[syzbot] [btrfs?] WARNING in prepare_to_merge

Hello,

syzbot found the following issue on:

HEAD commit:    b95f03f04d47 Merge tag 'mm-hotfixes-stable-2026-07-20-11-3..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=148b4ab9580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=20c9876b0f77b546
dashboard link: https://syzkaller.appspot.com/bug?extid=bb14fd4ddd0269623f20
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-b95f03f0.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/46cc4d5161ad/vmlinux-b95f03f0.xz
kernel image: https://storage.googleapis.com/syzbot-assets/4cb305677dbb/bzImage-b95f03f0.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: [email protected]

BTRFS info (device loop0): balance: start -d -m
BTRFS info (device loop0): relocating block group 6881280 flags data|metadata
------------[ cut here ]------------
btrfs_abort_should_print_stack(__error)
WARNING: fs/btrfs/relocation.c:1850 at prepare_to_merge+0x11a7/0x1740 fs/btrfs/relocation.c:1850, CPU#0: syz.0.0/5325
Modules linked in:
CPU: 0 UID: 0 PID: 5325 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:prepare_to_merge+0x11a7/0x1740 fs/btrfs/relocation.c:1850
Code: fc ef ff ff 4c 89 e7 e8 d7 24 0c fe e9 ef ef ff ff e8 1d 4b a8 07 e8 f8 f8 9c fd eb 10 e8 f1 f8 9c fd eb 09 e8 ea f8 9c fd 90 <0f> 0b 90 44 89 e1 f7 d9 4c 8b 6c 24 38 e9 5c f9 ff ff 89 f9 80 e1
RSP: 0018:ffffc9000e63f700 EFLAGS: 00010287
RAX: ffffffff842988e6 RBX: ffff888052c2c001 RCX: 0000000000100000
RDX: ffffc90020001000 RSI: 00000000000723ec RDI: 00000000000723ed
RBP: ffffc9000e63f810 R08: ffff88801f4d2540 R09: 0000000000000003
R10: 00000000fffffffb R11: 0000000000000002 R12: 00000000ffffffe4
R13: ffff8880122865e8 R14: dffffc0000000000 R15: ffff88803fbda538
FS:  00007f43e4dd66c0(0000) GS:ffff88808c549000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 000000004478f000 CR4: 0000000000352ef0
Call Trace:
 <TASK>
 relocate_block_group+0xbc6/0xe90 fs/btrfs/relocation.c:3700
 do_nonremap_reloc+0xa7/0x560 fs/btrfs/relocation.c:5322
 btrfs_relocate_block_group+0x6e2/0xaf0 fs/btrfs/relocation.c:5489
 btrfs_relocate_chunk+0x114/0x810 fs/btrfs/volumes.c:3647
 __btrfs_balance+0x1b5f/0x29d0 fs/btrfs/volumes.c:4586
 btrfs_balance+0xaa6/0x1180 fs/btrfs/volumes.c:4973
 btrfs_ioctl_balance+0x3da/0x640 fs/btrfs/ioctl.c:3480
 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:63 [inline]
 do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f43e3f9de99
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:00007f43e4dd5fe8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f43e4226090 RCX: 00007f43e3f9de99
RDX: 0000200000000180 RSI: 00000000c4009420 RDI: 0000000000000008
RBP: 00007f43e4033eaf R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f43e4226128 R14: 00007f43e4226090 R15: 00007ffdeedc4388
 </TASK>


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