Re: [PATCH v5 2/3] arm64: mmu: avoid allocating pages while splitting the linear mapping

Yang Shi
2026-01-21T22:57:36Z

On 1/21/26 2:20 AM, Ryan Roberts wrote:
> On 21/01/2026 08:32, Yeoreum Yun wrote:
>>>>> My concern is that if a secondary CPU can race and cause a split, that is
>>>>> unsound because we have determined that although the primary CPU supports BBML2,
>>>>> at least one of the secondary CPUs does not. So splitting a live mapping is unsafe.
>>>>>
>>>>> I just had a brief chat with Rutland, and he agrees that this _could_ be a
>>>>> problem. Basically there is a window between onlining the secondary cpus and
>>>>> entering the stop_machine() where one of those cpus _could_ end up doing
>>>>> something that causes us to split the linear map.
>>> If I remember correctly, split_kernel_leaf_mapping() does call
>>> system_supports_bbml2_noabort() before doing real split. So we basically
>>> should fall into two categories:
>>>
>>> 1. bbml2_noabort is supported on all cpus. Everything is fine.
>>> 2. bbml2_noabort is not supported on all cpus. split_kernel_leaf_mapping()
>>> just returns 0. Kernel doesn't split page table, so there won't be TLB
>>> conflict issue. But the following page prot update may see unexpected block
>>> mapping, then a   WARN  will be raised and it will return -EINVAL. So the
>>> worst case is the caller will fail (IIRC all the callers of set_memory_*()
>>> handle the failure), and we can know who is trying to change linear mapping
>>> before the linear mapping gets                    finalized. AFAICT I
>>> haven't seen such WARN yet.
> Ahh good point! So this isn't quite as terrible as I was thinking.

Yeah.

>
>> Thanks for the great detail :)
>> I've missed system_supports_bbml2_noabort() in split_kernel_leaf_mapping().
>>
>>>>> I'm not immediately sure how to solve that.
>>> Do we need some synchronization mechanism? If the linear mapping is not
>>> finalized yet, split_kernel_leaf_mapping() will spin. For example, something
>>> like this off the top of my head,
>>>
>>> DEFINE_STATIC_KEY_FALSE(linear_mapping_finalized);
>>>
>>> Once the linear mapping is finalized, we can call
>>> static_branch_enable(&linear_mapping_finalized);
>>>
>>> In split_kernel_leaf_mapping(), we can just do:
>>>
>>> retry:
>>>     if (!static_branch_likely(&linear_mapping_finalized))
>>>         goto retry;
>>>
> Yuck... But I guess it might work as long as the primary thread never does
> anything that would cause an attempt to split; otherwise we have a deadlock.
>
>>> There may be better way to handle it. But this case should be very unlikely
>>> IMHO. It sounds crazy to have such complicated kernel threads run so early.
>>> I'm not sure whether we should pay immediate attention to it or not.
> I think we need to figure out if this is actually possible. We bring up the
> secondary cpus, set system caps and finalize the linear map in smp_init().
> That's called from kernel_init_freeable() which is called from kernel_init(),
> which is invoked as a thread pinned to the boot cpu.
>
> sched_init_smp() is called after smp_init() (i.e. after the linear map is
> finalized). I'm guessing (based on the name of sched_init_smp()) that nothing
> other than the idle thread will run on any secondaries until after
> sched_init_smp() is called? (I'd be greatful if anyone can confirm that).
>
> Rutland suggested that it's probably too early for any PM type stuff to be
> running in the idle loop, so based on all of that, perhaps this is not a problem
> after all and there is basically zero chance of a secondary cpu doing anything
> that could cause a linear map split during this window?
>
> I'm inclined to leave this as is for now.

I agree. I don't think this would be a real problem.

Thanks,
Yang

>
> Thanks,
> Ryan
>
>> Thinking about it again, I’m not sure whether
>> it is acceptable to use a sleepable synchronization mechanism at this stage,
>> like split_kernel_leaf_mapping() does with mutex_lock()
>> (even though it may be technically possible).
>> It also feels odd that this function can be called at this point in time.
>>
>> If this is indeed considered problematic,
>> I think it would be better to simply return -EINVAL immediately
>> when linear_mapping_finalized has not yet been completed.
>>
>> --
>> Sincerely,
>> Yeoreum Yun