Re: [PATCH v2] mm/fake-numa: fix under-allocation detection in uniform split

[Mike Rapoport <rppt@kernel.org>]

Hi,

On Fri, Apr 17, 2026 at 10:58:05PM +0900, Sang-Heon Jeon wrote:
> When split NUMA node uniformly, split_nodes_size_interleave_uniform()
> returns the next absolute node ID, not the number of nodes created.
> 
> The existing under-allocation detection logic compares next absolute node
> ID (ret) and request count (n), which only works when nid starts at 0.
> 
> For example, on a system with 2 physical NUMA nodes (node 0: 2GB, node
> 1: 128MB) and numa=fake=8U, 8 fake nodes are successfully created from
> node 0 and split_nodes_size_interleave_uniform() returns 8. For node 1,
> fake node nid starts at 8, but only 4 fake nodes are created due to
> current FAKE_NODE_MIN_SIZE being 32MB, and
> split_nodes_size_interleave_uniform() returns 12. By existing
> under-allocation detection logic, "ret < n" (12 < 8) is false, so the

In this example it would be 11, won't it?
I'll update when applying.

> under-allocation will not be detected.
> 
> Fix under-allocation detection logic to compare the number of actually
> created nodes (ret - nid) against the request count (n). Also skip
> under-allocation detection logic for memoryless physical nodes where no
> fake nodes are created.
> 
> Also, fix the outdated comment to match the actual return value.
> 
> Signed-off-by: Sang-Heon Jeon <ekffu200098@gmail.com>
> Reported-by: Donghyeon Lee <asd142513@gmail.com>
> Reported-by: Munhui Chae <mochae@student.42seoul.kr>
> Fixes: cc9aec03e58f ("x86/numa_emulation: Introduce uniform split capability") # 4.19

...

> @@ -416,9 +416,18 @@ void __init numa_emulation(struct numa_meminfo *numa_meminfo, int numa_dist_cnt)
>  					n, &pi.blk[0], nid);
>  			if (ret < 0)
>  				break;
> -			if (ret < n) {
> +
> +			/*
> +			 * If no memory was found for this physical node,
> +			 * skip the under-allocation check. 

checkpatch complains about trailing white space here.
I'll fix it up when applying.

> +			 */
> +			if (ret == nid)
> +				continue;
> +
> +			nr_created = ret - nid;
> +			if (nr_created < n) {
>  				pr_info("%s: phys: %d only got %d of %ld nodes, failing\n",
> -						__func__, i, ret, n);
> +						__func__, i, nr_created, n);
>  				ret = -1;
>  				break;
>  			}
> -- 
> 2.43.0
> 

-- 
Sincerely yours,
Mike.