Re: [PATCH 4/5] mm: enable map_anon_folio_pmd_nopf to handle unshare

From: kernel test robot

Date: Sat May 23 2026 - 10:27:29 EST

Hello,

kernel test robot noticed "BUG:kernel_NULL_pointer_dereference,address" on:

commit: 419ac88f7d747a174b48e12d2fd2178a128f54de ("[PATCH 4/5] mm: enable map_anon_folio_pmd_nopf to handle unshare")
url: https://github.com/intel-lab-lkp/linux/commits/Luka-Bai/mm-add-basic-madvise-helpers-and-branch-for-THP-setup/20260502-230731
patch link: https://lore.kernel.org/all/20260501-thp_cow-v1-4-005377483738@xxxxxxxxxxx/
patch subject: [PATCH 4/5] mm: enable map_anon_folio_pmd_nopf to handle unshare

in testcase: boot

config: x86_64-kexec
compiler: clang-20
test machine: qemu-system-x86_64 -enable-kvm -cpu SandyBridge -smp 2 -m 32G

(please refer to attached dmesg/kmsg for entire log/backtrace)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
| Closes: https://lore.kernel.org/oe-lkp/202605231645.88096ca9-lkp@xxxxxxxxx

[ 15.354503][ T37] BUG: kernel NULL pointer dereference, address: 0000000000000000
[ 15.355458][ T37] #PF: supervisor write access in kernel mode
[ 15.356077][ T37] #PF: error_code(0x0002) - not-present page
[ 15.356690][ T37] PGD 0 P4D 0
[ 15.357088][ T37] Oops: Oops: 0002 [#1] SMP PTI
[ 15.357612][ T37] CPU: 0 UID: 0 PID: 37 Comm: khugepaged Not tainted 7.1.0-rc1-00099-g419ac88f7d74 #1 PREEMPT(lazy)
[ 15.358688][ T37] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[ 15.362222][ T37] RIP: 0010:map_anon_folio_pmd_nopf (x86/include/asm/pgtable_64.h:79 x86/include/asm/pgtable.h:1210 huge_memory.c:1448)
[ 15.364849][ T37] Code: e0 ff 48 89 df 4c 89 f6 4c 89 c2 b9 01 00 00 00 e8 b7 a2 fa ff 48 89 df 4c 89 f6 e8 6c 93 f4 ff 4c 89 64 24 20 48 8b 44 24 20 <49> 89 07 48 89 df 31 f6 48 83 c4 28 5b 41 5c 41 5d 41 5e 41 5f 5d
All code
========
0: e0 ff loopne 0x1
2: 48 89 df mov %rbx,%rdi
5: 4c 89 f6 mov %r14,%rsi
8: 4c 89 c2 mov %r8,%rdx
b: b9 01 00 00 00 mov $0x1,%ecx
10: e8 b7 a2 fa ff call 0xfffffffffffaa2cc
15: 48 89 df mov %rbx,%rdi
18: 4c 89 f6 mov %r14,%rsi
1b: e8 6c 93 f4 ff call 0xfffffffffff4938c
20: 4c 89 64 24 20 mov %r12,0x20(%rsp)
25: 48 8b 44 24 20 mov 0x20(%rsp),%rax
2a:* 49 89 07 mov %rax,(%r15) <-- trapping instruction
2d: 48 89 df mov %rbx,%rdi
30: 31 f6 xor %esi,%esi
32: 48 83 c4 28 add $0x28,%rsp
36: 5b pop %rbx
37: 41 5c pop %r12
39: 41 5d pop %r13
3b: 41 5e pop %r14
3d: 41 5f pop %r15
3f: 5d pop %rbp

Code starting with the faulting instruction
===========================================
0: 49 89 07 mov %rax,(%r15)
3: 48 89 df mov %rbx,%rdi
6: 31 f6 xor %esi,%esi
8: 48 83 c4 28 add $0x28,%rsp
c: 5b pop %rbx
d: 41 5c pop %r12
f: 41 5d pop %r13
11: 41 5e pop %r14
13: 41 5f pop %r15
15: 5d pop %rbp
[ 15.372240][ T37] RSP: 0000:ffffc9000013fb70 EFLAGS: 00010282
[ 15.374712][ T37] RAX: 8000000143a000e7 RBX: ffffea00050e8000 RCX: 0000000000000000
[ 15.377932][ T37] RDX: 000000000000021f RSI: ffff88881fc271b0 RDI: ffff88881fc30640
[ 15.381250][ T37] RBP: 0000000000000000 R08: 0000000000071ce9 R09: 000000000000b2e9
[ 15.384563][ T37] R10: 0000000000000001 R11: 000000000000003f R12: 8000000143a000e7
[ 15.387726][ T37] R13: 8000000000000025 R14: ffff88810e098900 R15: 0000000000000000
[ 15.391175][ T37] FS: 0000000000000000(0000) GS:ffff88889c101000(0000) knlGS:0000000000000000
[ 15.394883][ T37] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 15.397492][ T37] CR2: 0000000000000000 CR3: 0000000163644000 CR4: 00000000000406f0
[ 15.400508][ T37] Call Trace:
[ 15.403414][ T37] <TASK>
[ 15.404723][ T37] ? lruvec_stat_mod_folio (memcontrol.c:993)
[ 15.406687][ T37] collapse_single_pmd (khugepaged.c:1411)
[ 15.408449][ T37] ? __pfx_wq_barrier_func (workqueue.c:1144)
[ 15.410197][ T37] ? __thp_vma_allowable_orders (huge_memory.c:124)
[ 15.412115][ T37] khugepaged (khugepaged.c:2901)
[ 15.413596][ T37] ? __pfx_khugepaged (khugepaged.c:3113)
[ 15.415388][ T37] kthread (kthread.c:436)
[ 15.416719][ T37] ? __pfx_kthread (kthread.c:1738)
[ 15.418155][ T37] ret_from_fork (x86/kernel/process.c:158)
[ 15.419600][ T37] ? __pfx_kthread (kthread.c:1738)
[ 15.421001][ T37] ret_from_fork_asm (x86/entry/entry_64.S:245)
[ 15.422510][ T37] </TASK>
[ 15.423538][ T37] Modules linked in: sr_mod cdrom sg ata_generic fuse
[ 15.425454][ T37] CR2: 0000000000000000
[ 15.426674][ T37] ---[ end trace 0000000000000000 ]---
[ 15.428142][ T37] RIP: 0010:map_anon_folio_pmd_nopf (x86/include/asm/pgtable_64.h:79 x86/include/asm/pgtable.h:1210 huge_memory.c:1448)
[ 15.429793][ T37] Code: e0 ff 48 89 df 4c 89 f6 4c 89 c2 b9 01 00 00 00 e8 b7 a2 fa ff 48 89 df 4c 89 f6 e8 6c 93 f4 ff 4c 89 64 24 20 48 8b 44 24 20 <49> 89 07 48 89 df 31 f6 48 83 c4 28 5b 41 5c 41 5d 41 5e 41 5f 5d
All code
========
0: e0 ff loopne 0x1
2: 48 89 df mov %rbx,%rdi
5: 4c 89 f6 mov %r14,%rsi
8: 4c 89 c2 mov %r8,%rdx
b: b9 01 00 00 00 mov $0x1,%ecx
10: e8 b7 a2 fa ff call 0xfffffffffffaa2cc
15: 48 89 df mov %rbx,%rdi
18: 4c 89 f6 mov %r14,%rsi
1b: e8 6c 93 f4 ff call 0xfffffffffff4938c
20: 4c 89 64 24 20 mov %r12,0x20(%rsp)
25: 48 8b 44 24 20 mov 0x20(%rsp),%rax
2a:* 49 89 07 mov %rax,(%r15) <-- trapping instruction
2d: 48 89 df mov %rbx,%rdi
30: 31 f6 xor %esi,%esi
32: 48 83 c4 28 add $0x28,%rsp
36: 5b pop %rbx
37: 41 5c pop %r12
39: 41 5d pop %r13
3b: 41 5e pop %r14
3d: 41 5f pop %r15
3f: 5d pop %rbp

Code starting with the faulting instruction
===========================================
0: 49 89 07 mov %rax,(%r15)
3: 48 89 df mov %rbx,%rdi
6: 31 f6 xor %esi,%esi
8: 48 83 c4 28 add $0x28,%rsp
c: 5b pop %rbx
d: 41 5c pop %r12
f: 41 5d pop %r13
11: 41 5e pop %r14
13: 41 5f pop %r15
15: 5d pop %rbp

The kernel config and materials to reproduce are available at:
https://download.01.org/0day-ci/archive/20260523/202605231645.88096ca9-lkp@xxxxxxxxx

--
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki