Re: [PATCH 0/7] Nesting support for lazy MMU mode

[Alexander Gordeev <agordeev@xxxxxxxxxxxxx>]

On Thu, Sep 04, 2025 at 01:57:29PM +0100, Kevin Brodsky wrote:

Hi Kevin,

> When the lazy MMU mode was introduced eons ago, it wasn't made clear
> whether such a sequence was legal:
> 
>       arch_enter_lazy_mmu_mode()
>       ...
>               arch_enter_lazy_mmu_mode()
>               ...
>               arch_leave_lazy_mmu_mode()
>       ...
>       arch_leave_lazy_mmu_mode()

I did not take too deep - sorry if you already answered this.
Quick question - whether a concern Ryan expressed is addressed
in general case?

https://lore.kernel.org/all/3cad01ea-b704-4156-807e-7a83643917a8@xxxxxxx/

        enter_lazy_mmu
                for_each_pte {
                        read/modify-write pte

                        alloc_page
                                enter_lazy_mmu
                                        make page valid
                                exit_lazy_mmu

                        write_to_page
                }
        exit_lazy_mmu

<quote>
This example only works because lazy_mmu doesn't support nesting. The "make page
valid" operation is completed by the time of the inner exit_lazy_mmu so that the
page can be accessed in write_to_page. If nesting was supported, the inner
exit_lazy_mmu would become a nop and write_to_page would explode.
</quote>

...

Thanks!