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

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

On 12/09/2025 16:25, David Hildenbrand wrote:
>
>>
>> But I do not really expect it ever, since arch_enter_lazy_mmu_mode_pte()
>> is only to be called in PTE walkers that never span more than one page
>> table and follow the pattern:
>
> Well, the cover letter here states:
>
> "Unfortunately, a corner case (DEBUG_PAGEALLOC) may still cause
> nesting to occur on arm64. Ryan proposed [2] to address that corner
> case at the generic level but this approach received pushback; [3]
> then attempted to solve the issue on arm64 only, but it was deemed too
> fragile."
>
> So I guess we should support nesting cleanly, at least on the core-mm
> side.

Nesting remains a rare occurrence though. I think it would be plausible
to require this new interface to be used in a region where no nesting
can occur, just like pause()/resume().

In fact, I think this is a requirement if we go for the approach we have
been discussing, because nested enter()/leave() calls are not meant to
call arch_enter()/arch_leave(), and I really wouldn't want to use a
different logic for this variant.

>
> I guess we could start with saying "well, s390x doesn't fully support
> nesting yet but doing so just requires changing the way we manage this
> per-nesting-level state internally".
>
> s390 is trying to do something different than the other archs here, so
> that naturally concerns me :)
>
> But if it's really just about forwarding that data and having s390
> store it somewhere (task_struct, percpu variable, etc), fine with me. 

Yes I think this is fine, with the restriction above. The extra
arguments are directly forwarded to arch code and otherwise ignored by
core code, and unless the arch defines some __HAVE_ARCH... or CONFIG,
the extended interface falls back to regular enter()/leave().

- Kevin