium.com.cn>,
 linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 1/1] mm: numa_memblks: Identify the accurate NUMA ID of
 CFMW

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.

On 1/9/2026 1:48 AM, Andrew Morton wrote:
> On Tue,  6 Jan 2026 11:10:42 +0800 Cui Chao <cuichao1753@...tium.com.cn> wrote:
>
>> In some physical memory layout designs, the address space of CFMW
>> 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.
>>
>> 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.
> Thanks.
>
> Can you please help us understand the userspace-visible runtime effects
> of this incorrect assignment?

-- 
Best regards,
Cui Chao.