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

[Sang-Heon Jeon <ekffu200098@gmail.com>]

On Fri, Apr 17, 2026 at 8:41 PM Sang-Heon Jeon <ekffu200098@gmail.com> 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 Also, fix the outdated comment to match the actual return value.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
> ---
> Changes from RFC v1 [1]
> - Merge patchset into once.
> - Change base from linux-next to mm-unstable
>
> Changes from RFC v2 [2]
> - Fix error message to use the number of created node instead of
>   returned node ID.
> - Define nr_created variable to explicitly show the number of created
>   noeds.
>
> [1] https://lore.kernel.org/all/20260413154438.396031-1-ekffu200098@gmail.com/
> [2] https://lore.kernel.org/all/20260416102558.575210-1-ekffu200098@gmail.com/
> ---
> QEMU-based test results for the commit message scenario.
>
> 1) AS-IS (before fix)
> [    0.001878] NUMA: Node 0 [mem 0x00001000-0x0009ffff] + [mem 0x00100000-0x7fffffff] ]
> [    0.001881] Fake node size 255MB too small, increasing to 256MB
> [    0.001882] Faking node 0 at [mem 0x0000000000001000-0x0000000010000fff] (256MB)
> [    0.001883] Faking node 1 at [mem 0x0000000010001000-0x0000000020000fff] (256MB)
> [    0.001883] Faking node 2 at [mem 0x0000000020001000-0x0000000030000fff] (256MB)
> [    0.001884] Faking node 3 at [mem 0x0000000030001000-0x0000000040000fff] (256MB)
> [    0.001884] Faking node 4 at [mem 0x0000000040001000-0x0000000050000fff] (256MB)
> [    0.001884] Faking node 5 at [mem 0x0000000050001000-0x0000000060000fff] (256MB)
> [    0.001885] Faking node 6 at [mem 0x0000000060001000-0x0000000070000fff] (256MB)
> [    0.001885] Faking node 7 at [mem 0x0000000070001000-0x000000007fffffff] (255MB)
> [    0.001885] Fake node size 15MB too small, increasing to 32MB
> [    0.001886] Faking node 8 at [mem 0x0000000080000000-0x0000000081ffffff] (32MB)
> [    0.001886] Faking node 9 at [mem 0x0000000082000000-0x0000000083ffffff] (32MB)
> [    0.001887] Faking node 10 at [mem 0x0000000084000000-0x0000000087fdcfff] (63MB)
> [    0.001924] NODE_DATA(0) allocated [mem 0x0fffd6c0-0x10000fff]
> [    0.019852] NODE_DATA(1) allocated [mem 0x1fffd6c0-0x20000fff]
> [    0.022458] NODE_DATA(2) allocated [mem 0x2dffc6c0-0x2dffffff]
> [    0.023293] NODE_DATA(3) allocated [mem 0x3fffd6c0-0x40000fff]
> [    0.028522] NODE_DATA(4) allocated [mem 0x4fffd6c0-0x50000fff]
> [    0.032397] NODE_DATA(5) allocated [mem 0x5fffd6c0-0x60000fff]
> [    0.036552] NODE_DATA(6) allocated [mem 0x6fffd6c0-0x70000fff]
> [    0.038746] NODE_DATA(7) allocated [mem 0x7fffc6c0-0x7fffffff]
> [    0.040286] NODE_DATA(8) allocated [mem 0x81ffc6c0-0x81ffffff]
> [    0.041517] NODE_DATA(9) allocated [mem 0x83ffc6c0-0x83ffffff]
> [    0.043678] NODE_DATA(10) allocated [mem 0x87fd86c0-0x87fdbfff]
>
> 2) TO-BE (after fix)
> [    0.001858] NUMA: Node 0 [mem 0x00001000-0x0009ffff] + [mem 0x00100000-0x7fffffff] ]
> [    0.001860] Fake node size 255MB too small, increasing to 256MB
> [    0.001861] Faking node 0 at [mem 0x0000000000001000-0x0000000010000fff] (256MB)
> [    0.001861] Faking node 1 at [mem 0x0000000010001000-0x0000000020000fff] (256MB)
> [    0.001862] Faking node 2 at [mem 0x0000000020001000-0x0000000030000fff] (256MB)
> [    0.001862] Faking node 3 at [mem 0x0000000030001000-0x0000000040000fff] (256MB)
> [    0.001863] Faking node 4 at [mem 0x0000000040001000-0x0000000050000fff] (256MB)
> [    0.001863] Faking node 5 at [mem 0x0000000050001000-0x0000000060000fff] (256MB)
> [    0.001863] Faking node 6 at [mem 0x0000000060001000-0x0000000070000fff] (256MB)
> [    0.001864] Faking node 7 at [mem 0x0000000070001000-0x000000007fffffff] (255MB)
> [    0.001864] Fake node size 15MB too small, increasing to 32MB
> [    0.001864] Faking node 8 at [mem 0x0000000080000000-0x0000000081ffffff] (32MB)
> [    0.001865] Faking node 9 at [mem 0x0000000082000000-0x0000000083ffffff] (32MB)
> [    0.001865] Faking node 10 at [mem 0x0000000084000000-0x0000000087fdcfff] (63MB)
> [    0.001866] numa_emulation: phys: 1 only got 3 of 8 nodes, failing
> [    0.001867] NODE_DATA(0) allocated [mem 0x7fffc6c0-0x7fffffff]
> [    0.001940] NODE_DATA(1) allocated [mem 0x87fd96c0-0x87fdcfff]
>
> And also there is other scenario tested by Donghyeon. [1]
>
> [1] https://lore.kernel.org/all/CAFPTC5e1OLpHa3HqwhtSPjS_PTQz+iG=ovM2cZ=VnOZ_5z7oxg@mail.gmail.com/
>
> ---
>  mm/numa_emulation.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/mm/numa_emulation.c b/mm/numa_emulation.c
> index 703c8fa05048..bc2f163e7c45 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,
> @@ -398,7 +398,7 @@ void __init numa_emulation(struct numa_meminfo *numa_meminfo, int numa_dist_cnt)
>          */
>         if (strchr(emu_cmdline, 'U')) {
>                 unsigned long n;
> -               int nid = 0;
> +               int nid = 0, nr_created;
>
>                 n = simple_strtoul(emu_cmdline, &emu_cmdline, 0);
>                 ret = -1;
> @@ -416,9 +416,11 @@ 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) {
> +
> +                       nr_created = ret - nid;
> +                       if (nr_created < n) {

sashiko [1] found a false failure point with memoryless nodes, I'll
fix it and send v2 patch soon.

[1] https://sashiko.dev/#/patchset/20260417114127.1664283-1-ekffu200098%40gmail.com

>                                 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
>

Best Regards,
Sang-Heon Jeon