[syzbot] [mm?] WARNING: bad unlock balance in folios_put_refs

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: c1f49dea2b8f Merge tag 'mm-hotfixes-stable-2026-04-19-00-1..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=140711ba580000
kernel config: https://syzkaller.appspot.com/x/.config?x=507c1c0a12a79510
dashboard link: https://syzkaller.appspot.com/bug?extid=c4f8158debe41f3bca37
compiler: gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
userspace arch: i386

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-c1f49dea.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/98ce9fed1a97/vmlinux-c1f49dea.xz
kernel image: https://storage.googleapis.com/syzbot-assets/b02e163ec959/bzImage-c1f49dea.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+c4f815...@syzkaller.appspotmail.com

=====================================
WARNING: bad unlock balance detected!
syzkaller #0 Not tainted
-------------------------------------
dhcpcd/5934 is trying to release lock (rcu_read_lock) at:
[<ffffffff8248262a>] rcu_lock_release include/linux/rcupdate.h:310 [inline]
[<ffffffff8248262a>] rcu_read_unlock include/linux/rcupdate.h:869 [inline]
[<ffffffff8248262a>] lruvec_unlock_irqrestore include/linux/memcontrol.h:1493 [inline]
[<ffffffff8248262a>] folios_put_refs+0x50a/0xa90 mm/swap.c:1000
but there are no more locks to release!

other info that might help us debug this:
1 lock held by dhcpcd/5934:
#0: ffff8880134bf178 (&mm->mmap_lock){++++}-{4:4}, at: mmap_write_lock include/linux/mmap_lock.h:536 [inline]
#0: ffff8880134bf178 (&mm->mmap_lock){++++}-{4:4}, at: exit_mmap+0x22c/0xa10 mm/mmap.c:1308

stack backtrace:
CPU: 2 UID: 0 PID: 5934 Comm: dhcpcd 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
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
print_unlock_imbalance_bug.part.0+0xfb/0x106 kernel/locking/lockdep.c:5298
print_unlock_imbalance_bug kernel/locking/lockdep.c:5278 [inline]
__lock_release kernel/locking/lockdep.c:5537 [inline]
lock_release kernel/locking/lockdep.c:5889 [inline]
lock_release+0x28d/0x310 kernel/locking/lockdep.c:5875
rcu_lock_release include/linux/rcupdate.h:310 [inline]
rcu_read_unlock include/linux/rcupdate.h:869 [inline]
lruvec_unlock_irqrestore include/linux/memcontrol.h:1493 [inline]
folios_put_refs+0x50f/0xa90 mm/swap.c:1000
free_pages_and_swap_cache+0x22d/0x3b0 mm/swap_state.c:401
__tlb_batch_free_encoded_pages+0xe9/0x280 mm/mmu_gather.c:138
tlb_batch_pages_flush mm/mmu_gather.c:151 [inline]
tlb_flush_mmu_free mm/mmu_gather.c:417 [inline]
tlb_flush_mmu mm/mmu_gather.c:424 [inline]
tlb_finish_mmu+0x1b0/0x810 mm/mmu_gather.c:549
exit_mmap+0x454/0xa10 mm/mmap.c:1313
__mmput+0x12a/0x410 kernel/fork.c:1178
mmput+0x67/0x80 kernel/fork.c:1201
exit_mm kernel/exit.c:581 [inline]
do_exit+0x833/0x2a60 kernel/exit.c:963
do_group_exit+0xd5/0x2a0 kernel/exit.c:1117
__do_sys_exit_group kernel/exit.c:1128 [inline]
__se_sys_exit_group kernel/exit.c:1126 [inline]
__x64_sys_exit_group+0x3e/0x50 kernel/exit.c:1126
x64_sys_call+0x102c/0x1530 arch/x86/include/generated/asm/syscalls_64.h:232
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x10b/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fc4db37a6c5
Code: Unable to access opcode bytes at 0x7fc4db37a69b.
RSP: 002b:00007fffbd1b3848 EFLAGS: 00000206 ORIG_RAX: 00000000000000e7
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007fc4db37a6c5
RDX: 00000000000000e7 RSI: ffffffffffffff88 RDI: 0000000000000001
RBP: 00007fffbd1b3e58 R08: 000055eb8d7db2c0 R09: 0000000000000002
R10: 00000000000000e0 R11: 0000000000000206 R12: 00007fffbd1b3890
R13: 000055eb8d7dc8a0 R14: 00007fffbd1b3ad0 R15: 00007fffbd1b3880
</TASK>
------------[ cut here ]------------
rrln < 0 || rrln > RCU_NEST_PMAX
WARNING: kernel/rcu/tree_plugin.h:443 at __rcu_read_unlock kernel/rcu/tree_plugin.h:443 [inline], CPU#2: dhcpcd/5934
WARNING: kernel/rcu/tree_plugin.h:443 at __rcu_read_unlock+0x235/0x5e0 kernel/rcu/tree_plugin.h:430, CPU#2: dhcpcd/5934
Modules linked in:
CPU: 2 UID: 0 PID: 5934 Comm: dhcpcd 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:__rcu_read_unlock kernel/rcu/tree_plugin.h:443 [inline]
RIP: 0010:__rcu_read_unlock+0x235/0x5e0 kernel/rcu/tree_plugin.h:430
Code: 74 11 c7 45 58 01 00 00 00 bf 09 00 00 00 e8 a2 a4 da ff e8 8d e0 22 00 9c 58 f6 c4 02 0f 85 dd 02 00 00 fb e9 57 fe ff ff 90 <0f> 0b 90 5b 5d 41 5c 41 5d 41 5e 41 5f c3 cc cc cc cc e8 14 62 87
RSP: 0018:ffffc900045df718 EFLAGS: 00010286
RAX: 00000000ffffffff RBX: ffff88802a092500 RCX: ffffffff81e80bfe
RDX: 0000000000000000 RSI: ffffffff8df2c0ea RDI: ffff88802a0929c4
RBP: 000000000000001f R08: 0000000000000005 R09: 0000000000000000
R10: 0000000080000000 R11: 0000000000000001 R12: ffffc900045df8fc
R13: dffffc0000000000 R14: ffffc900045df920 R15: ffffc900045dfa20
FS: 0000000000000000(0000) GS:ffff8880972ee000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fc4db4814c0 CR3: 000000004fd57000 CR4: 0000000000352ef0
Call Trace:
<TASK>
rcu_read_unlock include/linux/rcupdate.h:871 [inline]
lruvec_unlock_irqrestore include/linux/memcontrol.h:1493 [inline]
folios_put_refs+0x514/0xa90 mm/swap.c:1000
free_pages_and_swap_cache+0x22d/0x3b0 mm/swap_state.c:401
__tlb_batch_free_encoded_pages+0xe9/0x280 mm/mmu_gather.c:138
tlb_batch_pages_flush mm/mmu_gather.c:151 [inline]
tlb_flush_mmu_free mm/mmu_gather.c:417 [inline]
tlb_flush_mmu mm/mmu_gather.c:424 [inline]
tlb_finish_mmu+0x1b0/0x810 mm/mmu_gather.c:549
exit_mmap+0x454/0xa10 mm/mmap.c:1313
__mmput+0x12a/0x410 kernel/fork.c:1178
mmput+0x67/0x80 kernel/fork.c:1201
exit_mm kernel/exit.c:581 [inline]
do_exit+0x833/0x2a60 kernel/exit.c:963
do_group_exit+0xd5/0x2a0 kernel/exit.c:1117
__do_sys_exit_group kernel/exit.c:1128 [inline]
__se_sys_exit_group kernel/exit.c:1126 [inline]
__x64_sys_exit_group+0x3e/0x50 kernel/exit.c:1126
x64_sys_call+0x102c/0x1530 arch/x86/include/generated/asm/syscalls_64.h:232
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x10b/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fc4db37a6c5
Code: Unable to access opcode bytes at 0x7fc4db37a69b.
RSP: 002b:00007fffbd1b3848 EFLAGS: 00000206 ORIG_RAX: 00000000000000e7
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007fc4db37a6c5
RDX: 00000000000000e7 RSI: ffffffffffffff88 RDI: 0000000000000001
RBP: 00007fffbd1b3e58 R08: 000055eb8d7db2c0 R09: 0000000000000002
R10: 00000000000000e0 R11: 0000000000000206 R12: 00007fffbd1b3890
R13: 000055eb8d7dc8a0 R14: 00007fffbd1b3ad0 R15: 00007fffbd1b3880
</TASK>


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