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Subjectmm: divide by zero in percpu_pagelist_fraction_sysctl_handler()
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Hi all,

I've stumbled on the following after some fuzzing using trinity inside a KVM tools guest, using a recent linux-next.

It appears as though it caused a divide by zero in mm/page_alloc.c:5230:

high = zone->present_pages / percpu_pagelist_fraction;

Here's the relevant disassembly:

2360: 48 8b 83 30 20 00 00 mov 0x2030(%rbx),%rax
2367: 31 d2 xor %edx,%edx
2369: 89 c9 mov %ecx,%ecx
236b: 4c 63 05 00 00 00 00 movslq 0x0(%rip),%r8 # 2372 <percpu_pagelist_fraction_sysctl_handler+0x72>
236e: R_X86_64_PC32 percpu_pagelist_fraction+0xfffffffffffffffc
2372: 49 f7 f0 div %r8

And finally, the dump:

[ 1208.152452] divide error: 0000 [#1] PREEMPT SMP
[ 1208.154780] CPU 0
[ 1208.154780] Pid: 25153, comm: trinity Tainted: G W 3.4.0-rc3-next-20120419-sasha-dirty #86
[ 1208.154780] RIP: 0010:[<ffffffff81179632>] [<ffffffff81179632>] percpu_pagelist_fraction_sysctl_handler+0x72/0xf0
[ 1208.154780] RSP: 0018:ffff88003133dd48 EFLAGS: 00010246
[ 1208.154780] RAX: 0000000000000f4a RBX: ffff88000dfcf000 RCX: 0000000000000000
[ 1208.154780] RDX: 0000000000000000 RSI: 0000000000000005 RDI: 0000000000000000
[ 1208.154780] RBP: ffff88003133dd68 R08: 0000000000000000 R09: 0000000000000000
[ 1208.154780] R10: 0000000000000000 R11: 0000000000000001 R12: ffffffff83746760
[ 1208.154780] R13: 0000000000000060 R14: 0000000000000001 R15: 00000000017f7100
[ 1208.154780] FS: 00007f98fb7ff700(0000) GS:ffff88000d800000(0000) knlGS:0000000000000000
[ 1208.154780] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1208.154780] CR2: 0000000000000000 CR3: 0000000031409000 CR4: 00000000000406f0
[ 1208.154780] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 1208.154780] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[ 1208.154780] Process trinity (pid: 25153, threadinfo ffff88003133c000, task ffff880031320000)
[ 1208.154780] Stack:
[ 1208.154780] ffff88000d45d388 ffffffff8323c2a0 0000000000000001 ffff88003133df48
[ 1208.154780] ffff88003133ddc8 ffffffff8124fb9e ffff88003133ddb8 0000000000000000
[ 1208.154780] ffff88003133ddb8 00000000017f7100 000000000116a08f 0000000000000001
[ 1208.154780] Call Trace:
[ 1208.154780] [<ffffffff8124fb9e>] proc_sys_call_handler.clone.11+0x8e/0xc0
[ 1208.154780] [<ffffffff8124fbd0>] ? proc_sys_call_handler.clone.11+0xc0/0xc0
[ 1208.154780] [<ffffffff8124fbe3>] proc_sys_write+0x13/0x20
[ 1208.154780] [<ffffffff811dc8db>] do_loop_readv_writev+0x4b/0x90
[ 1208.154780] [<ffffffff811dcb76>] do_readv_writev+0x106/0x1e0
[ 1208.154780] [<ffffffff810b5e2a>] ? do_setitimer+0x1aa/0x1f0
[ 1208.154780] [<ffffffff8269f77b>] ? _raw_spin_unlock_irq+0x2b/0x80
[ 1208.154780] [<ffffffff810e45c1>] ? get_parent_ip+0x11/0x50
[ 1208.154780] [<ffffffff810e478e>] ? sub_preempt_count+0xae/0xe0
[ 1208.154780] [<ffffffff826a0e29>] ? sysret_check+0x22/0x5d
[ 1208.154780] [<ffffffff811dcce6>] vfs_writev+0x46/0x60
[ 1208.154780] [<ffffffff811dcdff>] sys_writev+0x4f/0xb0
[ 1208.154780] [<ffffffff826a0dfd>] system_call_fastpath+0x1a/0x1f
[ 1208.154780] Code: 00 00 0f 1f 80 00 00 00 00 48 83 bb 30 20 00 00 00 74 64 eb 7d 0f 1f 40 00 48 8b 83 30 20 00 00 31 d2 89 c9 4c 63 05 f6 c9 33 03 <49> f7 f0 48 8b 53 60 48 03 14 cd a0 c2 73 83 48 89 c1 89 42 04
[ 1208.154780] RIP [<ffffffff81179632>] percpu_pagelist_fraction_sysctl_handler+0x72/0xf0
[ 1208.154780] RSP <ffff88003133dd48>
[ 1208.315517] ---[ end trace a307b3ed40206b4b ]---



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