Re: [PATCH v2 2/7] mm: introduce local state for lazy_mmu sections

Kevin Brodsky
2025-09-12T08:48:40Z

On 12/09/2025 10:04, David Hildenbrand wrote:
>>>
>>> struct lazy_mmu_state {
>>>      uint8_t enabled_count;
>>>      bool paused;
>>
>> Looking at the arm64 implementation, I'm thinking: instead of the paused
>> member, how about a PF_LAZY_MMU task flag? It would be set when lazy_mmu
>> is actually enabled (i.e. inside an enter()/leave() section, and not
>> inside a pause()/resume() section). This way, architectures could use
>> that flag directly to tell if lazy_mmu is enabled instead of reinventing
>> the wheel, all in slightly different ways. Namely:
>>
>> * arm64 uses a thread flag (TIF_LAZY_MMU) - this is trivially replaced
>> with PF_LAZY_MMU
>> * powerpc and sparc use batch->active where batch is a per-CPU variable;
>> I expect this can also be replaced with PF_LAZY_MMU
>> * x86/xen is more complex as it has xen_lazy_mode which tracks both
>> LAZY_MMU and LAZY_CPU modes. I'd probably leave that one alone, unless a
>> Xen expert is motivated to refactor it.
>>
>> With that approach, the implementation of arch_enter() and arch_leave()
>> becomes very simple (no tracking of lazy_mmu status) on arm64, powerpc
>> and sparc.
>>
>> (Of course we could also have an "enabled" member in lazy_mmu_state
>> instead of PF_LAZY_MMU, there is no functional difference.)
>>
>
> No strong opinion, but to me it feels like PF_LAZY_MMU is rather "the
> effective state when combining nested+paused", and might complicate
> the code + sanity checks?
>
> So we could maintain that in addition fairly easily of course from the
> core instead of letting archs do that manually.
>
> I would probably have to see the end result to judge whether removing
> the "paused" bool makes things look more complicated or not.

Agreed, it is a little difficult to consider all the cases ahead of
time. What is clear to me though is that [paused] can be inferred from
[count + enabled], and conversely [enabled] from [count + paused]. As a
result I really wouldn't store both paused and enabled in task_struct -
duplicating information is how you create inconsistent states.

We can very easily introduce helpers to get the enabled/paused status
regardless of how they're stored. Since "enabled" is what we need to
know in most cases (arch checking the status), I would rather store
"enabled" than "paused". But indeed, let's see how it turns out in practice.

>
>>> }
>>>
>>> c) With that config, common-code lazy_mmu_*() functions implement the
>>> updating of the lazy_mmu_state in task_struct and call into arch code
>>> on the transition from 0->1, 1->0 etc.
>>
>> Indeed, this is how I thought about it. There is actually quite a lot
>> that can be moved to the generic functions:
>> * Updating lazy_mmu_state
>> * Sanity checks on lazy_mmu_state (e.g. underflow/overflow)
>> * Bailing out if in_interrupt() (not done consistently across arch's at
>> the moment)
>>
>>>
>>> Maybe that can be done through exiting
>>> arch_enter_lazy_mmu_mode()/arch_leave_lazy_mmu_mode() callbacks, maybe
>>> we need more. I feel like
>>> we might be able to implement that through the existing helpers.
>>
>> We might want to rename them to align with the new generic helpers, but
>> yes otherwise the principle should remain unchanged.
>>
>> In fact, we will also need to revive arch_flush_lazy_mmu_mode().
>
> That's okay if it's all hidden behaind a sane core API.
>
>> Indeed,
>> in the nested situation, we need the following arch calls:
>>
>> enter() -> arch_enter()
>>      enter() -> [nothing]
>>      leave() -> arch_flush()
>> leave() -> arch_leave()
>>
>> leave() must always flush whatever arch state was batched, as may be
>> expected by the caller.
>>
>> How does all that sound?
>
> I am no expert on the "always flush when leaving", but it sounds
> reasonable to me.

This is a core expectation for lazy_mmu: when leave() is called, any
batched state is flushed. The fact it currently happens unconditionally
when calling leave() is in fact what stops nesting from exploding on
arm64 with DEBUG_PAGEALLOC [1].

[1] https://lore.kernel.org/all/aEhKSq0zVaUJkomX@arm.com/

>
> Which arch operations would you call from
>
> pause()
> continue()

I also wondered about that. I think the safest is to make them
respectively arch_leave() and arch_enter() - the flushing entailed by
arch_leave() might not be required, but it is safer. Additionally,
powerpc/sparc disable preemption while in lazy_mmu, so it seems like a
good idea to re-enable it while paused (by calling arch_leave()).

- Kevin