Re: [PATCH v3 06/13] mm: introduce generic lazy_mmu helpers

David Hildenbrand
2025-10-27T16:25:05Z

On 24.10.25 16:32, Kevin Brodsky wrote:
> On 24/10/2025 15:27, David Hildenbrand wrote:
>> On 24.10.25 14:13, Kevin Brodsky wrote:
>>> On 23/10/2025 21:52, David Hildenbrand wrote:
>>>> On 15.10.25 10:27, Kevin Brodsky wrote:
>>>>> [...]
>>>>>
>>>>> * madvise_*_pte_range() call arch_leave() in multiple paths, some
>>>>>     followed by an immediate exit/rescheduling and some followed by a
>>>>>     conditional exit. These functions assume that they are called
>>>>>     with lazy MMU disabled and we cannot simply use pause()/resume()
>>>>>     to address that. This patch leaves the situation unchanged by
>>>>>     calling enable()/disable() in all cases.
>>>>
>>>> I'm confused, the function simply does
>>>>
>>>> (a) enables lazy mmu
>>>> (b) does something on the page table
>>>> (c) disables lazy mmu
>>>> (d) does something expensive (split folio -> take sleepable locks,
>>>>      flushes tlb)
>>>> (e) go to (a)
>>>
>>> That step is conditional: we exit right away if pte_offset_map_lock()
>>> fails. The fundamental issue is that pause() must always be matched with
>>> resume(), but as those functions look today there is no situation where
>>> a pause() would always be matched with a resume().
>>
>> We have matches enable/disable, so my question is rather "why" you are
>> even thinking about using pause/resume?
>>
>> What would be the benefit of that? If there is no benefit then just
>> drop this from the patch description as it's more confusing than just
>> ... doing what the existing code does :)
>
> Ah sorry I misunderstood, I guess you originally meant: why would we use
> pause()/resume()?
>
> The issue is the one I mentioned in the commit message: using
> enable()/disable(), we assume that the functions are called with lazy
> MMU mode is disabled. Consider:
>
>   lazy_mmu_mode_enable()
>   madvise_cold_or_pageout_pte_range():
>     lazy_mmu_mode_enable()
>     ...
>     if (need_resched()) {
>       lazy_mmu_mode_disable()
>       cond_resched() // lazy MMU still enabled
>     }
>
> This will explode on architectures that do not allow sleeping while in
> lazy MMU mode. I'm not saying this is an actual problem - I don't see
> why those functions would be called with lazy MMU mode enabled. But it
> does go against the notion that nesting works everywhere.

I would tackle it from a different direction: if code calls with lazy
MMU enabled into random other code that might sleep, that caller would
be wrong.

It's not about changing functions like this to use pause/resume.

Maybe the rule is simple: if you enable the lazy MMU, don't call any
functions that might sleep.

Maybe we could support that later by handling it before/after sleeping,
if ever required?

Or am I missing something regarding your point on pause()/resume()?

--
Cheers

David / dhildenb