Re: [syzbot] [kvm?] WARNING: locking bug in __refill_objects_node

[Steven Rostedt]

On Tue, 27 Oct 2026 18:48:28 +0000
Harry Yoo <harry@xxxxxxxxxx> wrote:

> [+Cc RT folks]
> 
> On Tue, Oct 27, 2026 at 07:00:16AM -0700, Sean Christopherson wrote:
> > +slub folks
> > 
> > On Tue, Oct 27, 2026, Hillf Danton wrote:  
> > > Ask the locking people to shed light on the warning.  
> > 
> > get_partial_node_bulk() is waiting on a non-raw spinlock while in hard IRQ context,
> > and it looks like __refill_objects_node() would do the same later on.  On PREEMPT_RT,
> > those spinlocks become sleepable locks, which is what triggers the splat.
> > 
> > The alloction coming from KVM-as-a-guest code is GFP_ATOMIC, so I think that wonky
> > code is fine.  
> 
> But memory allocation w/ GFP_ATOMIC is fine only on non-RT?

Correct. GFP_ATOMIC is meaningless on RT. It still cannot be called with
preemption disabled. Most places that disable preemption in the non-RT case
do not disable it in the RT case. For example, spin_lock() disables
preemption on non-RT but does not in RT. Same for interrupt handlers, as in
RT interrupt handlers are run in task context.

That is, in RT only "special" places disable preemption for real.

> 
> >From the documentation:  
> https://docs.kernel.org/core-api/real-time/differences.html#memory-allocation
> > The memory allocation APIs, such as kmalloc() and alloc_pages(),
> > require a gfp_t flag to indicate the allocation context.
> > On non-PREEMPT_RT kernels, it is necessary to use GFP_ATOMIC when
> > allocating memory from interrupt context or from sections where
> > preemption is disabled. This is because the allocator must not
> > sleep in these contexts waiting for memory to become available.  
> 
> Here it mentions GFP_ATOMIC allocations in preemption disabled
> contexts on non-RT, and...
> 
> > However, this approach does not work on PREEMPT_RT kernels.
> > The memory allocator in PREEMPT_RT uses sleeping locks internally,
> > which cannot be acquired when preemption is disabled.  
> 
> it says we shoud not allocate memory in such a context on RT?
> 
> > Fortunately,
> > this is generally not a problem, because PREEMPT_RT moves most contexts
> > that would traditionally run with preemption or interrupts disabled
> > into threaded context, where sleeping is allowed.  
> 
> Not sure if this applies here as I don't understand what's
> going in in KVM world.
> 
> > > > Date: Mon, 26 Oct 2026 03:19:37 -0700	[thread overview]
> > > > Hello,
> > > > 
> > > > syzbot found the following issue on:
> > > > 
> > > > HEAD commit:    c03c1a14507d Linux 7.3
> > > > git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
> > > > console output: https://syzkaller.appspot.com/x/log.txt?x=128d8ca5580000
> > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=129d675874dbf896
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=f11512d1e5878ba5e60e
> > > > compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
> > > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=168d8ca5580000
> > > > 
> > > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > > Reported-by: syzbot+f11512d1e5878ba5e60e@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > > 
> > > > =============================
> > > > [ BUG: Invalid wait context ]
> > > > syzkaller #0 Tainted: G S                 
> > > > -----------------------------
> > > > syz-executor177/6041 is trying to lock:
> > > > ffff88801d040618 (&n->list_lock){-.-.}-{3:3}, at: get_partial_node_bulk mm/slub.c:3831 [inline]
> > > > ffff88801d040618 (&n->list_lock){-.-.}-{3:3}, at: __refill_objects_node+0x104/0xab0 mm/slub.c:7279
> > > > other info that might help us debug this:
> > > > context-{2:2}
> > > > locks held by syz-executor177/6041: 1, last CPU#1:
> > > >  #0: ffffffff9090b438 (tomoyo_ss){.+.+}-{0:0}, at: srcu_lock_acquire include/linux/srcu.h:198 [inline]
> > > >  #0: ffffffff9090b438 (tomoyo_ss){.+.+}-{0:0}, at: srcu_read_lock include/linux/srcu.h:305 [inline]
> > > >  #0: ffffffff9090b438 (tomoyo_ss){.+.+}-{0:0}, at: tomoyo_read_lock security/tomoyo/common.h:1112 [inline]
> > > >  #0: ffffffff9090b438 (tomoyo_ss){.+.+}-{0:0}, at: tomoyo_check_open_permission+0x169/0x3c0 security/tomoyo/file.c:772
> > > > stack backtrace:
> > > > CPU: 1 UID: 0 PID: 6041 Comm: syz-executor177 Tainted: G S                  syzkaller #0 PREEMPT(full) 
> > > > Tainted: [S]=CPU_OUT_OF_SPEC
> > > > Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> > > > Call Trace:
> > > >  <IRQ>
> > > >  __dump_stack lib/dump_stack.c:94 [inline]
> > > >  dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
> > > >  print_lock_invalid_wait_context kernel/locking/lockdep.c:4874 [inline]
> > > >  check_wait_context kernel/locking/lockdep.c:4946 [inline]
> > > >  __lock_acquire+0xfcc/0x1f40 kernel/locking/lockdep.c:5238
> > > >  lock_acquire kernel/locking/lockdep.c:5942 [inline]
> > > >  lock_acquire+0x1d1/0x380 kernel/locking/lockdep.c:5899
> > > >  __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:155 [inline]
> > > >  _raw_spin_lock_irqsave+0x3a/0x60 kernel/locking/spinlock.c:181
> > > >  get_partial_node_bulk mm/slub.c:3831 [inline]
> > > >  __refill_objects_node+0x104/0xab0 mm/slub.c:7279
> > > >  refill_objects+0x58/0x410 mm/slub.c:7405
> > > >  refill_sheaf mm/slub.c:2885 [inline]
> > > >  __pcs_replace_empty_main+0x35e/0x650 mm/slub.c:4774
> > > >  alloc_from_pcs mm/slub.c:4850 [inline]
> > > >  slab_alloc_node mm/slub.c:4984 [inline]
> > > >  __kmalloc_cache_noprof+0x4f4/0x6d0 mm/slub.c:5559
> > > >  _kmalloc_noprof include/linux/slab.h:991 [inline]
> > > >  _kzalloc_noprof include/linux/slab.h:1312 [inline]
> > > >  kvm_async_pf_task_wake arch/x86/kernel/kvm.c:224 [inline]

Hmm, this part is OK:

                if (!dummy) {
                        raw_spin_unlock(&b->lock);
                        dummy = kzalloc_obj(*dummy, GFP_ATOMIC);

As it releases the raw_spin_lock() before calling kzalloc_obj().

So something else must have disabled preemption/interrupts.

> > > >  __sysvec_kvm_asyncpf_interrupt+0x334/0x610 arch/x86/kernel/kvm.c:338

DEFINE_IDTENTRY_SYSVEC(sysvec_kvm_asyncpf_interrupt)
{       
        struct pt_regs *old_regs = set_irq_regs(regs);
        u32 token;
                
        apic_eoi();

        inc_irq_stat(HYPERVISOR_CALLBACK);

        if (__this_cpu_read(async_pf_enabled)) {
                token = __this_cpu_read(apf_reason.token);
                kvm_async_pf_task_wake(token);
                __this_cpu_write(apf_reason.token, 0);
                wrmsrq(MSR_KVM_ASYNC_PF_ACK, 1);
        }

        set_irq_regs(old_regs);
}


Is that run in hard interrupt context? If so, we need to make that a thread
for RT kernels if you want to call kzalloc_obj().

-- Steve


> > > >  instr_sysvec_kvm_asyncpf_interrupt arch/x86/kernel/kvm.c:327 [inline]
> > > >  sysvec_kvm_asyncpf_interrupt+0x9e/0xc0 arch/x86/kernel/kvm.c:327
> > > >  </IRQ>
> > > >  <TASK>
> > > >  asm_sysvec_kvm_asyncpf_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:753
> > > > RIP: 0010:memcmp+0x156/0x1d0 lib/string.c:665
> > > > Code: 0f 86 fb fe ff ff 48 89 d8 48 89 de 48 c1 e8 03 83 e6 07 42 0f b6 14 28 48 8d 43 07 48 89 c1 48 c1 e9 03 40 38 f2 40 0f 9e c6 <84> d2 42 0f b6 0c 29 0f 95 c2 40 84 d6 0f 84 45 ff ff ff be 08 00
> > > > RSP: 0018:ffffc90003ab7538 EFLAGS: 00000297
> > > > RAX: ffff8880220be413 RBX: ffff8880220be40c RCX: 1ffff11004417c82
> > > > RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffff8880220be40c
> > > > RBP: ffffffff8c58a940 R08: 0000000000000001 R09: 0000000000000066
> > > > R10: 0000000000000066 R11: 0000000000000001 R12: 0000000000000012
> > > > R13: dffffc0000000000 R14: dffffc0000000000 R15: 0000000000000000
> > > >  strstr+0x109/0x170 lib/string.c:743
> > > >  tomoyo_add_entry security/tomoyo/common.c:2157 [inline]
> > > >  tomoyo_supervisor+0xf47/0x1340 security/tomoyo/common.c:2238
> > > >  tomoyo_audit_path_log security/tomoyo/file.c:169 [inline]
> > > >  tomoyo_path_permission security/tomoyo/file.c:592 [inline]
> > > >  tomoyo_path_permission+0x270/0x3b0 security/tomoyo/file.c:577
> > > >  tomoyo_check_open_permission+0x37f/0x3c0 security/tomoyo/file.c:782
> > > >  tomoyo_file_open+0x6b/0x90 security/tomoyo/tomoyo.c:334
> > > >  security_file_open+0xb5/0x1e0 security/security.c:2739
> > > >  do_dentry_open+0x588/0x14d0 fs/open.c:973
> > > >  vfs_open+0x82/0x3f0 fs/open.c:1101
> > > >  do_open fs/namei.c:4836 [inline]
> > > >  path_openat+0x1a23/0x2450 fs/namei.c:4999
> > > >  do_file_open+0x20e/0x430 fs/namei.c:5028
> > > >  do_sys_openat2+0x10f/0x1e0 fs/open.c:1417
> > > >  do_sys_open fs/open.c:1423 [inline]
> > > >  __do_sys_openat fs/open.c:1439 [inline]
> > > >  __se_sys_openat fs/open.c:1434 [inline]
> > > >  __x64_sys_openat+0x12d/0x210 fs/open.c:1434
> > > >  do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
> > > >  do_syscall_64+0x123/0x790 arch/x86/entry/syscall_64.c:84
> > > >  entry_SYSCALL_64_after_hwframe+0x77/0x7f  
> > >   
>