Re: [syzbot] [kernel?] WARNING in sched_mm_cid_remote_clear (3)

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2025-07-28 10:46, Thomas Gleixner wrote:
> On Mon, Jul 28 2025 at 06:22, syzbot wrote:
> 
> CC+ Mathieu and leaving the context intact.
> 
>> Hello,
>>
>> syzbot found the following issue on:
>>
>> HEAD commit:    25fae0b93d1d Merge tag 'drm-fixes-2025-07-24' of https://g..
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=136930a2580000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=9f175a9275d2cdd7
>> dashboard link: https://syzkaller.appspot.com/bug?extid=c3be4001853e1add0a03
>> compiler:       gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
>>
>> Unfortunately, I don't have any reproducer for this issue yet.
>>
>> Downloadable assets:
>> disk image: https://storage.googleapis.com/syzbot-assets/7acf9d19f311/disk-25fae0b9.raw.xz
>> vmlinux: https://storage.googleapis.com/syzbot-assets/532080ae9996/vmlinux-25fae0b9.xz
>> kernel image: https://storage.googleapis.com/syzbot-assets/47473831ee3b/bzImage-25fae0b9.xz
>>
>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>> Reported-by: syzbot+c3be4001853e1add0a03@syzkaller.appspotmail.com
>>
>> ------------[ cut here ]------------
>> WARNING: CPU: 1 PID: 12208 at ./include/linux/cpumask.h:135 cpu_max_bits_warn include/linux/cpumask.h:135 [inline]
> 
> That means __mm_cid_put() is invoked with cid >= nr_cpu_ids ....

I'm trying to figure out how this can happen. One potential
scenario I see is if we have mm->nr_cpus_allowed > num_possible_cpus().
Is that possible ?

We strongly rely on having mm->nr_cpus_allowed <= num_possible_cpus()
in __mm_cid_try_get().

When a process is created, it takes the main thread's p->nr_cpus_allowed
as initial value. Then it is updated by mm_set_cpus_allowed(). It is
basically a union each thread allowed CPUs masks.

Can we expect all scheduler classes (including ext) to guarantee the
affinity context only includes CPUs that are present in the possible
CPU mask ?

Thanks,

Mathieu

> 
>> WARNING: CPU: 1 PID: 12208 at ./include/linux/cpumask.h:135 cpumask_check include/linux/cpumask.h:142 [inline]
>> WARNING: CPU: 1 PID: 12208 at ./include/linux/cpumask.h:135 cpumask_clear_cpu include/linux/cpumask.h:620 [inline]
>> WARNING: CPU: 1 PID: 12208 at ./include/linux/cpumask.h:135 __mm_cid_put kernel/sched/sched.h:3622 [inline]
>> WARNING: CPU: 1 PID: 12208 at ./include/linux/cpumask.h:135 sched_mm_cid_remote_clear+0x488/0x4f0 kernel/sched/core.c:10543
>> Modules linked in:
>> CPU: 1 UID: 0 PID: 12208 Comm: syz.4.2719 Not tainted 6.16.0-rc7-syzkaller-00034-g25fae0b93d1d #0 PREEMPT(full)
>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
>> RIP: 0010:cpu_max_bits_warn include/linux/cpumask.h:135 [inline]
>> RIP: 0010:cpumask_check include/linux/cpumask.h:142 [inline]
>> RIP: 0010:cpumask_clear_cpu include/linux/cpumask.h:620 [inline]
>> RIP: 0010:__mm_cid_put kernel/sched/sched.h:3622 [inline]
>> RIP: 0010:sched_mm_cid_remote_clear+0x488/0x4f0 kernel/sched/core.c:10543
>> Code: 0f 01 e8 3b 71 0e 00 eb 9d 4c 89 ef e8 f1 73 97 00 e9 67 fc ff ff e8 e7 73 97 00 e9 47 fd ff ff 89 44 24 48 e9 17 fe ff ff 90 <0f> 0b 90 e9 f0 fd ff ff e8 8b b3 fc 09 e9 06 ff ff ff 48 89 d6 48
>> RSP: 0018:ffffc9000480fcc8 EFLAGS: 00010012
>> RAX: 0000000000000093 RBX: ffffe8ffffc476c0 RCX: ffffffff8189c611
>> RDX: 00000000ffffffff RSI: 0000000000000004 RDI: ffffc9000480fd10
>> RBP: 1ffff92000901f9a R08: 0000000000000001 R09: fffff52000901fa2
>> R10: 0000000000000003 R11: 0000000000000001 R12: ffff88807b22ed50
>> R13: ffffe8ffffc476c8 R14: 0000000000000200 R15: ffff8880b843a2c0
>> FS:  000055556f5a3500(0000) GS:ffff888124820000(0000) knlGS:0000000000000000
>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> CR2: 000055558d727588 CR3: 00000000668d0000 CR4: 00000000003526f0
>> Call Trace:
>>   <TASK>
>>   sched_mm_cid_remote_clear_old kernel/sched/core.c:10576 [inline]
>>   task_mm_cid_work+0x3b4/0x910 kernel/sched/core.c:10626
>>   task_work_run+0x14d/0x240 kernel/task_work.c:227
>>   resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
>>   exit_to_user_mode_loop+0xeb/0x110 kernel/entry/common.c:114
>>   exit_to_user_mode_prepare include/linux/entry-common.h:330 [inline]
>>   syscall_exit_to_user_mode_work include/linux/entry-common.h:414 [inline]
>>   syscall_exit_to_user_mode include/linux/entry-common.h:449 [inline]
>>   do_syscall_64+0x3f6/0x4c0 arch/x86/entry/syscall_64.c:100
>>   entry_SYSCALL_64_after_hwframe+0x77/0x7f
>> RIP: 0033:0x7f8ef99c1265
>> Code: 24 0c 89 3c 24 48 89 4c 24 18 e8 f6 54 ff ff 4c 8b 54 24 18 48 8b 54 24 10 41 89 c0 8b 74 24 0c 8b 3c 24 b8 e6 00 00 00 0f 05 <44> 89 c7 48 89 04 24 e8 4f 55 ff ff 48 8b 04 24 48 83 c4 28 f7 d8
>> RSP: 002b:00007ffe741fc2d0 EFLAGS: 00000293 ORIG_RAX: 00000000000000e6
>> RAX: 0000000000000000 RBX: 00007f8ef9bb5fa0 RCX: 00007f8ef99c1265
>> RDX: 00007ffe741fc310 RSI: 0000000000000000 RDI: 0000000000000000
>> RBP: 00007f8ef9bb7ba0 R08: 0000000000000000 R09: 00007f8efa7eb000
>> R10: 0000000000000000 R11: 0000000000000293 R12: 0000000000048110
>> R13: 00007f8ef9bb6080 R14: ffffffffffffffff R15: 00007ffe741fc450
>>   </TASK>
>>
>>
>> ---
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-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com