Hi Tejun,

When testing

tree:   git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git blkcg-cfq-hierarchy
head:   945010db297e9dbe05453af789490f21b99ea8a0 [18/18] cfq-iosched: add hierarchical cfq_group statistics

I got the following divide error, and it's bisected down to

edc3f94f6425e014b6628c1dc5390209bdd9431c is the first bad commit
commit edc3f94f6425e014b6628c1dc5390209bdd9431c
Author: Tejun Heo <tj@kernel.org>
Date:   Fri Dec 14 14:36:07 2012 -0800

    cfq-iosched: implement hierarchy-ready cfq_group charge scaling


[   67.512488] modprobe (1197) used greatest stack depth: 3072 bytes left
[   76.529600] S01glibc.sh (1202) used greatest stack depth: 2880 bytes left
[  161.176483] divide error: 0000 [#1] SMP DEBUG_PAGEALLOC
[  161.176483] Modules linked in:
[  161.176483] CPU 0 
[  161.176483] Pid: 1369, comm: blkid Not tainted 3.7.0-03735-g945010d #4177 Bochs Bochs
[  161.176483] RIP: 0010:[<ffffffff812f7ec9>]  [<ffffffff812f7ec9>] cfq_group_service_tree_add+0xae/0xc5
[  161.176483] RSP: 0018:ffff8800194e99d8  EFLAGS: 00000046
[  161.176483] RAX: 0000000000000000 RBX: ffff88001b9e9fb0 RCX: 0000000000000000
[  161.176483] RDX: 0000000000000000 RSI: ffff88001b6bba20 RDI: ffff88001b9e9fb0
[  161.176483] RBP: ffff8800194e99e8 R08: 0000000000000001 R09: 0000000000000000
[  161.176483] R10: 0000000000000000 R11: 0000000000000000 R12: ffff88001b9e9fb0
[  161.176483] R13: ffff88001b6bba20 R14: 00000000ffff7859 R15: ffff88001b9ea088
[  161.176483] FS:  00007f18d51c9740(0000) GS:ffff88001f200000(0000) knlGS:0000000000000000
[  161.176483] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[  161.176483] CR2: 00007f6c8e8f46c0 CR3: 0000000019f5d000 CR4: 00000000000006f0
[  161.176483] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  161.176483] DR3: 0000000000000000 DR6: 0000000000000000 DR7: 0000000000000000
[  161.176483] Process blkid (pid: 1369, threadinfo ffff8800194e8000, task ffff8800194e0000)
[  161.176483] Stack:
[  161.176483]  0000000000000000 ffff88001b6bba18 ffff8800194e9a48 ffffffff812f8891
[  161.176483]  ffff8800194e9a28 ffffffff00000001 ffffffff00000001 00000019004e06a0
[  161.176483]  0000000200000000 ffff88001cedf5e0 ffff88001b6bba18 ffff88001bb2c540
[  161.176483] Call Trace:
[  161.176483]  [<ffffffff812f8891>] cfq_service_tree_add+0x25d/0x26c
[  161.176483]  [<ffffffff812fab55>] cfq_add_rq_rb+0xdc/0x12b
[  161.176483]  [<ffffffff812fac72>] cfq_insert_request+0xce/0x50f
[  161.176483]  [<ffffffff81149fad>] ? I_BDEV+0xd/0xd
[  161.176483]  [<ffffffff812e1f64>] __elv_add_request+0x193/0x204
[  161.176483]  [<ffffffff812e88f8>] blk_flush_plug_list+0x187/0x1e2
[  161.176483]  [<ffffffff812e8968>] blk_finish_plug+0x15/0x36
[  161.176483]  [<ffffffff810e3312>] __do_page_cache_readahead+0x22d/0x24f
[  161.176483]  [<ffffffff810e3189>] ? __do_page_cache_readahead+0xa4/0x24f
[  161.176483]  [<ffffffff817ef7b3>] ? retint_restore_args+0x13/0x13
[  161.176483]  [<ffffffff81085789>] ? trace_hardirqs_off+0xd/0xf
[  161.176483]  [<ffffffff810e3514>] force_page_cache_readahead+0x6a/0x97
[  161.176483]  [<ffffffff810e38cc>] page_cache_sync_readahead+0x2e/0x42
[  161.176483]  [<ffffffff810db3b4>] generic_file_aio_read+0x276/0x650
[  161.176483]  [<ffffffff81085b60>] ? save_trace+0x3d/0xaf
[  161.176483]  [<ffffffff8114a158>] blkdev_aio_read+0x5f/0x66
[  161.176483]  [<ffffffff8111c68c>] do_sync_read+0x96/0xd6
[  161.176483]  [<ffffffff8111cc74>] vfs_read+0xa3/0x12f
[  161.176483]  [<ffffffff8111cd4c>] sys_read+0x4c/0x80
[  161.176483]  [<ffffffff817f6999>] system_call_fastpath+0x16/0x1b
[  161.176483] Code: 08 00 00 00 00 48 89 fe 48 c7 43 10 00 00 00 00 48 89 df 48 89 1a e8 1e 95 00 00 8b 43 38 8b 4b 24 31 d2 ff 43 20 01 c1 c1 e0 0c <f7> f1 89 4b 24 b9 01 00 00 00 85 c0 0f 45 c8 89 4b 28 41 59 5b 
[  161.176483] RIP  [<ffffffff812f7ec9>] cfq_group_service_tree_add+0xae/0xc5
[  161.176483]  RSP <ffff8800194e99d8>

Thanks,
Fengguang