Re: [PATCH v2 0/7] Nesting support for lazy MMU mode

On Fri, Sep 12, 2025 at 05:25:27PM +0200, Kevin Brodsky wrote:

Hi Kevin,

Based on the outcome of the discussion with David on patch 2 [1p], there
is indeed an alternative approach that we should seriously consider. In
summary:

* Keep the API stateless, handle nesting with a counter in task_struct
* Introduce new functions to temporarily disable lazy_mmu without
impacting nesting, track that with a bool in task_struct (addresses the
situation in mm/kasan/shadow.c and possibly some x86 cases too)
* Move as much handling from arch_* to generic functions

What the new generic infrastructure would look like:

struct task_struct {
    ...
#ifdef CONFIG_ARCH_LAZY_MMU
    struct {
        uint8_t count;
        bool enabled; /* or paused, see below */
    } lazy_mmu_state;
#endif
}

* lazy_mmu_mode_enable():

This helper is parameter-free, assuming the MMU unit does not need any
configuration other than turning it on/off. That is currently true, but
(as I noted in my other mail) I am going to introduce a friend enable
function that accepts parameters, creates an arch-specific state and
uses it while the lazy mmu mode is active.

That does not impact your design (AFAICT), except one change below.

    if (!lazy_mmu_state.count) {
        arch_enter_lazy_mmu_mode();
        lazy_mmu_state.enabled = true;
    }
    lazy_mmu_state.count++;

* lazy_mmu_mode_disable():
    lazy_mmu_count--;
    if (!lazy_mmu_state.count) {
        lazy_mmu_state.enabled = false;
        arch_leave_lazy_mmu_mode();
    } else {
        arch_flush_lazy_mmu_mode();
    }

* lazy_mmu_mode_pause():
    lazy_mmu_state.enabled = false;
    arch_leave_lazy_mmu_mode();

This needs to be arch_pause_lazy_mmu_mode(), otherwise the arch-specific
state will be lost.

* lazy_mmu_mode_resume();
    arch_enter_lazy_mmu_mode();

Conversely, this needs to be arch_resume_lazy_mmu_mode(). And it can not
be arch_enter_lazy_mmu_mode(), since a lazy_mmu_mode_resume() caller does
not know the parameters passed to the lazy_mmu_mode_enable(...)-friend.

    lazy_mmu_state.enabled = true;

...

Thanks!