[BUG] x86/smpboot: WARN_ON in set_cpu_sibling_map triggered by numa=fake=2

w15303746062
2026-04-29T03:42:47Z

Hi x86 maintainers,

While fuzzing the v7.0 kernel, I encountered a persistent WARNING in `set_cpu_sibling_map()` during boot when using the `numa=fake=2` command-line parameter.

The issue appears to be a logic gap in the topology consistency check. When `numa=fake=N` is used, it artificially divides a single physical package into multiple software NUMA nodes. However, the existing check in `set_cpu_sibling_map()` does not account for this fake NUMA state:

if (match_pkg(c, o) && !topology_same_node(c, o))
WARN_ON_ONCE(topology_num_nodes_per_package() == 1);

Since `numa=fake` forces `!topology_same_node(c, o)` to be true for CPUs on the same package, the WARN_ON_ONCE is falsely triggered. With `panic_on_warn=1` enabled in many fuzzing and testing environments, this leads to an early boot panic.

Here is the relevant part of the crash log:

------------[ cut here ]------------
WARNING: arch/x86/kernel/smpboot.c:698 at set_cpu_sibling_map+0x1206/0x1f20
Modules linked in:
CPU: 1 UID: 0 PID: 0 Comm: swapper/1 Not tainted 7.0.0 #1 PREEMPT(full)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009)
RIP: 0010:set_cpu_sibling_map+0x1206/0x1f20
Call Trace:
<TASK>
ap_starting arch/x86/kernel/smpboot.c:196 [inline]
start_secondary+0xd8/0x2d0 arch/x86/kernel/smpboot.c:280
common_startup_64+0x13e/0x148
</TASK>
---[ end trace ]---

I am reporting this to bring it to your attention, as it might require a small adjustment to bypass this strict topology check when `numa=fake` is active.

Best regards,
Mingyu Wang