On 08/05/2016 06:12 PM, Chris Mason wrote:
>
> On 08/05/2016 07:08 AM, Nikolay Borisov wrote:
>> Hello,
>>
>> Recently I started getting the following crashes on some servers,
>> running btrfs:
>>
>> [340435.480338] BTRFS info (device loop7): disk space caching is enabled
>> [340435.480509] BTRFS: has skinny extents
>> [340441.716174] BTRFS: checking UUID tree
>> [340441.912070] BUG: unable to handle kernel NULL pointer dereference at 0000000000000098
>> [340441.912463] IP: [<ffffffffa081f774>] btrfs_uuid_tree_iterate+0xf4/0x2d0 [btrfs]
>> [340441.912823] PGD 0
>> [340441.913035] Oops: 0000 [#1] SMP
>> [340441.913302] Modules linked in:
>> [340441.916996] CPU: 10 PID: 24990 Comm: btrfs-uuid Tainted: P W O 4.4.14-clouder1 #55
>> [340441.917287] Hardware name: Supermicro X9DRD-iF/LF/X9DRD-iF, BIOS 3.2 01/16/2015
>> [340441.917573] task: ffff8801b95c1b80 ti: ffff88034e504000 task.ti: ffff88034e504000
>> [340441.917859] RIP: 0010:[<ffffffffa081f774>] [<ffffffffa081f774>] btrfs_uuid_tree_iterate+0xf4/0x2d0 [btrfs]
>> [340441.918212] RSP: 0018:ffff88034e507e20 EFLAGS: 00010246
>> [340441.918382] RAX: 0000000000000000 RBX: 0000160000000000 RCX: ffff880000000000
>> [340441.918665] RDX: 0000000000000001 RSI: ffff8801e3abd140 RDI: ffff88046f027f00
>> [340441.918952] RBP: ffff88034e507ea8 R08: 000060fb80001760 R09: ffffffffa07ac1de
>> [340441.919236] R10: ffffe8ffffd41760 R11: ffffea00078eaf40 R12: ffff8801b98ab750
>> [340441.919521] R13: 00000000fffffffe R14: ffff8801e3abd140 R15: ffff880049586000
>> [340441.919810] FS: 0000000000000000(0000) GS:ffff88047fd40000(0000) knlGS:0000000000000000
>> [340441.920097] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> [340441.920267] CR2: 0000000000000098 CR3: 0000000001c0a000 CR4: 00000000000406e0
>> [340441.920554] Stack:
>> [340441.920717] ffff880049586000 ffff8801b98ab750 00003f7b00014fc0 ffff8803711dec08
>> [340441.921186] ffffffffa07d0c40 ffff880332342000 0000000000000114 1b7088046d7612f8
>> [340441.921655] 8cfb42689378e508 70157e0ade97f5d6 8c42689378e5081b 15157e0ade97f5d6
>> [340441.922126] Call Trace:
>> [340441.922315] [<ffffffffa07d0c40>] ? find_live_mirror.isra.18+0xc0/0xc0 [btrfs]
>> [340441.922614] [<ffffffffa07d0ae0>] ? btrfs_uuid_scan_kthread+0x3c0/0x3c0 [btrfs]
>> [340441.922917] [<ffffffffa07d0afb>] btrfs_uuid_rescan_kthread+0x1b/0x60 [btrfs]
>> [340441.923197] [<ffffffff8107161f>] kthread+0xef/0x110
>> [340441.923363] [<ffffffff81071530>] ? kthread_park+0x60/0x60
>> [340441.923531] [<ffffffff816149ff>] ret_from_fork+0x3f/0x70
>> [340441.923697] [<ffffffff81071530>] ? kthread_park+0x60/0x60
>> [340441.923863] Code: 0f 86 a0 00 00 00 48 bb 00 00 00 00 00 16 00 00 41 8b 44 24 40 48 b9 00 00 00 00 00 88 ff ff 8d 50 01 49 8b 04 24 41 89 54 24 40 <48> 03 98 98 00 00 00 48 89 d8 48 c1 f8 06 48 c1 e0 0c 3b 54 08
>> [340441.927296] RIP [<ffffffffa081f774>] btrfs_uuid_tree_iterate+0xf4/0x2d0 [btrfs]
>> [340441.927641] RSP <ffff88034e507e20>
>> [340441.927806] CR2: 0000000000000098
>>
>>
>> ffffffffa081f774 is in the heavily inlined btrfs_next_item. Here
>> is the decoded instructions, right before the crash with annotations:
>>
>> 0: 0f 86 a0 00 00 00 jbe 0xa6
>> 6: 48 bb 00 00 00 00 00 mov $0x160000000000,%rbx
>> d: 16 00 00
>> 10: 41 8b 44 24 40 mov 0x40(%r12),%eax ; r12 is btrfs_path, eax points to first slot
>> 15: 48 b9 00 00 00 00 00 mov $0xffff880000000000,%rcx
>> 1c: 88 ff ff
>> 1f: 8d 50 01 lea 0x1(%rax),%edx ; incr slot
>> 22: 49 8b 04 24 mov (%r12),%rax ; load first extent_buffer in rax
>> 26: 41 89 54 24 40 mov %edx,0x40(%r12) ; save incremented slot
>> 2b:* 48 03 98 98 00 00 00 add 0x98(%rax),%rbx <-- trapping instruction ; load the first page from the extent_buffer
>> 32: 48 89 d8 mov %rbx,%rax
>> 35: 48 c1 f8 06 sar $0x6,%rax
>> 39: 48 c1 e0 0c shl $0xc,%rax
>> 3d: 3b .byte 0x3b
>> 3e: 54 push %rsp
>> 3f: 08 .byte 0x8
>>
>> So as can be seen rax is zero and naturally dereferencing it is
>> also zero. What's interesting is the content of the btrf_path:
>>
>> struct btrfs_path {
>> nodes = {0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0},
>> slots = {1, 0, 0, 0, 0, 0, 0, 0},
>> locks = {0, 0, 0, 0, 0, 0, 0, 0},
>> reada = 0,
>> lowest_level = 0,
>> search_for_split = 0,
>> keep_locks = 0,
>> skip_locking = 0,
>> leave_spinning = 0,
>> search_commit_root = 0,
>> need_commit_sem = 0,
>> skip_release_on_error = 0
>> }
>>
>> Any ideas how come btrfs_path can be all zero, the one in
>> the first slot comes from the increment in btrfs_next_old_item.
>
> Thanks for all the extra details. It really must be this:
>
> if (ret > 0) {
> btrfs_release_path(path);
> ret = btrfs_uuid_iter_rem(root, uuid, key.type,
> subid_cpu);
> if (ret == 0) {
> /*
> * this might look inefficient, but the
> * justification is that it is an
> * exception that check_func returns 1,
> * and that in the regular case only one
> * entry per UUID exists.
> */
> goto again_search_slot;
> }
> if (ret < 0 && ret != -ENOENT)
> goto out;
> }
> item_size -= sizeof(subid_le);
> offset += sizeof(subid_le);
>
>
> We've released the path, which would explain why its full of NULL. ret
> was ENOENT, so it kept on going, and we fell through to
> btrfs_next_item()
>
> Once the path is released, we should either be searching again or
> exiting. A goto again_search_slot would probably fix it, but I'd want
> to also bump the key so we don't just process the same item over and
> over again.
>
> Can you reproduce this reliably? I'd hate to patch it now and make more
> problems later just because we didn't fully understand the items we were
> tripping over.
Hello Chris,
Indeed it seems that btrfs_uuid_iter_rem returned a ENOENT:
callq 0xffffffffa081f450 <btrfs_uuid_tree_rem>
mov %eax,%r13d
je 0xffffffffa081f882 <btrfs_uuid_tree_iterate+514> ; if uuid_iter_rem returned -ENOENT; else fall through.
I checked and r13d is not being touched between the invocation of
btrfs_uuid_iter_rem and the btrfs_next_item:
RIP: ffffffffa081f774 RSP: ffff88034e507e20 RFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000160000000000 RCX: ffff880000000000
RDX: 0000000000000001 RSI: ffff8801e3abd140 RDI: ffff88046f027f00
RBP: ffff88034e507ea8 R8: 000060fb80001760 R9: ffffffffa07ac1de
R10: ffffe8ffffd41760 R11: ffffea00078eaf40 R12: ffff8801b98ab750
R13: 00000000fffffffe R14: ffff8801e3abd140 R15: ffff880049586000
ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018
r13 is clearly -ENOENT. So your assumption was correct.
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