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

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Message-ID: <5a0818bb-75d4-47df-925c-0102f7d598f4-agordeev@linux.ibm.com>
Date: Mon, 15 Sep 2025 08:28:52 +0200
From: Alexander Gordeev <agordeev@...ux.ibm.com>
To: Kevin Brodsky <kevin.brodsky@....com>
Cc: David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Andreas Larsson <andreas@...sler.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "David S. Miller" <davem@...emloft.net>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jann Horn <jannh@...gle.com>, Juergen Gross <jgross@...e.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
        Madhavan Srinivasan <maddy@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>, Michal Hocko <mhocko@...e.com>,
        Mike Rapoport <rppt@...nel.org>, Nicholas Piggin <npiggin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ryan Roberts <ryan.roberts@....com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>, Vlastimil Babka <vbabka@...e.cz>,
        Will Deacon <will@...nel.org>, Yeoreum Yun <yeoreum.yun@....com>,
        linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
        sparclinux@...r.kernel.org, xen-devel@...ts.xenproject.org,
        Mark Rutland <Mark.Rutland@....com>
Subject: 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!