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

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Message-ID: <d407a381-099b-4ec6-a20e-aeff4f3d750f@arm.com>
Date: Mon, 15 Sep 2025 13:19:32 +0200
From: Kevin Brodsky <kevin.brodsky@....com>
To: Alexander Gordeev <agordeev@...ux.ibm.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 15/09/2025 08:28, Alexander Gordeev wrote:
> 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.

Yes I think that's fine.

> 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.

Got it, that makes sense. Even without your proposal, it is probably a
good idea to allow arch's to behave differently on pause/resume.

I hope we can avoid forcing all arch's to define arch_pause/arch_resume
though, since only s390 will use it for the foreseeable future. Using
optional macros should do the trick.

- Kevin