NULL inode->i_mapping in generic_sync_sb_inodes

[Jeff Moyer <jmoyer@redhat.com>]

Hi,

I've come across a problem in 2.6.31 whereby the umount path on shutdown
Oopses like so:

BUG: unable to handle kernel NULL pointer dereference at 00000070
IP: [<c0510e19>] generic_sync_sb_inodes+0x2ca/0x34b
*pdpt = 00000000220b1001 *pde = 0000000099419067 
Oops: 0000 [#1] SMP 
last sysfs file:
/sys/devices/pci0000:00/0000:00:07.0/0000:0d:00.0/0000:0e:08.0/host0/target0:1:0/0:1:0:0/block/sda/removable
Modules linked in: fcoe libfcoe libfc scsi_transport_fc scsi_tgt ipv6 xts lrw
gf128mul sha256_generic cbc dm_crypt dm_round_robin dm_multipath dm_snapshot
dm_mirror dm_region_hash dm_log dm_zero dm_mod linear raid10 raid456 raid6_pq
async_xor async_memcpy async_tx xor raid1 raid0 nfs lockd fscache nfs_acl
auth_rpcgss sunrpc radeon mptsas ttm drm_kms_helper mptscsih drm mptbase
i2c_algo_bit i2c_core scsi_transport_sas bnx2 iscsi_ibft pcspkr edd iscsi_tcp
libiscsi_tcp libiscsi scsi_transport_iscsi squashfs cramfs

Pid: 5082, comm: grub Tainted: G        W  (2.6.31-27.el6.i686 #1) PowerEdge
1955
EIP: 0060:[<c0510e19>] EFLAGS: 00010246 CPU: 0
EIP is at generic_sync_sb_inodes+0x2ca/0x34b
EAX: ec45ae14 EBX: 00000000 ECX: 00000000 EDX: c0510e4f
ESI: ec45ae04 EDI: ec45b1c4 EBP: f25fdf38 ESP: f25fdf10
 DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
Process grub (pid: 5082, ti=f25fc000 task=ef0cc6f0 task.ti=f25fc000)
Stack:
 00000246 00000001 f422fa6c 000abc70 f422fa64 f422fa54 8fd7ae23 f422f970
<0> 00000001 f25fdf68 f25fdf74 c0510f0e 00000000 00000001 00000000 7fffffff
<0> 00000000 00000000 00000000 ffffffff 7fffffff 00000000 8fd7ae23 f422f970
Call Trace:
 [<c0510f0e>] ? sync_inodes_sb+0x74/0x8c
 [<c051407b>] ? __sync_filesystem+0x41/0x74
 [<c0514144>] ? sync_filesystems+0x96/0xed
 [<c051420a>] ? sys_sync+0x27/0x4a
 [<c04095ab>] ? sysenter_do_call+0x12/0x38
Code: 0f 85 83 00 00 00 8b b3 e4 00 00 00 81 c3 e4 00 00 00 31 ff 89 5d ec 83
ee 10 eb 4b f6 86 6c 02 00 00 78 75 3c 8b 9e 3c 01 00 00 <83> 7b 70 00 74 30 89
f0 e8 d8 61 ff ff b8 cc f4 a0 c0 e8 e0 dd 
EIP: [<c0510e19>] generic_sync_sb_inodes+0x2ca/0x34b SS:ESP 0068:f25fdf10
CR2: 0000000000000070
---[ end trace 8171140d16b04470 ]---

The Oops is in fs/fs-writeback.c:568:

               list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
                        struct address_space *mapping;

                        if (inode->i_state &
                                        (I_FREEING|I_CLEAR|I_WILL_FREE|I_NEW))
                                continue;
                        mapping = inode->i_mapping;
                        if (mapping->nrpages == 0)    <==== BUG

Any idea how that can happen?  Maybe a race in the umount path?

Cheers,
Jeff