On 26.12.2012 12:22, Zhouping Liu wrote:

Hello everyone,

The latest mainline(637704cbc95c) would trigger the following error when the system was under
some pressure condition(in my testing, I used oom01 case inside LTP test suite to trigger the issue):

[ 5462.920151] BUG: unable to handle kernel NULL pointer dereference at 0000000000000500
[ 5462.927991] IP: [<ffffffff811542d9>] wait_iff_congested+0x59/0x140
[ 5462.934176] PGD 0
[ 5462.936191] Oops: 0000 [#2] SMP
[ 5462.939428] Modules linked in: lockd sunrpc iptable_mangle ipt_REJECT nf_conntrack_ipv4 nf_defrag_ipv4 xt_conntrack nf_conntrack ebtable_filter ebtables ip6table_filter ip6_tables iptable_filter ip_tabled
[ 5462.984261] CPU 13
[ 5462.986184] Pid: 117, comm: kswapd3 Tainted: G      D      3.8.0-rc1+ #1 Dell Inc. PowerEdge M905/0D413F
[ 5462.995814] RIP: 0010:[<ffffffff811542d9>]  [<ffffffff811542d9>] wait_iff_congested+0x59/0x140
[ 5463.004411] RSP: 0018:ffff88007c97fd48  EFLAGS: 00010202
[ 5463.009701] RAX: 0000000000000001 RBX: 0000000000000064 RCX: 0000000000000001
[ 5463.016818] RDX: 0000000000000064 RSI: 0000000000000000 RDI: 0000000000000000
[ 5463.023926] RBP: ffff88007c97fd98 R08: 0000000000000000 R09: ffff88022ffd9d80
[ 5463.031033] R10: 0000000000003189 R11: 0000000000000000 R12: 00000001004ee87e
[ 5463.038140] R13: 0000000000000002 R14: 0000000000000000 R15: ffff88022ffd9000
[ 5463.045258] FS:  00007f3e570de740(0000) GS:ffff88022fcc0000(0000) knlGS:0000000000000000
[ 5463.053317] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[ 5463.059041] CR2: 0000000000000500 CR3: 00000000018dc000 CR4: 00000000000007e0
[ 5463.066157] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 5463.073276] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[ 5463.080400] Process kswapd3 (pid: 117, threadinfo ffff88007c97e000, task ffff88007c981970)
[ 5463.088633] Stack:
[ 5463.090646]  ffff88007c97fd98 0000000000000000 ffff88007c981970 ffffffff81086080
[ 5463.098090]  ffff88007c97fd68 ffff88007c97fd68 ffff88022ffd9d80 0000000000000002
[ 5463.105527]  0000000000000002 0000000000000000 ffff88007c97feb8 ffffffff8114b0e3
[ 5463.112998] Call Trace:
[ 5463.115446]  [<ffffffff81086080>] ? wake_up_bit+0x40/0x40
[ 5463.120826]  [<ffffffff8114b0e3>] kswapd+0x6c3/0xa50
[ 5463.125775]  [<ffffffff8114aa20>] ? zone_reclaim+0x270/0x270
[ 5463.131415]  [<ffffffff81085680>] kthread+0xc0/0xd0
[ 5463.136278]  [<ffffffff810855c0>] ? kthread_create_on_node+0x120/0x120
[ 5463.142786]  [<ffffffff8160a0ac>] ret_from_fork+0x7c/0xb0
[ 5463.148166]  [<ffffffff810855c0>] ? kthread_create_on_node+0x120/0x120
[ 5463.154668] Code: 4e 6d 88 00 48 c7 45 b8 00 00 00 00 48 83 c0 18 48 c7 45 c8 80 60 08 81 48 89 45 d0 48 89 45 d8 8b 04 b5 a0 9a cd 81 85 c0 74 0f <48> 8b 87 00 05 00 00 a8 04 0f 85 98 00 00 00 e8 b3 c3
[ 5463.174097] RIP  [<ffffffff811542d9>] wait_iff_congested+0x59/0x140
[ 5463.180352]  RSP <ffff88007c97fd48>
[ 5463.183824] CR2: 0000000000000500
[ 5463.203717] ---[ end trace 9ff4ff9087c13a36 ]---

I attached the config file, hope it can make some help.

Thanks,
Zhouping

If I'm decoding it properly, this translates to:

0xffffffff811542e9 is in wait_iff_congested 
(/usr/src/linux/arch/x86/include/asm/bitops.h:321).
316	}
317	
318	static __always_inline int constant_test_bit(unsigned int nr, const 
volatile unsigned long *addr)
319	{
320		return ((1UL << (nr % BITS_PER_LONG)) &
321			(addr[nr / BITS_PER_LONG])) != 0;
322	}
323	
324	static inline int variable_test_bit(int nr, volatile const unsigned 
long *addr)
325	{

0xffffffff811542e8 is in wait_iff_congested (mm/backing-dev.c:815).
810		/*
811		 * If there is no congestion, or heavy congestion is not being
812		 * encountered in the current zone, yield if necessary instead
813		 * of sleeping on the congestion queue
814		 */
815		if (atomic_read(&nr_bdi_congested[sync]) == 0 ||
816				!zone_is_reclaim_congested(zone)) {
817			cond_resched();
818	
819			/* In case we scheduled, work out time remaining */

All code
========
    0:	4e 6d                	rex.WRX insl (%dx),%es:(%rdi)
    2:	88 00                	mov    %al,(%rax)
    4:	48 c7 45 b8 00 00 00 	movq   $0x0,-0x48(%rbp)
    b:	00
    c:	48 83 c0 18          	add    $0x18,%rax
   10:	48 c7 45 c8 80 60 08 	movq   $0xffffffff81086080,-0x38(%rbp)
   17:	81
   18:	48 89 45 d0          	mov    %rax,-0x30(%rbp)
   1c:	48 89 45 d8          	mov    %rax,-0x28(%rbp)
   20:	8b 04 b5 a0 9a cd 81 	mov    -0x7e326560(,%rsi,4),%eax
   27:	85 c0                	test   %eax,%eax
   29:	74 0f                	je     0x3a
   2b:*	48 8b 87 00 05 00 00 	mov    0x500(%rdi),%rax     <-- trapping 
instruction
   32:	a8 04                	test   $0x4,%al
   34:	0f 85 98 00 00 00    	jne    0xd2
   3a:	e8                   	.byte 0xe8
   3b:	b3 c3                	mov    $0xc3,%bl

I remember when I was instrumenting vmscan.c to see which of the 
congestion_wait() calls was making trouble, the only place that really 
called it many times (other counters being zero all the time!) was the 
one that I eventually replaced with wait_iff_congested().

So, I wonder did we now uncover a subtle bug in wait_iff_congested() 
that has gone unnoticed for a long time?
-- 
Zlatko

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