[syzbot] [btrfs?] kernel BUG in btrfs_backref_release_cache (4)

[syzbot <syzbot+918038c3f9425b05dcd3@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    d266640c6c76 Merge tag 'driver-core-7.3-rc5' of git://git...
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=16cc3315580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=2b924f079099ba6e
dashboard link: https://syzkaller.appspot.com/bug?extid=918038c3f9425b05dcd3
compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-d266640c.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/25e4cb4d0962/vmlinux-d266640c.xz
kernel image: https://storage.googleapis.com/syzbot-assets/ea6eec983242/bzImage-d266640c.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+918038c3f9425b05dcd3@syzkaller.appspotmail.com

BTRFS info (device loop0): balance: start -d -m
BTRFS info (device loop0): relocating block group 6881280 flags data|metadata
BTRFS info (device loop0): relocating block group 5242880 flags data|metadata
BTRFS info (device loop0): found 9 extents, stage: move data extents
assertion failed: !cache->nr_nodes, in fs/btrfs/backref.c:3163 (cache->nr_nodes=2)
------------[ cut here ]------------
kernel BUG at fs/btrfs/backref.c:3163!
Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI
CPU: 0 UID: 0 PID: 5328 Comm: syz.0.0 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010:btrfs_backref_release_cache+0x1fe/0x230 fs/btrfs/backref.c:3163
Code: 0f 0b e8 75 da 90 fd 48 c7 c7 20 c8 5b 8c 48 c7 c6 80 c8 5b 8c 48 c7 c2 80 bf 5b 8c b9 5b 0c 00 00 41 89 e8 e8 23 9e f0 fc 90 <0f> 0b e8 4b da 90 fd 48 c7 c7 c0 c8 5b 8c 48 c7 c6 20 c9 5b 8c 48
RSP: 0018:ffffc9000eb7f7e8 EFLAGS: 00010246
RAX: 0000000000000052 RBX: ffff88801fa82020 RCX: 980914ae5784ef00
RDX: 0000000000000000 RSI: 8000000000000000 RDI: 0000000000000000
RBP: 0000000000000002 R08: ffffc9000eb7f567 R09: 1ffff92001d6feac
R10: dffffc0000000000 R11: fffff52001d6fead R12: dffffc0000000000
R13: 1ffff11003f50404 R14: ffff88801fa820f0 R15: ffff8880525fd3b0
FS:  00007facfea6b6c0(0000) GS:ffff88808c2bc000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000001000 CR3: 000000001235c000 CR4: 0000000000352ef0
Call Trace:
 <TASK>
 relocate_block_group+0xb84/0xeb0 fs/btrfs/relocation.c:3756
 do_nonremap_reloc+0xa7/0x560 fs/btrfs/relocation.c:5391
 btrfs_relocate_block_group+0x6e2/0xaf0 fs/btrfs/relocation.c:5558
 btrfs_relocate_chunk+0x114/0x810 fs/btrfs/volumes.c:3729
 __btrfs_balance+0x1b5f/0x29d0 fs/btrfs/volumes.c:4668
 btrfs_balance+0xaa6/0x1180 fs/btrfs/volumes.c:5055
 btrfs_ioctl_balance+0x3da/0x640 fs/btrfs/ioctl.c:3494
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
 __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7facfdb9e159
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007facfea6afe8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007facfde26090 RCX: 00007facfdb9e159
RDX: 0000200000000180 RSI: 00000000c4009420 RDI: 0000000000000006
RBP: 00007facfdc3506b R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007facfde26128 R14: 00007facfde26090 R15: 00007fff41f35e48
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:btrfs_backref_release_cache+0x1fe/0x230 fs/btrfs/backref.c:3163
Code: 0f 0b e8 75 da 90 fd 48 c7 c7 20 c8 5b 8c 48 c7 c6 80 c8 5b 8c 48 c7 c2 80 bf 5b 8c b9 5b 0c 00 00 41 89 e8 e8 23 9e f0 fc 90 <0f> 0b e8 4b da 90 fd 48 c7 c7 c0 c8 5b 8c 48 c7 c6 20 c9 5b 8c 48
RSP: 0018:ffffc9000eb7f7e8 EFLAGS: 00010246
RAX: 0000000000000052 RBX: ffff88801fa82020 RCX: 980914ae5784ef00
RDX: 0000000000000000 RSI: 8000000000000000 RDI: 0000000000000000
RBP: 0000000000000002 R08: ffffc9000eb7f567 R09: 1ffff92001d6feac
R10: dffffc0000000000 R11: fffff52001d6fead R12: dffffc0000000000
R13: 1ffff11003f50404 R14: ffff88801fa820f0 R15: ffff8880525fd3b0
FS:  00007facfea6b6c0(0000) GS:ffff88808c2bc000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000001000 CR3: 000000001235c000 CR4: 0000000000352ef0


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