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

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

On 04/12/2025 12:52, David Hildenbrand (Red Hat) wrote:
> 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.

I think it is consistent to remove that comment in this patch, since
nesting is fully supported from this patch onwards. Subsequent patches
are cleanups/optimisations that aren't functionally required.

Patch 7 takes the same approach: add handling in the generic layer,
remove anything now superfluous from arm64.

>
>>
>>>   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. 

Agreed, #ifdef'ing types should only be done if necessary.

>
>> 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.

Indeed.

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

In fact it would almost certainly make no difference (depending on
randomized_struct) since almost all members in task_struct have an
alignment of at least 2.

>
> [...]
>
>>> +/**
>>> + * 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.

Agreed, in fact I'd rather not expose helpers that should only be used
in the lazy_mmu implementation itself.

- Kevin