Re: BUG at fs/btrfs/relocation.c:794! Still happening on misc-next and 5.8.3

Qu Wenruo
2020-09-02T01:47:42Z

On 2020/9/2 上午8:14, Zygo Blaxell wrote:
> On Wed, Sep 02, 2020 at 07:33:21AM +0800, Qu Wenruo wrote:
>> This looks like a race between some reloc tree creation from some other
>> part.
>>
>> If you could add debug output for create_reloc_root() and its callers,
>> we may have a chance to debug it.
>
> The callers are always the same:
>
> btrfs_init_reloc_root+0x1b0
> record_root_in_trans+0x18c
> record_root_in_trans+0x8b
> start_transaction+0x189
>
> (gdb) l *(create_reloc_root+0x468)
> 0xffffffff81930848 is in create_reloc_root (fs/btrfs/relocation.c:1503).
> 1498 btrfs_tree_unlock(eb);
> 1499 free_extent_buffer(eb);
> 1500
> 1501 ret = btrfs_insert_root(trans, fs_info->tree_root,
> 1502 &root_key, root_item);
> 1503 BUG_ON(ret);
> 1504 kfree(root_item);
> 1505
> 1506 reloc_root = btrfs_read_tree_root(fs_info->tree_root, &root_key);
> 1507 BUG_ON(IS_ERR(reloc_root));
> (gdb) l *(btrfs_init_reloc_root+0x1b0)
> 0xffffffff81937db0 is in btrfs_init_reloc_root (fs/btrfs/relocation.c:1567).
> 1562 if (!trans->reloc_reserved) {
> 1563 rsv = trans->block_rsv;
> 1564 trans->block_rsv = rc->block_rsv;
> 1565 clear_rsv = 1;
> 1566 }
> 1567 reloc_root = create_reloc_root(trans, root, root->root_key.objectid);
> 1568 if (clear_rsv)
> 1569 trans->block_rsv = rsv;
> 1570
> 1571 ret = __add_reloc_root(reloc_root);
> (gdb) l *(record_root_in_trans+0x18c)
> 0xffffffff81889bfc is in record_root_in_trans (./include/asm-generic/bitops/instrumented-atomic.h:41).
> 36 *
> 37 * This is a relaxed atomic operation (no implied memory barriers).
> 38 */
> 39 static inline void clear_bit(long nr, volatile unsigned long *addr)
> 40 {
> 41 kasan_check_write(addr + BIT_WORD(nr), sizeof(long));
> 42 arch_clear_bit(nr, addr);
> 43 }
> 44
> 45 /**
> (gdb) l *(start_transaction+0x189)
> 0xffffffff8188f0d9 is in start_transaction (fs/btrfs/transaction.c:697).
> 692 * Thus it need to be called after current->journal_info initialized,
> 693 * or we can deadlock.
> 694 */
> 695 btrfs_record_root_in_trans(h, root);
> 696
> 697 return h;
> 698
> 699 join_fail:
> 700 if (type & __TRANS_FREEZABLE)
> 701 sb_end_intwrite(fs_info->sb);
> (gdb) quit
>
> It seems to be very early in the transaction. Is there anything to
> output here? Or are we more interested in what is left over from
> the previous transaction?

What I mean is, I want to see who else created the reloc tree, not only
who caused the EEXIST BUG_ON().

That's why I hope to add enough debug pr_info or whatever for
create_reloc_root(), so that we can catch the ordinary calls that seems
safe but may be unsafe for other callers.

Thanks,
Qu

>
>> But for the first step, we can hunt down the BUG_ON()s first and make it
>> exist more gracefully.
>>
>> I'll try to spare some time to do this in the following week.
>>
>> Thanks,
>> Qu
>>
>> On 2020/9/2 上午6:53, Zygo Blaxell wrote:
>>> On Fri, Aug 28, 2020 at 04:42:55PM -0400, Zygo Blaxell wrote:
>>>> On Fri, Aug 28, 2020 at 09:34:31AM +0300, Nikolay Borisov wrote:
>>>>> On 28.08.20 г. 3:08 ч., Zygo Blaxell wrote:
>>>>>> On Thu, Aug 27, 2020 at 08:03:13PM -0400, Zygo Blaxell wrote:
>>>>>>> On Tue, Aug 04, 2020 at 12:16:26PM -0400, Zygo Blaxell wrote:
>>>>>
>>>>> <snip>
>>>>>
>>>>> Since you can repro reliably could you modify the code in
>>>>> create_reloc_root so it prints what's the returned error value, I'd
>>>>> speculate it's EEXIST from
>>>>>
>>>>> btrfs_insert_root
>>>>> btrfs_insert_item
>>>>> btrfs_insert_empty_item
>>>>> btrfs_insert_empty_items
>>>>> btrfs_search_slot
>>>>>
>>>>> But better be sure.
>>>>
>>>> Here you go, EEXIST == 17:
>>>>
>>>> Aug 28 15:30:55 regress kernel: [18452.845182][T31546] BTRFS info (device dm-0): balance: start -dlimit=9
>>>> Aug 28 15:30:55 regress kernel: [18452.874627][T31546] BTRFS info (device dm-0): relocating block group 15873413742592 flags data
>>>> Aug 28 15:30:55 regress kernel: [18453.097516][ T2100] btrfs_search_slot ret = 0
>>>> Aug 28 15:30:55 regress kernel: [18453.104865][ T2100] btrfs_search_slot ret = 0
>>>> Aug 28 15:30:55 regress kernel: [18453.109751][ T2100] btrfs_search_slot ret = 0
>>>> Aug 28 15:30:56 regress kernel: [18454.453135][ T2100] btrfs_search_slot ret = 0
>>>> Aug 28 15:30:56 regress kernel: [18454.453955][ T2100] btrfs_insert_empty_item ret = -17
>>>> Aug 28 15:30:56 regress kernel: [18454.455022][ T2100] btrfs_insert_root ret = -17
>>>> Aug 28 15:30:56 regress kernel: [18454.456229][ T2100] ------------[ cut here ]------------
>>>> Aug 28 15:30:56 regress kernel: [18454.457622][ T2100] kernel BUG at fs/btrfs/relocation.c:795!
>>>
>>> I did a low-resolution bisect for this issue. I dug up 5.4, 5.5, 5.6,
>>> and 5.7 kernel sources, backported btrfs fixes from 5.4 to the obsolete
>>> kernels, and ran the tests on each kernel. Results:
>>>
>>> 5.8: kernel BUG at fs/btrfs/relocation.c:794
>>>
>>> 5.7: kernel BUG (same code but different line number)
>>>
>>> 5.6: kernel BUG (same as the others)
>>>
>>> 5.5: assertion failure (stack trace below)
>>>
>>> 5.4: kernel BUG (!)
>>>
>>> The 5.4 result is interesting--I've been running 5.4 for some time and
>>> not hit this before. So there are 3 possible theories:
>>>
>>> 1. It's because of sending signals to balance. That has been
>>> added to my test workload after 5.7 was released, so earlier
>>> tests on 5.4 would not have triggered it.
>>>
>>> 2. There's a regression in 5.4-stable, which I've cherry-picked
>>> to all the other kernels during my test setup. (On the other
>>> hand, if I don't backport some fixes, kernels 5.5..5.7 crash
>>> before they get to this bug.)
>>>
>>> 3. There's something rotten in my test filesystem, and the
>>> BUG will go away for a while if I do a mkfs. Qu asked for
>>> a dump earlier in this thread, and I provided one.
>>>
>>> All three of these theories are testable to some extent, so I'll have
>>> my test VM run some variations.
>>>
>>> The full test workload is:
>>>
>>> balance metadata or data at random intervals
>>>
>>> scrub, scrub cancel at random intervals
>>>
>>> 20x rsync updating files
>>>
>>> snapshot create, delete at random intervals
>>>
>>> bees dedupe (lots of EXTENT_SAME and LOGICAL_INO calls)
>>>
>>> balance cancel at random intervals
>>>
>>> kill -9 $(pidof btrfs balance) at random intervals (NEW - added
>>> when 5.7 came out)
>>>
>>> This is the 5.5 root assertion failure:
>>>
>>> Sep 1 04:48:48 regress kernel: [10642.537825][T24161] assertion failed: root, in fs/btrfs/relocation.c:837
>>> Sep 1 04:48:48 regress kernel: [10642.538911][T24161] ------------[ cut here ]------------
>>> Sep 1 04:48:48 regress kernel: [10642.539704][T24161] kernel BUG at fs/btrfs/ctree.h:3125!
>>> Sep 1 04:48:48 regress kernel: [10642.540621][T24161] invalid opcode: 0000 [#1] SMP KASAN PTI
>>> Sep 1 04:48:48 regress kernel: [10642.540624][ T4639] irq event stamp: 49626809
>>> Sep 1 04:48:48 regress kernel: [10642.540632][ T4639] hardirqs last enabled at (49626809): [<ffffffff8a00481a>] trace_hardirqs_on_thunk+0x1a/0x1c
>>> Sep 1 04:48:48 regress kernel: [10642.541451][T24161] CPU: 1 PID: 24161 Comm: btrfs Tainted: G W 5.5.19-76348822ab91+ #14
>>> Sep 1 04:48:48 regress kernel: [10642.542114][ T4639] hardirqs last disabled at (49626808): [<ffffffff8a004836>] trace_hardirqs_off_thunk+0x1a/0x1c
>>> Sep 1 04:48:48 regress kernel: [10642.543693][T24161] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
>>> Sep 1 04:48:48 regress kernel: [10642.545017][ T4639] softirqs last enabled at (49626726): [<ffffffff8bc0048b>] __do_softirq+0x48b/0x5be
>>> Sep 1 04:48:48 regress kernel: [10642.545023][ T4639] softirqs last disabled at (49626715): [<ffffffff8a1258a2>] irq_exit+0x112/0x120
>>> Sep 1 04:48:48 regress kernel: [10642.546536][T24161] RIP: 0010:assertfail.constprop.42+0x1c/0x1e
>>> Sep 1 04:48:49 regress kernel: [10642.551589][T24161] Code: 48 c7 c6 c0 90 03 8c e8 89 29 f1 ff 0f 0b 55 89 f1 48 c7 c2 40 82 03 8c 48 89 fe 48 c7 c7 60 83 03 8c 48 89 e5 e8 41 b0 90 ff <0f> 0b 0f 1f 44 00 00 55 48 89 e5 41 54 49 89 f4 53 48 89 fb 48 83
>>> Sep 1 04:48:49 regress kernel: [10642.554495][T24161] RSP: 0018:ffffc90002327150 EFLAGS: 00010282
>>> Sep 1 04:48:49 regress kernel: [10642.555371][T24161] RAX: 0000000000000034 RBX: 000004513701c000 RCX: ffffffff8a264242
>>> Sep 1 04:48:49 regress kernel: [10642.556515][T24161] RDX: 0000000000000000 RSI: 0000000000000008 RDI: ffff8881f580004c
>>> Sep 1 04:48:49 regress kernel: [10642.557680][T24161] RBP: ffffc90002327150 R08: ffffed103eb017e1 R09: ffffed103eb017e1
>>> Sep 1 04:48:49 regress kernel: [10642.558895][T24161] R10: ffffed103eb017e0 R11: ffff8881f580bf07 R12: ffff88800d1ea5c0
>>> Sep 1 04:48:49 regress kernel: [10642.560139][T24161] R13: ffffc900023273e0 R14: 0000000000000000 R15: ffff8880bbf238f0
>>> Sep 1 04:48:49 regress kernel: [10642.561391][T24161] FS: 00007f03f48488c0(0000) GS:ffff8881f5600000(0000) knlGS:0000000000000000
>>> Sep 1 04:48:49 regress kernel: [10642.562779][T24161] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>> Sep 1 04:48:49 regress kernel: [10642.563801][T24161] CR2: 00007f1cab76f718 CR3: 0000000189a5e004 CR4: 00000000001606e0
>>> Sep 1 04:48:49 regress kernel: [10642.565046][T24161] Call Trace:
>>> Sep 1 04:48:49 regress kernel: [10642.565565][T24161] build_backref_tree+0x186b/0x2590
>>> Sep 1 04:48:49 regress kernel: [10642.566389][T24161] ? relocate_data_extent+0x1a0/0x1a0
>>> Sep 1 04:48:49 regress kernel: [10642.567295][T24161] ? lock_downgrade+0x3d0/0x3d0
>>> Sep 1 04:48:49 regress kernel: [10642.568142][T24161] ? match_held_lock+0x20/0x260
>>> Sep 1 04:48:49 regress kernel: [10642.568925][T24161] ? do_raw_spin_unlock+0xa8/0x140
>>> Sep 1 04:48:49 regress kernel: [10642.569765][T24161] ? _raw_spin_trylock_bh+0x60/0x80
>>> Sep 1 04:48:49 regress kernel: [10642.570605][T24161] ? release_extent_buffer+0x13b/0x230
>>> Sep 1 04:48:49 regress kernel: [10642.571480][T24161] ? free_extent_buffer.part.45+0xd7/0x140
>>> Sep 1 04:48:49 regress kernel: [10642.572406][T24161] relocate_tree_blocks+0x204/0xa50
>>> Sep 1 04:48:49 regress kernel: [10642.573244][T24161] ? build_backref_tree+0x2590/0x2590
>>> Sep 1 04:48:49 regress kernel: [10642.574103][T24161] ? rb_insert_color+0x3af/0x400
>>> Sep 1 04:48:49 regress kernel: [10642.574896][T24161] ? kmem_cache_alloc_trace+0x5af/0x740
>>> Sep 1 04:48:49 regress kernel: [10642.575785][T24161] ? tree_insert+0x90/0xb0
>>> Sep 1 04:48:49 regress kernel: [10642.576495][T24161] ? add_tree_block.isra.38+0x1d6/0x230
>>> Sep 1 04:48:49 regress kernel: [10642.577387][T24161] relocate_block_group+0x528/0x9d0
>>> Sep 1 04:48:49 regress kernel: [10642.578220][T24161] ? merge_reloc_roots+0x470/0x470
>>> Sep 1 04:48:49 regress kernel: [10642.579047][T24161] btrfs_relocate_block_group+0x26e/0x4c0
>>> Sep 1 04:48:49 regress kernel: [10642.579968][T24161] btrfs_relocate_chunk+0x52/0xf0
>>> Sep 1 04:48:49 regress kernel: [10642.580773][T24161] btrfs_balance+0xe5b/0x1800
>>> Sep 1 04:48:49 regress kernel: [10642.581542][T24161] ? btrfs_relocate_chunk+0xf0/0xf0
>>> Sep 1 04:48:49 regress kernel: [10642.582381][T24161] ? kmem_cache_alloc_trace+0x5af/0x740
>>> Sep 1 04:48:49 regress kernel: [10642.583270][T24161] ? _copy_from_user+0xaa/0xd0
>>> Sep 1 04:48:49 regress kernel: [10642.584022][T24161] btrfs_ioctl_balance+0x3de/0x4c0
>>> Sep 1 04:48:49 regress kernel: [10642.584819][T24161] btrfs_ioctl+0x3122/0x4470
>>> Sep 1 04:48:49 regress kernel: [10642.585540][T24161] ? __asan_loadN+0xf/0x20
>>> Sep 1 04:48:49 regress kernel: [10642.586229][T24161] ? __asan_loadN+0xf/0x20
>>> Sep 1 04:48:49 regress kernel: [10642.586920][T24161] ? btrfs_ioctl_get_supported_features+0x30/0x30
>>> Sep 1 04:48:49 regress kernel: [10642.587935][T24161] ? __asan_loadN+0xf/0x20
>>> Sep 1 04:48:49 regress kernel: [10642.588649][T24161] ? pvclock_clocksource_read+0xeb/0x190
>>> Sep 1 04:48:49 regress kernel: [10642.589566][T24161] ? __asan_loadN+0xf/0x20
>>> Sep 1 04:48:49 regress kernel: [10642.590254][T24161] ? pvclock_clocksource_read+0xeb/0x190
>>> Sep 1 04:48:49 regress kernel: [10642.591128][T24161] ? __kasan_check_read+0x11/0x20
>>> Sep 1 04:48:49 regress kernel: [10642.591913][T24161] ? check_chain_key+0x1e6/0x2e0
>>> Sep 1 04:48:49 regress kernel: [10642.592707][T24161] ? __asan_loadN+0xf/0x20
>>> Sep 1 04:48:49 regress kernel: [10642.593409][T24161] ? pvclock_clocksource_read+0xeb/0x190
>>> Sep 1 04:48:49 regress kernel: [10642.594312][T24161] ? kvm_sched_clock_read+0x18/0x30
>>> Sep 1 04:48:49 regress kernel: [10642.595139][T24161] ? check_chain_key+0x1e6/0x2e0
>>> Sep 1 04:48:49 regress kernel: [10642.595929][T24161] ? sched_clock_cpu+0x1b/0x120
>>> Sep 1 04:48:49 regress kernel: [10642.596712][T24161] do_vfs_ioctl+0x13e/0xad0
>>> Sep 1 04:48:49 regress kernel: [10642.597432][T24161] ? btrfs_ioctl_get_supported_features+0x30/0x30
>>> Sep 1 04:48:49 regress kernel: [10642.598455][T24161] ? do_vfs_ioctl+0x13e/0xad0
>>> Sep 1 04:48:49 regress kernel: [10642.599202][T24161] ? compat_ioctl_preallocate+0x170/0x170
>>> Sep 1 04:48:49 regress kernel: [10642.600128][T24161] ? __kasan_check_write+0x14/0x20
>>> Sep 1 04:48:49 regress kernel: [10642.600949][T24161] ? up_read+0x176/0x4f0
>>> Sep 1 04:48:49 regress kernel: [10642.601648][T24161] ? down_write_nested+0x2d0/0x2d0
>>> Sep 1 04:48:49 regress kernel: [10642.602476][T24161] ? handle_mm_fault+0x211/0x480
>>> Sep 1 04:48:49 regress kernel: [10642.603263][T24161] ? __kasan_check_read+0x11/0x20
>>> Sep 1 04:48:49 regress kernel: [10642.604062][T24161] ? __fget_light+0xb2/0x110
>>> Sep 1 04:48:49 regress kernel: [10642.604805][T24161] ksys_ioctl+0x67/0x90
>>> Sep 1 04:48:49 regress kernel: [10642.605471][T24161] __x64_sys_ioctl+0x43/0x50
>>> Sep 1 04:48:49 regress kernel: [10642.606203][T24161] do_syscall_64+0x77/0x2d0
>>> Sep 1 04:48:49 regress kernel: [10642.606933][T24161] entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>> Sep 1 04:48:49 regress kernel: [10642.607875][T24161] RIP: 0033:0x7f03f493b427
>>> Sep 1 04:48:49 regress kernel: [10642.608588][T24161] Code: 00 00 90 48 8b 05 69 aa 0c 00 64 c7 00 26 00 00 00 48 c7 c0 ff ff ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 b8 10 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 39 aa 0c 00 f7 d8 64 89 01 48
>>> Sep 1 04:48:49 regress kernel: [10642.611697][T24161] RSP: 002b:00007ffd6bd78fb8 EFLAGS: 00000206 ORIG_RAX: 0000000000000010
>>> Sep 1 04:48:49 regress kernel: [10642.613035][T24161] RAX: ffffffffffffffda RBX: 00007ffd6bd79058 RCX: 00007f03f493b427
>>> Sep 1 04:48:49 regress kernel: [10642.614313][T24161] RDX: 00007ffd6bd79058 RSI: 00000000c4009420 RDI: 0000000000000003
>>> Sep 1 04:48:49 regress kernel: [10642.615605][T24161] RBP: 0000000000000003 R08: 0000000000000003 R09: 0000000000000078
>>> Sep 1 04:48:49 regress kernel: [10642.616877][T24161] R10: fffffffffffff5ab R11: 0000000000000206 R12: 0000000000000001
>>> Sep 1 04:48:49 regress kernel: [10642.618132][T24161] R13: 0000000000000000 R14: 00007ffd6bd7aa46 R15: 0000000000000001
>>> Sep 1 04:48:49 regress kernel: [10642.619378][T24161] Modules linked in:
>>> Sep 1 04:48:49 regress kernel: [10642.620153][T24161] ---[ end trace a33c17a7d43dd973 ]---
>>>