[syzbot] [bcachefs?] BUG: unable to handle kernel NULL pointer dereference in mempool_alloc_noprof (2)

[syzbot <syzbot+56edda805363e0a093b8@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    4cb6c8af8591 selftests/filesystems: Fix build of anon_inod..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=16a8a00c580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=43b7075a5c42ffca
dashboard link: https://syzkaller.appspot.com/bug?extid=56edda805363e0a093b8
compiler:       Debian clang version 20.1.6 (++20250514063057+1e4d39e07757-1~exp1~20250514183223.118), Debian LLD 20.1.6
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11c67ed4580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=147a300c580000

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-4cb6c8af.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/91b28032d866/vmlinux-4cb6c8af.xz
kernel image: https://storage.googleapis.com/syzbot-assets/7cf2a9f8c096/bzImage-4cb6c8af.xz
mounted in repro #1: https://storage.googleapis.com/syzbot-assets/9c0eef7ba9b5/mount_0.gz
mounted in repro #2: https://storage.googleapis.com/syzbot-assets/0c5386ea7a4e/mount_4.gz
  fsck result: failed (log: https://syzkaller.appspot.com/x/fsck.log?x=12a8a00c580000)

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+56edda805363e0a093b8@syzkaller.appspotmail.com

BUG: kernel NULL pointer dereference, address: 0000000000000000
#PF: supervisor instruction fetch in kernel mode
#PF: error_code(0x0010) - not-present page
PGD 0 P4D 0 
Oops: Oops: 0010 [#1] SMP KASAN NOPTI
CPU: 0 UID: 0 PID: 5317 Comm: syz-executor257 Not tainted 6.15.0-syzkaller-10402-g4cb6c8af8591 #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
RIP: 0010:0x0
Code: Unable to access opcode bytes at 0xffffffffffffffd6.
RSP: 0018:ffffc9000d3fedb8 EFLAGS: 00010246
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffff888044583ee8
RDX: 1ffff110088b07dd RSI: 0000000000000000 RDI: 0000000000092800
RBP: ffffc9000d3fef10 R08: ffffc9000d3fee87 R09: 0000000000000000
R10: ffffc9000d3fee60 R11: 0000000000000000 R12: 1ffff92001a7fdc8
R13: ffff888044583ee8 R14: 0000000000092c40 R15: 0000000000092800
FS:  0000000000000000(0000) GS:ffff88808d265000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffd6 CR3: 0000000042a6b000 CR4: 0000000000352ef0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 mempool_alloc_noprof+0x1a7/0x510 mm/mempool.c:402
 bch2_btree_update_start+0x549/0x1480 fs/bcachefs/btree_update_interior.c:1194
 bch2_btree_node_rewrite+0x17e/0x1120 fs/bcachefs/btree_update_interior.c:2208
 bch2_move_btree+0x6f0/0xc70 fs/bcachefs/move.c:1093
 bch2_scan_old_btree_nodes+0x95/0x240 fs/bcachefs/move.c:1215
 bch2_data_job+0x646/0x910 fs/bcachefs/move.c:1354
 bch2_data_thread+0x8f/0x1d0 fs/bcachefs/chardev.c:315
 kthread+0x711/0x8a0 kernel/kthread.c:464
 ret_from_fork+0x3fc/0x770 arch/x86/kernel/process.c:148
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>
Modules linked in:
CR2: 0000000000000000
---[ end trace 0000000000000000 ]---
RIP: 0010:0x0
Code: Unable to access opcode bytes at 0xffffffffffffffd6.
RSP: 0018:ffffc9000d3fedb8 EFLAGS: 00010246
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffff888044583ee8
RDX: 1ffff110088b07dd RSI: 0000000000000000 RDI: 0000000000092800
RBP: ffffc9000d3fef10 R08: ffffc9000d3fee87 R09: 0000000000000000
R10: ffffc9000d3fee60 R11: 0000000000000000 R12: 1ffff92001a7fdc8
R13: ffff888044583ee8 R14: 0000000000092c40 R15: 0000000000092800
FS:  0000000000000000(0000) GS:ffff88808d265000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffd6 CR3: 0000000042a6b000 CR4: 0000000000352ef0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


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