powerpc/e500: WARNING: at mm/hugetlb.c:4755 hugetlb_add_hstate

[Sourabh Jain <sourabhjain@linux.ibm.com>]

Kernel is printing below warning while booting:


WARNING: CPU: 0 PID: 1 at mm/hugetlb.c:4753 hugetlb_add_hstate+0xc0/0x180
Modules linked in:
CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 
6.18.0-rc1-01400-ga297f72c4951 #6 NONE
Hardware name: QEMU ppce500 e5500 0x80240020 QEMU e500
NIP:  c000000001370800 LR: c000000001357740 CTR: 0000000000000005
REGS: c000000080183890 TRAP: 0700   Not tainted 
(6.18.0-rc1-01400-ga297f72c4951)
MSR:  0000000080029002 <CE,EE,ME>  CR: 48000242  XER: 20000000
IRQMASK: 0
GPR00: c000000001357740 c000000080183b30 c000000001352000 000000000000000e
GPR04: c0000000011d1c4f 0000000000000002 000000000000001a 0000000000000000
GPR08: 0000000000000000 0000000000000002 0000000000000001 0000000000000005
GPR12: c0000000013576a4 c0000000015ad000 c00000000000210c 0000000000000000
GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
GPR20: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
GPR24: 0000000000000000 c0000000015876e8 0000000000000002 c000000001587500
GPR28: c000000001587578 000000000000000e 0000000004000000 0000000000000170
NIP [c000000001370800] hugetlb_add_hstate+0xc0/0x180
LR [c000000001357740] hugetlbpage_init+0x9c/0xf0
Call Trace:
hugetlb_add_hstate+0x148/0x180 (unreliable)
hugetlbpage_init+0x9c/0xf0
do_one_initcall+0x84/0x308
kernel_init_freeable+0x2e4/0x380
kernel_init+0x30/0x15c
ret_from_kernel_user_thread+0x14/0x1c

Kernel commit causing these warning:
commit 7b4f21f5e0386dfe02c68c009294d8f26e3c1bad (HEAD)
Author: David Hildenbrand <david@redhat.com>
Date:   Mon Sep 1 17:03:29 2025 +0200

     mm/hugetlb: check for unreasonable folio sizes when registering hstate

     Let's check that no hstate that corresponds to an unreasonable 
folio size
     is registered by an architecture.  If we were to succeed 
registering, we
     could later try allocating an unsupported gigantic folio size.

...

         BUG_ON(order < order_base_2(__NR_USED_SUBPAGE));
+       WARN_ON(order > MAX_FOLIO_ORDER);
         h = &hstates[hugetlb_max_hstate++];

snip...


Command to create kernel config:
make ARCH=powerpc corenet64_smp_defconfig

Qemu command:
qemu-system-ppc64 -nographic -vga none -M ppce500 -smp 2 -m 4G -accel 
tcg -kernel ./vmlinux -nic user -initrd ./ppc64-novsx-rootfs.cpio.gz 
-cpu e5500 -append "noreboot"


Root cause:
The MAX_FOLIO_ORDER  for e500 platform is MAX_PAGE_ORDER which is
nothing but CONFIG_ARCH_FORCE_MAX_ORDER which dependent of page-size
which was 4k. So value of MAX_FOLIO_ODER is 12 for this case.

As per arch/powerpc/mm/nohash/tlb.c the following page size are supported on
e500 platform:

struct mmu_psize_def mmu_psize_defs[MMU_PAGE_COUNT] = {
     [MMU_PAGE_4K] = {
         .shift    = 12,
     },
     [MMU_PAGE_2M] = {
         .shift    = 21,
     },
     [MMU_PAGE_4M] = {
         .shift    = 22,
     },
     [MMU_PAGE_16M] = {
         .shift    = 24,
     },
     [MMU_PAGE_64M] = {
         .shift    = 26,
     },
     [MMU_PAGE_256M] = {
         .shift    = 28,
     },
     [MMU_PAGE_1G] = {
         .shift    = 30,
     },
};

With the above MAX_FOLIO_ORDER and page sizes, hugetlbpage_init() in
arch/powerpc/mm/hugetlbpage.c tries to call hugetlb_add_hstate() with
an order higher than 12, causing the kernel to print the above warning.

Things I tried:
I enabled CONFIG_ARCH_HAS_GIGANTIC_PAGE for the e500 platform. With that,
MAX_FOLIO_ORDER was set to 16, but that was not sufficient for MMU_PAGE_1G.

This is because with CONFIG_ARCH_HAS_GIGANTIC_PAGE enabled,
MAX_FOLIO_ORDER was set to 16 = PUD_ORDER = (PMD_INDEX_SIZE (7) + 
PTE_INDEX_SIZE (9)),
while the order for MMU_PAGE_1G was 18.

Thanks,
Sourabh Jain