[PATCH] riscv: require vmemmap for NUMA balancing

[Jia Wang via B4 Relay]

From: Jia Wang <wangjia@xxxxxxxxxxxxx>

RISC-V randconfig can enable NUMA balancing while selecting
SPARSEMEM_EXTREME without SPARSEMEM_VMEMMAP. In that layout, the
section and node fields can consume enough page flag bits that
LAST_CPUPID cannot fit in page->flags.

The generic MM code then stores last_cpupid in struct page and emits
the "Unfortunate NUMA and NUMA Balancing config" warning from
mm/memory.c.

NUMA balancing needs LAST_CPUPID storage, but this fallback grows
struct page for a configuration that randconfig can select
accidentally. Keep NUMA support available, but advertise RISC-V NUMA
balancing support only when SPARSEMEM_VMEMMAP is enabled, where the
page flag layout leaves room for LAST_CPUPID.

This was originally seen with a RISC-V randconfig generated from
KCONFIG_SEED=550012.

Signed-off-by: Jia Wang <wangjia@xxxxxxxxxxxxx>
---
RISC-V randconfig can enable NUMA balancing with SPARSEMEM_EXTREME while
leaving SPARSEMEM_VMEMMAP disabled. In that layout, LAST_CPUPID may not
fit in page->flags, so the generic MM code falls back to growing struct
page and emits the "Unfortunate NUMA and NUMA Balancing config" warning.

Limit RISC-V NUMA balancing support to SPARSEMEM_VMEMMAP configurations.
This keeps NUMA available while preventing randconfig from selecting the
known-bad NUMA balancing layout.

The issue was originally seen with a RISC-V randconfig generated from
KCONFIG_SEED=550012.
---
 arch/riscv/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index c0a6992933e4..681c5a2eb006 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -481,7 +481,7 @@ endchoice
 config NUMA
 	bool "NUMA Memory Allocation and Scheduler Support"
 	depends on SMP && MMU
-	select ARCH_SUPPORTS_NUMA_BALANCING
+	select ARCH_SUPPORTS_NUMA_BALANCING if SPARSEMEM_VMEMMAP
 	select GENERIC_ARCH_NUMA
 	select HAVE_SETUP_PER_CPU_AREA
 	select NEED_PER_CPU_EMBED_FIRST_CHUNK

---
base-commit: 37e2f878a7a660a216cc7a60459995fefd150f25
change-id: 20260716-numa_kconfig-123e5990558f

Best regards,
--  
Jia Wang <wangjia@xxxxxxxxxxxxx>