Re: [RFC PATCH 1/2] mm: make lazy MMU mode context-aware

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

On 25/03/2026 17:20, Alexander Gordeev wrote:
> On Wed, Mar 25, 2026 at 10:55:23AM +0100, David Hildenbrand (Arm) wrote:
>
> Hi David,
>
>>> +/**
>>> + * lazy_mmu_mode_enable_pte() - Enable the lazy MMU mode with parameters
>> You have to be a lot clearer about implications. For example, what
>> happens if we would bail out and not process all ptes? What are the
>> exact semantics.
> The only implication is "only this address/PTE range could be updated
> and that range may span one page table at most".
>
> Whether all or portion of PTEs were actually updated is not defined,
> just like in case of lazy_mmu_mode_enable_pte().
>
> Makes sense?

I also feel that the comment needs to be much more specific. From a
brief glance at patch 2, it seems that __ipte_batch_set_pte() assumes
that all PTEs processed after this function is called are contiguous.
This should be documented.

>>> + * Enters a new lazy MMU mode section; if the mode was not already enabled,
>>> + * enables it and calls arch_enter_lazy_mmu_mode_pte().
>>> + *
>>> + * 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_pte(struct mm_struct *mm,
>>> +					    unsigned long addr,
>>> +					    unsigned long end,
>>> +					    pte_t *ptep)
>> It can be multiple ptes, so should this be some kind of "pte_range"/
>>
>> lazy_mmu_mode_enable_for_pte_range()
>>
>> A bit mouthful but clearer.
>>
>>> +{
>>> +	struct lazy_mmu_state *state = &current->lazy_mmu_state;
>>> +
>>> +	if (in_interrupt() || state->pause_count > 0)
>>> +		return;
>>> +
>>> +	VM_WARN_ON_ONCE(state->enable_count == U8_MAX);
>>> +
>>> +	if (state->enable_count++ == 0)
>>> +		arch_enter_lazy_mmu_mode_pte(mm, addr, end, ptep);
> I will also change arch_enter_lazy_mmu_mode_pte() to
> arch_enter_lazy_mmu_mode_for_pte_range() then.

Makes sense. The interface looks reasonable to me with this new name.

One more comment though: in previous discussions you mentioned the need
for arch_{pause,resume} hooks, is that no longer necessary simply
because {pause,resume} are not used on the paths where you make use of
the new enable function?

- Kevin