Re: [PATCH v4 06/12] mm: introduce generic lazy_mmu helpers

[Kevin Brodsky <kevin.brodsky@xxxxxxx>]

On 10/11/2025 11:45, Kevin Brodsky wrote:
>>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>>> index b8d37eb037fc..d9c8e94f140f 100644
>>> --- a/arch/arm64/mm/mmu.c
>>> +++ b/arch/arm64/mm/mmu.c
>>> @@ -731,7 +731,7 @@ int split_kernel_leaf_mapping(unsigned long start, 
>>> unsigned long end)
>>>             return -EINVAL;
>>>  
>>>     mutex_lock(&pgtable_split_lock);
>>> -   arch_enter_lazy_mmu_mode();
>>> +   lazy_mmu_mode_enable();
>>>  
>>>     /*
>>>      * The split_kernel_leaf_mapping_locked() may sleep, it is not a
>> This is a bit unfortunate, IMHO. The rest of this comment explains that 
>> although
>> you're not supposed to sleep inside lazy mmu mode, it's fine for arm64's
>> implementation. But we are no longer calling arm64's implementation; we are
>> calling a generic function, which does who knows what.
>>
>> I think it all still works, but we are no longer containing our assumptions 
>> in
>> arm64 code. We are relying on implementation details of generic code.
> I see your point. The change itself is still correct (and required
> considering patch 8), but maybe the documentation of the generic
> interface should be clarified to guarantee that the generic layer can
> itself cope with sleeping - without any guarantee regarding the
> behaviour of arch_*_lazy_mmu_mode.

Re-reading the existing comment in <linux/pgtable.h>, I think it already
makes clear that sleeping is not forbidden by design:

>  * In the general case, no lock is guaranteed to be held between entry
> and exit
>  * of the lazy mode. So the implementation must assume preemption may
> be enabled
>  * and cpu migration is possible; it must take steps to be robust
> against this.

The arch implementation may disable preemption, but arm64 code can rely
on the arm64 implementation allowing sleeping.

- Kevin