Re: BUG: unable to handle kernel NULL pointer dereference at (null)

[Josef Bacik <josef@redhat.com>]

On Tue, Apr 05, 2011 at 07:38:13PM +0200, Johannes Hirte wrote:
> With the latest btrfs changes, I got this Oops when doing rm on a large 
> directory:
> 
> BUG: unable to handle kernel NULL pointer dereference at   (null)
> IP: [<c101c838>] kunmap+0x46/0x46
> *pdpt = 0000000034a85001 *pde = 0000000000000000 
> Oops: 0000 [#1] PREEMPT SMP 
> last sysfs file: /sys/devices/virtual/vtconsole/vtcon1/uevent
> Modules linked in: snd_seq_oss snd_seq_midi_event snd_seq snd_seq_device 
> snd_pcm_oss snd_mixer_oss fuse dm_crypt dm_mod usbhid snd_intel8x0 
> snd_ac97_codec ac97_bus snd_pcm snd_timer sr_mod cdrom sg snd fschmd e1000 
> uhci_hcd snd_page_alloc i2c_i801 [last unloaded: microcode]
> 
> Pid: 1156, comm: btrfs-transacti Tainted: G        W   2.6.39-rc1-00262-
> gc53813f #20 FUJITSU SIEMENS SCENIC P / SCENICO P/D1561
> EIP: 0060:[<c101c838>] EFLAGS: 00010296 CPU: 1
> EIP is at kmap+0x0/0x38
> EAX: 00000000 EBX: 00000000 ECX: 00000000 EDX: 00000010
> ESI: f5bc6400 EDI: f3c75520 EBP: f3c755f0 ESP: f58f9e10
>  DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068
> Process btrfs-transacti (pid: 1156, ti=f58f8000 task=f6516f40 
> task.ti=f58f8000)
> Stack:
>  c1186d15 ffc22000 f58f9ec0 00000010 f3c75610 00000000 f5885780 f52339e8
>  00000009 f5bc6400 00010000 00000000 f6415800 f3c75638 000008bb f5bc63c0
>  f58857b4 f60b68a0 00000040 f52338e8 ffc22000 00000000 00000008 00000010
> Call Trace:
>  [<c1186d15>] ? btrfs_write_out_cache+0x60c/0xa3c
>  [<c114a815>] ? btrfs_write_dirty_block_groups+0x400/0x494
>  [<c11566a7>] ? commit_cowonly_roots+0xa9/0x180
>  [<c1157799>] ? btrfs_commit_transaction+0x2ee/0x59c
>  [<c1037c85>] ? wake_up_bit+0x16/0x16
>  [<c1152a83>] ? transaction_kthread+0x149/0x1d6
>  [<c101d1b9>] ? complete+0x28/0x36
>  [<c115293a>] ? btrfs_congested_fn+0x5d/0x5d
>  [<c10379c4>] ? kthread+0x63/0x68
>  [<c1037961>] ? kthread_worker_fn+0xeb/0xeb
>  [<c13cba36>] ? kernel_thread_helper+0x6/0xd
> Code: 8d 8a 00 e4 54 c1 2b 8a 8c e7 54 c1 81 f9 00 08 00 00 74 11 81 f9 00 0c 
> 00 00 75 0e 83 3d 10 2f 60 c1 02 75 05 e9 5e a3 04 00 c3 <8b> 10 c1 ea 1e c1 
> e2 0a 8d 8a 00 e4 54 c1 2b 8a 8c e7 54 c1 81 
> EIP: [<c101c838>] kmap+0x0/0x38 SS:ESP 0068:f58f9e10
> CR2: 0000000000000000
> ---[ end trace c8511126ee91dfdf ]---
> 
> This is the second Oops. On the first one I wasn't able to catch the backtrace, 
> but IIRC the bug happend on kmap not kunmap the first time.
>

Yeah I think I know what this is but I need somebody to verify it for me.  Can
you run with this patch and let me know what happens?  Thanks,

Josef

diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
index 74bc432..5e6f4b3 100644
--- a/fs/btrfs/free-space-cache.c
+++ b/fs/btrfs/free-space-cache.c
@@ -624,6 +624,7 @@ int btrfs_write_out_cache(struct btrfs_root *root,
 
 		next_page = false;
 
+		BUG_ON(index > last_index);
 		if (index == 0) {
 			start_offset = first_page_offset;
 			offset = start_offset;
@@ -732,6 +733,7 @@ int btrfs_write_out_cache(struct btrfs_root *root,
 		struct btrfs_free_space *entry =
 			list_entry(pos, struct btrfs_free_space, list);
 
+		BUG_ON(index > last_index);
 		page = find_get_page(inode->i_mapping, index);
 
 		addr = kmap(page);