Re: [PATCH v5 08/12] mm: enable lazy_mmu sections to nest

[Kevin Brodsky <kevin.brodsky@arm.com>]

On 27/11/2025 13:33, Alexander Gordeev wrote:
> On Mon, Nov 24, 2025 at 01:22:24PM +0000, Kevin Brodsky wrote:
>
> Hi Kevin,
>
> ...
>> +/**
>> + * lazy_mmu_mode_pause() - Pause the lazy MMU mode.
>> + *
>> + * Pauses the lazy MMU mode; if it is currently active, disables it and calls
>> + * arch_leave_lazy_mmu_mode().
>> + *
>> + * Must be paired with a call to lazy_mmu_mode_resume(). Calls to the
>> + * lazy_mmu_mode_* API have no effect until the matching resume() call.
> Sorry if it was discussed already - if yes, I somehow missed it. If I read
> the whole thing correctly enter()/pause() interleaving is not forbidden?

Correct, any call inside pause()/resume() is now allowed (but
effectively ignored). See discussion with Ryan in v4 [1].

[1]
https://lore.kernel.org/all/824bf705-e9d6-4eeb-9532-9059fa56427f@arm.com/

> lazy_mmu_mode_enable()
> 	lazy_mmu_mode_pause()
> 		lazy_mmu_mode_enable()
> 		...
> 		lazy_mmu_mode_disable()
> 	lazy_mmu_mode_resume()
> lazy_mmu_mode_disable()
>
>> + *
>> + * Has no effect if called:
>> + * - While paused (inside another pause()/resume() pair)
>> + * - In interrupt context
>> + */
>>  static inline void lazy_mmu_mode_pause(void)
>>  {
>> +	struct lazy_mmu_state *state = &current->lazy_mmu_state;
>> +
>>  	if (in_interrupt())
>>  		return;
>>  
>> -	arch_leave_lazy_mmu_mode();
>> +	VM_WARN_ON_ONCE(state->pause_count == U8_MAX);
>> +
>> +	if (state->pause_count++ == 0 && state->enable_count > 0)
>> +		arch_leave_lazy_mmu_mode();
>>  }
>>  
>> +/**
>> + * lazy_mmu_mode_pause() - Resume the lazy MMU mode.
>                     resume() ?

Good catch! One copy-paste too many...

- Kevin

>> + *
>> + * Resumes the lazy MMU mode; if it was active at the point where the matching
>> + * call to lazy_mmu_mode_pause() was made, re-enables it and calls
>> + * arch_enter_lazy_mmu_mode().
>> + *
>> + * Must match a call to lazy_mmu_mode_pause().
>> + *
>> + * Has no effect if called:
>> + * - While paused (inside another pause()/resume() pair)
>> + * - In interrupt context
>> + */
>>  static inline void lazy_mmu_mode_resume(void)
>>  {
>> +	struct lazy_mmu_state *state = &current->lazy_mmu_state;
>> +
>>  	if (in_interrupt())
>>  		return;
>>  
>> -	arch_enter_lazy_mmu_mode();
>> +	VM_WARN_ON_ONCE(state->pause_count == 0);
>> +
>> +	if (--state->pause_count == 0 && state->enable_count > 0)
>> +		arch_enter_lazy_mmu_mode();
>>  }
> ...
> Thanks!