Message-ID: <93d04ef8-0364-4013-8839-ba599d930cb2@kernel.org>
Date: Thu, 4 Dec 2025 12:52:32 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Anshuman Khandual <anshuman.khandual@....com>,
 Kevin Brodsky <kevin.brodsky@....com>, linux-mm@...ck.org
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Subject: Re: [PATCH v5 08/12] mm: enable lazy_mmu sections to nest

Some comments from my side:


>>   static inline void arch_enter_lazy_mmu_mode(void)
>>   {
>> -	/*
>> -	 * lazy_mmu_mode is not supposed to permit nesting. But in practice this
>> -	 * does happen with CONFIG_DEBUG_PAGEALLOC, where a page allocation
>> -	 * inside a lazy_mmu_mode section (such as zap_pte_range()) will change
>> -	 * permissions on the linear map with apply_to_page_range(), which
>> -	 * re-enters lazy_mmu_mode. So we tolerate nesting in our
>> -	 * implementation. The first call to arch_leave_lazy_mmu_mode() will
>> -	 * flush and clear the flag such that the remainder of the work in the
>> -	 * outer nest behaves as if outside of lazy mmu mode. This is safe and
>> -	 * keeps tracking simple.
>> -	 */
>> -
>>   	set_thread_flag(TIF_LAZY_MMU);>  }
> 
> Should not platform specific changes be deferred to subsequent patches until
> nesting is completely enabled in generic first ? Although no problem as such
> but would be bit cleaner.

This could indeed be done in a separate patch. But I also don't see a 
problem with updating the doc in this patch.

> 
>>   
>> diff --git a/include/linux/mm_types_task.h b/include/linux/mm_types_task.h
>> index a82aa80c0ba4..11bf319d78ec 100644
>> --- a/include/linux/mm_types_task.h
>> +++ b/include/linux/mm_types_task.h
>> @@ -88,4 +88,9 @@ struct tlbflush_unmap_batch {
>>   #endif
>>   };
>>   
>> +struct lazy_mmu_state {
>> +	u8 enable_count;
>> +	u8 pause_count;
>> +};
>> +
> 
> Should not this be wrapped with CONFIG_ARCH_HAS_LAZY_MMU_MODE as the task_struct
> element 'lazy_mmu_state' is only available with the feature.

No strong opinion; the compiler will ignore it either way. And less 
ifdef is good, right? :)

... and there is nothing magical in there that would result in other 
dependencies.

> Besides, is a depth
> of 256 really expected here ? 4 bits for each element would not be sufficient for
> a depth of 16 ?


We could indeed use something like

struct lazy_mmu_state {
	u8 enable_count : 4;
	u8 pause_count : 4;
};

but then, the individual operations on enable_count/pause_count need 
more instructions.

Further, as discussed, this 1 additional byte barely matters given the 
existing size of the task struct.

No strong opinion.

> 
>>    */
>>   #ifdef CONFIG_ARCH_HAS_LAZY_MMU_MODE
>> +/**
>> + * lazy_mmu_mode_enable() - Enable the lazy MMU mode.
>> + *
>> + * Enters a new lazy MMU mode section; if the mode was not already enabled,
>> + * enables it and calls arch_enter_lazy_mmu_mode().
>> + *
>> + * Must be paired with a call to lazy_mmu_mode_disable().
>> + *
>> + * Has no effect if called:
>> + * - While paused - see lazy_mmu_mode_pause()
>> + * - In interrupt context
>> + */
>>   static inline void lazy_mmu_mode_enable(void)
>>   {
>> -	if (in_interrupt())
>> +	struct lazy_mmu_state *state = &current->lazy_mmu_state;
>> +
>> +	if (in_interrupt() || state->pause_count > 0)
>>   		return;
>>   
>> -	arch_enter_lazy_mmu_mode();
>> +	VM_WARN_ON_ONCE(state->enable_count == U8_MAX);
>> +
>> +	if (state->enable_count++ == 0)
>> +		arch_enter_lazy_mmu_mode();
> 
> When lazy_mmu_mode_enable() gets called for the first time with state->enable_count as 0,
> then arch_enter_lazy_mmu_mode() will not get called ? Bit confused.


state->enable_count++ returns the old value (0). Are you thinking of
++state->enable_count?

But maybe I misudnerstood your concern.

[...]

>> +/**
>> + * lazy_mmu_mode_pause() - Resume the lazy MMU mode.
>> + *
>> + * 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();
>>   }
> 
> Should not state->pause/enable_count tests and increment/decrement be handled
> inside include/linux/sched via helpers like in_lazy_mmu_mode() ? This is will
> ensure cleaner abstraction with respect to task_struct.

I don't think this is required given that this code here implements
CONFIG_ARCH_HAS_LAZY_MMU_MODE support.

-- 
Cheers

David

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