Hi all, While fuzzing with trinity inside a KVM tools guest running the latest -next kernel I've stumbled on the following spew: [ 761.704089] BUG: unable to handle kernel NULL pointer dereference at (null) [ 761.704089] IP: mm_find_pmd (mm/rmap.c:570) [ 761.704089] PGD 51223067 PUD 50a09067 PMD 0 [ 761.704089] Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC [ 761.704089] Dumping ftrace buffer: [ 761.704089] (ftrace buffer empty) [ 761.704089] Modules linked in: [ 761.704089] CPU: 4 PID: 20723 Comm: trinity-c131 Tainted: G W 3.16.0-rc3-next-20140630-sasha-00023-g44434d4-dirty #756 [ 761.704089] task: ffff88004e3c0000 ti: ffff88004e0b8000 task.ti: ffff88004e0b8000 [ 761.704089] RIP: mm_find_pmd (mm/rmap.c:570) [ 761.704089] RSP: 0000:ffff88004e0bbaa8 EFLAGS: 00010246 [ 761.704089] RAX: 0000000000000000 RBX: 0000000000a65000 RCX: ffff88004e0bbb30 [ 761.704089] RDX: 0000000000000000 RSI: 0000000000a65000 RDI: ffff880000146000 [ 761.704089] RBP: ffff88004e0bbaa8 R08: 0000000000000000 R09: 0000000000000000 [ 761.704089] R10: ffff88004e3c0000 R11: 0000000000000000 R12: ffffea000d766e00 [ 761.704089] R13: ffff88004e0bbb30 R14: ffff880000146000 R15: 0000000000000000 [ 761.704089] FS: 00007f0293c61700(0000) GS:ffff880144e00000(0000) knlGS:0000000000000000 [ 761.704089] CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b [ 761.704089] CR2: 0000000000000000 CR3: 000000004e3be000 CR4: 00000000000006a0 [ 761.704089] Stack: [ 761.704089] ffff88004e0bbae8 ffffffff9c2d0815 800000035d9b8805 ffff880000146000 [ 761.704089] ffffea000d766e00 ffff88000b4c4e58 ffff880034d7d200 0000000000000302 [ 761.704089] ffff88004e0bbb68 ffffffff9c2d1491 ffff88004e0bbb28 ffffffff9f57c58a [ 761.704089] Call Trace: [ 761.704089] __page_check_address (mm/rmap.c:618) [ 761.704089] try_to_unmap_one (mm/rmap.c:1133) [ 761.704089] ? down_read (kernel/locking/rwsem.c:45 (discriminator 2)) [ 761.704089] ? page_lock_anon_vma_read (./arch/x86/include/asm/atomic.h:118 mm/rmap.c:491) [ 761.704089] ? page_lock_anon_vma_read (mm/rmap.c:448) [ 761.704089] rmap_walk (mm/rmap.c:1634 mm/rmap.c:1705) [ 761.704089] try_to_unmap (mm/rmap.c:1527) [ 761.704089] ? page_remove_rmap (mm/rmap.c:1124) [ 761.704089] ? invalid_migration_vma (mm/rmap.c:1483) [ 761.704089] ? try_to_unmap_one (mm/rmap.c:1391) [ 761.704089] ? anon_vma_prepare (mm/rmap.c:448) [ 761.704089] ? invalid_mkclean_vma (mm/rmap.c:1478) [ 761.704089] ? page_get_anon_vma (mm/rmap.c:405) [ 761.704089] migrate_pages (mm/migrate.c:912 mm/migrate.c:955 mm/migrate.c:1142) [ 761.704089] ? perf_trace_mm_numa_migrate_ratelimit (mm/migrate.c:1590) [ 761.704089] migrate_misplaced_page (mm/migrate.c:1750) [ 761.704089] __handle_mm_fault (mm/memory.c:3162 mm/memory.c:3212 mm/memory.c:3322) [ 761.704089] handle_mm_fault (include/linux/memcontrol.h:124 mm/memory.c:3348) [ 761.704089] ? __do_page_fault (arch/x86/mm/fault.c:1163) [ 761.704089] __do_page_fault (arch/x86/mm/fault.c:1230) [ 761.704089] ? vtime_account_user (kernel/sched/cputime.c:687) [ 761.704089] ? get_parent_ip (kernel/sched/core.c:2550) [ 761.704089] ? context_tracking_user_exit (include/linux/vtime.h:89 include/linux/jump_label.h:115 include/trace/events/context_tracking.h:47 kernel/context_tracking.c:180) [ 761.704089] ? preempt_count_sub (kernel/sched/core.c:2606) [ 761.704089] ? context_tracking_user_exit (kernel/context_tracking.c:184) [ 761.704089] ? __this_cpu_preempt_check (lib/smp_processor_id.c:63) [ 761.704089] ? trace_hardirqs_off_caller (kernel/locking/lockdep.c:2638 (discriminator 2)) [ 761.704089] trace_do_page_fault (arch/x86/mm/fault.c:1313 include/linux/jump_label.h:115 include/linux/context_tracking_state.h:27 include/linux/context_tracking.h:45 arch/x86/mm/fault.c:1314) [ 761.704089] do_async_page_fault (arch/x86/kernel/kvm.c:264) [ 761.704089] async_page_fault (arch/x86/kernel/entry_64.S:1322) [ 761.704089] Code: 00 48 8b 5d f0 4c 8b 65 f8 c9 c3 66 0f 1f 44 00 00 66 66 66 66 90 55 48 89 f2 48 8b 47 40 48 c1 ea 27 48 89 e5 81 e2 ff 01 00 00 <48> 8b 3c d0 40 f6 c7 01 75 0c 31 f6 e9 af 00 00 00 0f 1f 44 00 All code ======== 0: 00 48 8b add %cl,-0x75(%rax) 3: 5d pop %rbp 4: f0 4c 8b 65 f8 lock mov -0x8(%rbp),%r12 9: c9 leaveq a: c3 retq b: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1) 11: 66 66 66 66 90 data32 data32 data32 xchg %ax,%ax 16: 55 push %rbp 17: 48 89 f2 mov %rsi,%rdx 1a: 48 8b 47 40 mov 0x40(%rdi),%rax 1e: 48 c1 ea 27 shr $0x27,%rdx 22: 48 89 e5 mov %rsp,%rbp 25: 81 e2 ff 01 00 00 and $0x1ff,%edx 2b:* 48 8b 3c d0 mov (%rax,%rdx,8),%rdi <-- trapping instruction 2f: 40 f6 c7 01 test $0x1,%dil 33: 75 0c jne 0x41 35: 31 f6 xor %esi,%esi 37: e9 af 00 00 00 jmpq 0xeb 3c: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) Code starting with the faulting instruction =========================================== 0: 48 8b 3c d0 mov (%rax,%rdx,8),%rdi 4: 40 f6 c7 01 test $0x1,%dil 8: 75 0c jne 0x16 a: 31 f6 xor %esi,%esi c: e9 af 00 00 00 jmpq 0xc0 11: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) [ 761.704089] RIP mm_find_pmd (mm/rmap.c:570) [ 761.704089] RSP <ffff88004e0bbaa8> [ 761.704089] CR2: 0000000000000000 As I didn't see any code changes around that part I'm thinking that it's a locking issue that got messed up somewhere rather then a missing '!= NULL' check. Thanks, Sasha -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>