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

Alexander Gordeev
2025-09-12T14:05:42Z

On Fri, Sep 12, 2025 at 03:02:15PM +0200, David Hildenbrand wrote:
> How would that work with nesting? I feel like there is a fundamental problem
> with nesting with what you describe but I might be wrong.

My picture is - flush on each lazy_mmu_disable(), pause on lazy_mmu_pause()
and honour only top-level arch_enter_lazy_mmu_mode_pte(mm, start, end, ptep)
context on all nested levels.

In theory (and if I got it right, you leave the door open for this possibility)
every (mm, start, end, ptep) context could be stored for each nesting level
(as an opaque arch-specific data?).

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:

ptep = pte_offset_map_lock(...);
arch_enter_lazy_mmu_mode_pte(mm, start, end, ptep);

for (...; ptep++) {
/*
* set_pte(ptep, ...) or something
*/
}

arch_leave_lazy_mmu_mode();
pte_unmap_unlock(...);

As result, the lazy mmu mode is only "bound" to a single PTE table on s390,
while arch_enter_lazy_mmu_mode() is going to stay NOP.

So when you say you feel a fundamental problem - what that could be?

> David / dhildenb

Thanks!