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

[Mike Rapoport <rppt@kernel.org>]

On Thu, Apr 16, 2026 at 07:25:58PM +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
> 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 Also, fix the outdated comment to match the actual return value.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
> ---
>  mm/numa_emulation.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/numa_emulation.c b/mm/numa_emulation.c
> index 703c8fa05048..c1d0a76aef64 100644
> --- a/mm/numa_emulation.c
> +++ b/mm/numa_emulation.c
> @@ -214,7 +214,7 @@ static u64 uniform_size(u64 max_addr, u64 base, u64 hole, int nr_nodes)
>   * Sets up fake nodes of `size' interleaved over physical nodes ranging from
>   * `addr' to `max_addr'.
>   *
> - * Returns zero on success or negative on error.
> + * Returns absolute node ID on success or negative on error.
>   */
>  static int __init split_nodes_size_interleave_uniform(struct numa_meminfo *ei,
>  					      struct numa_meminfo *pi,
> @@ -416,7 +416,7 @@ 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 (ret - nid < n) {
>  				pr_info("%s: phys: %d only got %d of %ld nodes, failing\n",
>  						__func__, i, ret, n);

The error message also should be updated, now it prints the last node ID
rather than number of created nodes. I think it's worse creating a
temporary variable for ret - nid to make the code clearer.

I'd also recommend running qemu without and with your patch and verifying
it works as intended.

>  				ret = -1;
> -- 
> 2.43.0
> 

-- 
Sincerely yours,
Mike.