[syzbot] [bcachefs?] kernel BUG in bch2_evacuate_bucket

[syzbot <syzbot+bd56952613b5dae47ca4@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    9f16d5e6f220 Merge tag 'for-linus' of git://git.kernel.org..
git tree:       upstream
console+strace: https://syzkaller.appspot.com/x/log.txt?x=13260ee8580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=e92fc420ca55fe33
dashboard link: https://syzkaller.appspot.com/bug?extid=bd56952613b5dae47ca4
compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=124c25c0580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=164c25c0580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/c9f905470542/disk-9f16d5e6.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/5b4c9cc530ec/vmlinux-9f16d5e6.xz
kernel image: https://storage.googleapis.com/syzbot-assets/e0f262e4c35e/bzImage-9f16d5e6.xz
mounted in repro: https://storage.googleapis.com/syzbot-assets/8b2fe0894685/mount_0.gz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+bd56952613b5dae47ca4@syzkaller.appspotmail.com

------------[ cut here ]------------
kernel BUG at fs/bcachefs/bkey_types.h:210!
Oops: invalid opcode: 0000 [#1] PREEMPT SMP KASAN PTI
CPU: 1 UID: 0 PID: 5847 Comm: bch-copygc/loop Not tainted 6.12.0-syzkaller-09073-g9f16d5e6f220 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024
RIP: 0010:bkey_s_c_to_backpointer fs/bcachefs/bkey_types.h:210 [inline]
RIP: 0010:bch2_get_next_backpointer+0x1316/0x1320 fs/bcachefs/backpointers.c:257
Code: f9 fd e9 56 f9 ff ff e8 78 58 91 fd 90 0f 0b e8 d0 5a ba 07 e8 6b 58 91 fd 90 0f 0b e8 63 58 91 fd 90 0f 0b e8 5b 58 91 fd 90 <0f> 0b 0f 1f 84 00 00 00 00 00 90 90 90 90 90 90 90 90 90 90 90 90
RSP: 0018:ffffc90003dd6c80 EFLAGS: 00010293
RAX: ffffffff84048765 RBX: 00000000000000b3 RCX: ffff888033fc5a00
RDX: 0000000000000000 RSI: 00000000000000b3 RDI: 000000000000001c
RBP: ffffc90003dd6ff8 R08: ffffffff840480a8 R09: 0000000000000000
R10: 0000000000880000 R11: 0000000000000000 R12: ffff88807dae4000
R13: 1ffff920007bad9c R14: ffffc90003dd6ed0 R15: ffff88806f8c0160
FS:  0000000000000000(0000) GS:ffff8880b8700000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000055f337ebb000 CR3: 000000007db9a000 CR4: 00000000003526f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 bch2_evacuate_bucket+0x113c/0x3620 fs/bcachefs/move.c:708
 bch2_copygc+0x42c9/0x4ca0 fs/bcachefs/movinggc.c:240
 bch2_copygc_thread+0x737/0xc20 fs/bcachefs/movinggc.c:381
 kthread+0x2f0/0x390 kernel/kthread.c:389
 ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:bkey_s_c_to_backpointer fs/bcachefs/bkey_types.h:210 [inline]
RIP: 0010:bch2_get_next_backpointer+0x1316/0x1320 fs/bcachefs/backpointers.c:257
Code: f9 fd e9 56 f9 ff ff e8 78 58 91 fd 90 0f 0b e8 d0 5a ba 07 e8 6b 58 91 fd 90 0f 0b e8 63 58 91 fd 90 0f 0b e8 5b 58 91 fd 90 <0f> 0b 0f 1f 84 00 00 00 00 00 90 90 90 90 90 90 90 90 90 90 90 90
RSP: 0018:ffffc90003dd6c80 EFLAGS: 00010293
RAX: ffffffff84048765 RBX: 00000000000000b3 RCX: ffff888033fc5a00
RDX: 0000000000000000 RSI: 00000000000000b3 RDI: 000000000000001c
RBP: ffffc90003dd6ff8 R08: ffffffff840480a8 R09: 0000000000000000
R10: 0000000000880000 R11: 0000000000000000 R12: ffff88807dae4000
R13: 1ffff920007bad9c R14: ffffc90003dd6ed0 R15: ffff88806f8c0160
FS:  0000000000000000(0000) GS:ffff8880b8700000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000055f337c9a518 CR3: 000000007ce96000 CR4: 00000000003526f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


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