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

Kevin Brodsky
2025-10-24T14:32:50Z

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.

>
>>>
>>> Why would we use enable/disable instead?
>>>
>>>>
>>>> * 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.
>>>
>>> How does this interact with patch #11?
>>
>> It is a requirement for patch 11, in fact. If we called disable() when
>> switching out a task, then lazy_mmu_state.enabled would (most likely) be
>> false when scheduling it again.
>>
>> By calling the arch_* helpers when context-switching, we ensure
>> lazy_mmu_state remains unchanged. This is consistent with what happens
>> on all other architectures (which don't do anything about lazy_mmu when
>> context-switching). lazy_mmu_state is the lazy MMU status *when the task
>> is scheduled*, and should be preserved on a context-switch.
>
> Okay, thanks for clarifying. That whole XEN stuff here is rather horrible.

Can't say I disagree... I tried to simplify it further, but the
Xen-specific "LAZY_CPU" mode makes it just too difficult.

>
>>
>>>
>>>>
>>>> Note: 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.
>>>
>>> I can see a call in __kernel_map_pages() and
>>> arch_kmap_local_post_map()/arch_kmap_local_post_unmap().
>>>
>>> I guess that is ... harmless/irrelevant in the context of this series?
>>
>> It should be. arch_flush_lazy_mmu_mode() was only used by x86 before
>> this series; we're adding new calls to it from the generic layer, but
>> existing x86 calls shouldn't be affected.
>
> Okay, I'd like to understand the rules when arch_flush_lazy_mmu_mode()
> would actually be required in such arch code, but that's outside of
> the scope of your patch series.

Not too sure either. A little archaeology shows that the calls were
added by [1][2]. Chances are [1] is no longer relevant since lazy_mmu
isn't directly used in copy_page_range().

I think [2] is still required - __kernel_map_pages() can be called while
lazy MMU is already enabled, and AIUI the mapping changes should take
effect by the time __kernel_map_pages() returns. On arm64 we shouldn't
have this problem by virtue of __kernel_map_pages() using lazy_mmu
itself, meaning that the nested call to disable() will trigger a flush.
(This case is in fact the original motivation for supporting nesting.)

- Kevin

[1]
https://lore.kernel.org/all/1319573279-13867-2-git-send-email-konrad.wilk@oracle.com/
[2]