Re: [PATCH v5 06/12] mm: introduce generic lazy_mmu helpers

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

On 04/12/2025 05:17, Anshuman Khandual wrote:
> On 24/11/25 6:52 PM, Kevin Brodsky wrote:
>> The implementation of the lazy MMU mode is currently entirely
>> arch-specific; core code directly calls arch helpers:
>> arch_{enter,leave}_lazy_mmu_mode().
>>
>> We are about to introduce support for nested lazy MMU sections.
>> As things stand we'd have to duplicate that logic in every arch
>> implementing lazy_mmu - adding to a fair amount of logic
>> already duplicated across lazy_mmu implementations.
>>
>> This patch therefore introduces a new generic layer that calls the
>> existing arch_* helpers. Two pair of calls are introduced:
>>
>> * lazy_mmu_mode_enable() ... lazy_mmu_mode_disable()
>>     This is the standard case where the mode is enabled for a given
>>     block of code by surrounding it with enable() and disable()
>>     calls.
>>
>> * lazy_mmu_mode_pause() ... lazy_mmu_mode_resume()
>>     This is for situations where the mode is temporarily disabled
>>     by first calling pause() and then resume() (e.g. to prevent any
>>     batching from occurring in a critical section).
>>
>> The documentation in <linux/pgtable.h> will be updated in a
>> subsequent patch.
>>> No functional change should be introduced at this stage.
>> The implementation of enable()/resume() and disable()/pause() is
>> currently identical, but nesting support will change that.
>>
>> Most of the call sites have been updated using the following
>> Coccinelle script:
>>
>> @@
>> @@
>> {
>> ...
>> - arch_enter_lazy_mmu_mode();
>> + lazy_mmu_mode_enable();
>> ...
>> - arch_leave_lazy_mmu_mode();
>> + lazy_mmu_mode_disable();
>> ...
>> }
>>
>> @@
>> @@
>> {
>> ...
>> - arch_leave_lazy_mmu_mode();
>> + lazy_mmu_mode_pause();
>> ...
>> - arch_enter_lazy_mmu_mode();
>> + lazy_mmu_mode_resume();
>> ...
>> }
> At this point arch_enter/leave_lazy_mmu_mode() helpers are still
> present on a given platform but now being called from new generic
> helpers lazy_mmu_mode_enable/disable(). Well except x86, there is
> direct call sites for those old helpers.

Indeed, see notes below regarding x86. The direct calls to arch_flush()
are specific to x86 and there shouldn't be a need for a generic abstraction.

- Kevin

> arch/arm64/include/asm/pgtable.h:static inline void arch_enter_lazy_mmu_mode(void)
> arch/arm64/include/asm/pgtable.h:static inline void arch_leave_lazy_mmu_mode(void)
>
> arch/arm64/mm/mmu.c:    lazy_mmu_mode_enable();
> arch/arm64/mm/pageattr.c:       lazy_mmu_mode_enable();
>
> arch/arm64/mm/mmu.c:    lazy_mmu_mode_disable();
> arch/arm64/mm/pageattr.c:       lazy_mmu_mode_disable();
>
>> A couple of notes regarding x86:
>>
>> * Xen is currently the only case where explicit handling is required
>>   for lazy MMU when context-switching. This is purely an
>>   implementation detail and using the generic lazy_mmu_mode_*
>>   functions would cause trouble when nesting support is introduced,
>>   because the generic functions must be called from the current task.
>>   For that reason we still use arch_leave() and arch_enter() there.
>>
>> * x86 calls arch_flush_lazy_mmu_mode() unconditionally in a few
>>   places, but only defines it if PARAVIRT_XXL is selected, and we
>>   are removing the fallback in <linux/pgtable.h>. Add a new fallback
>>   definition to <asm/pgtable.h> to keep things building.
>>
>> [...]