Re: [syzbot] [kernel?] linux-next test error: WARNING in switch_mm_irqs_off

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Apr 14, 2025 at 03:28:27AM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    b425262c07a6 Add linux-next specific files for 20250414
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=10ddb398580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=cc26bd6fced8397d
> dashboard link: https://syzkaller.appspot.com/bug?extid=c2537ce72a879a38113e
> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/e04788e9f74f/disk-b425262c.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/32ac1bacc159/vmlinux-b425262c.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/99cc84c793ed/bzImage-b425262c.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+c2537ce72a879a38113e@syzkaller.appspotmail.com
> 
> ------------[ cut here ]------------
> WARNING: CPU: 0 PID: 9 at arch/x86/mm/tlb.c:919 switch_mm_irqs_off+0x686/0x810 arch/x86/mm/tlb.c:918
> Modules linked in:
> CPU: 0 UID: 0 PID: 9 Comm: kworker/0:0 Not tainted 6.15.0-rc2-next-20250414-syzkaller #0 PREEMPT(full) 
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2025
> Workqueue: events once_deferred
> RIP: 0010:switch_mm_irqs_off+0x686/0x810 arch/x86/mm/tlb.c:918
> Code: 90 41 f7 c5 00 08 00 00 0f 84 ee fa ff ff 90 0f 0b 90 e9 e5 fa ff ff 90 0f 0b 90 e9 76 fe ff ff 90 0f 0b 90 e9 cc fb ff ff 90 <0f> 0b 90 4d 39 f4 0f 85 eb fb ff ff e9 31 fc ff ff 90 0f 0b 90 e9
> RSP: 0000:ffffc900000e7680 EFLAGS: 00010056
> RAX: 0000000000000001 RBX: 0000000000000000 RCX: ffffffff816ffd4d
> RDX: 0000000000000000 RSI: 0000000000000008 RDI: ffff88801b070940
> RBP: ffffc900000e7750 R08: ffff88801b070947 R09: 1ffff1100360e128
> R10: dffffc0000000000 R11: ffffed100360e129 R12: ffffffff8ee49240
> R13: ffff88801b070940 R14: ffffffff8ee49240 R15: 0000000000000000
> FS:  0000000000000000(0000) GS:ffff888124faa000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: ffff88823ffff000 CR3: 000000001b078000 CR4: 00000000003506f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  <TASK>
>  unuse_temporary_mm+0x9f/0x100 arch/x86/mm/tlb.c:1038
>  __text_poke+0x7b6/0xb40 arch/x86/kernel/alternative.c:2214
>  text_poke arch/x86/kernel/alternative.c:2257 [inline]
>  smp_text_poke_batch_finish+0x3e7/0x12c0 arch/x86/kernel/alternative.c:2565
>  arch_jump_label_transform_apply+0x1c/0x30 arch/x86/kernel/jump_label.c:146
>  static_key_disable_cpuslocked+0xd2/0x1c0 kernel/jump_label.c:240
>  static_key_disable+0x1a/0x20 kernel/jump_label.c:248
>  once_deferred+0x70/0xb0 lib/once.c:20
>  process_one_work kernel/workqueue.c:3238 [inline]
>  process_scheduled_works+0xac3/0x18e0 kernel/workqueue.c:3319
>  worker_thread+0x870/0xd50 kernel/workqueue.c:3400
>  kthread+0x7b7/0x940 kernel/kthread.c:464
>  ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:153
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>  </TASK>

So I can reproduce, and I I think I see what happens, except I'm
confused as to why the recently merged patches show this.

AFAIU what happens is that unuse_temporary_mm() clears the mm_cpumask()
for the current CPU, while switch_mm_irqs_off() then checks that the
mm_cpumask() bit is set for the current CPU.

This behaviour hasn't really changed since 209954cbc7d0 ("x86/mm/tlb:
Update mm_cpumask lazily") introduced both.

I'm not entirely sure what the best way forward is.. we can simply
delete the warning, or make use_temporary_mm() tag the special MMs
somehow and exclude them from the warning.