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

[Ryan Roberts <ryan.roberts@arm.com>]

On 24/11/2025 12:47, Kevin Brodsky wrote:
> 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.

Yeah ok, I buy that!

> 
> - Kevin
>