Re: [PATCH v4 07/12] mm: enable lazy_mmu sections to nest

[Kevin Brodsky <kevin.brodsky@xxxxxxx>]

On 05/11/2025 16:12, Alexander Gordeev wrote:
> On Wed, Nov 05, 2025 at 02:19:03PM +0530, Ritesh Harjani wrote:
>>> + * in_lazy_mmu_mode() can be used to check whether the lazy MMU mode is
>>> + * currently enabled.
>>>   */
>>>  #ifdef CONFIG_ARCH_HAS_LAZY_MMU_MODE
>>>  static inline void lazy_mmu_mode_enable(void)
>>>  {
>>> -   arch_enter_lazy_mmu_mode();
>>> +   struct lazy_mmu_state *state = &current->lazy_mmu_state;
>>> +
>>> +   VM_WARN_ON_ONCE(state->nesting_level == U8_MAX);
>>> +   /* enable() must not be called while paused */
>>> +   VM_WARN_ON(state->nesting_level > 0 && !state->active);
>>> +
>>> +   if (state->nesting_level++ == 0) {
>>> +           state->active = true;
>>> +           arch_enter_lazy_mmu_mode();
>>> +   }
>>>  }
>> Some architectures disables preemption in their
>> arch_enter_lazy_mmu_mode(). So shouldn't the state->active = true should
>> happen after arch_enter_lazy_mmu_mode() has disabled preemption()? i.e.
> Do you have some scenario in mind that could cause an issue?
> IOW, what could go wrong if the process is scheduled to another
> CPU before preempt_disable() is called?

I'm not sure I understand the issue either.

>>   static inline void lazy_mmu_mode_enable(void)
>>   {
>>  -   arch_enter_lazy_mmu_mode();
>>  +   struct lazy_mmu_state *state = &current->lazy_mmu_state;
>>  +
>>  +   VM_WARN_ON_ONCE(state->nesting_level == U8_MAX);
>>  +   /* enable() must not be called while paused */
>>  +   VM_WARN_ON(state->nesting_level > 0 && !state->active);
>>  +
>>  +   if (state->nesting_level++ == 0) {
>>  +           arch_enter_lazy_mmu_mode();
>>  +           state->active = true;
>>  +   }
>>   }
>>
>> ... I think it make more sense to enable the state after the arch_**
>> call right.
> But then in_lazy_mmu_mode() would return false if called from
> arch_enter_lazy_mmu_mode(). Not big problem, but still..

The ordering of nesting_level/active was the way you expected in v3, but
the conclusion of the discussion with David H [1] is that it doesn't
really matter so I simplified the ordering in v4 - the arch hooks
shouldn't call in_lazy_mmu_mode() or inspect lazy_mmu_state.
arch_enter()/arch_leave() shouldn't need it anyway since they're called
once per outer section (not in nested sections). arch_flush() could
potentially do something different when nested, but that seems unlikely.

- Kevin

[1]
https://lore.kernel.org/all/af4414b6-617c-4dc8-bddc-3ea00d1f6f3b@xxxxxxxxxx/