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

Kevin Brodsky
2025-10-24T12:13:46Z

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().

Alternatively it should be possible to pause(), unconditionally resume()
after the expensive operations are done and then leave() right away in
case of failure. It requires restructuring and might look a bit strange,
but can be done if you think it's justified.

>
> 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.

>
>>
>> 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.

- Kevin