linux-kernel - Re: [PATCH v5 06/12] mm: introduce generic lazy

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Message-ID: <573881f1-60f7-4eb3-a484-1df4858aa1b4@arm.com>
Date: Fri, 5 Dec 2025 13:47:12 +0100
From: Kevin Brodsky <kevin.brodsky@....com>
To: Anshuman Khandual <anshuman.khandual@....com>, linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org, Alexander Gordeev <agordeev@...ux.ibm.com>,
 Andreas Larsson <andreas@...sler.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 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 Hildenbrand <david@...hat.com>, "David S. Miller"
 <davem@...emloft.net>, David Woodhouse <dwmw2@...radead.org>,
 "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>,
 "Ritesh Harjani (IBM)" <ritesh.list@...il.com>,
 Ryan Roberts <ryan.roberts@....com>, Suren Baghdasaryan <surenb@...gle.com>,
 Thomas Gleixner <tglx@...utronix.de>,
 Venkat Rao Bagalkote <venkat88@...ux.ibm.com>,
 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, x86@...nel.org
Subject: Re: [PATCH v5 06/12] mm: introduce generic lazy_mmu helpers

On 04/12/2025 05:17, Anshuman Khandual wrote:
> On 24/11/25 6:52 PM, Kevin Brodsky wrote:
>> The implementation of the lazy MMU mode is currently entirely
>> arch-specific; core code directly calls arch helpers:
>> arch_{enter,leave}_lazy_mmu_mode().
>>
>> We are about to introduce support for nested lazy MMU sections.
>> As things stand we'd have to duplicate that logic in every arch
>> implementing lazy_mmu - adding to a fair amount of logic
>> already duplicated across lazy_mmu implementations.
>>
>> This patch therefore introduces a new generic layer that calls the
>> existing arch_* helpers. Two pair of calls are introduced:
>>
>> * lazy_mmu_mode_enable() ... lazy_mmu_mode_disable()
>>     This is the standard case where the mode is enabled for a given
>>     block of code by surrounding it with enable() and disable()
>>     calls.
>>
>> * lazy_mmu_mode_pause() ... lazy_mmu_mode_resume()
>>     This is for situations where the mode is temporarily disabled
>>     by first calling pause() and then resume() (e.g. to prevent any
>>     batching from occurring in a critical section).
>>
>> The documentation in <linux/pgtable.h> will be updated in a
>> subsequent patch.
>>> No functional change should be introduced at this stage.
>> The implementation of enable()/resume() and disable()/pause() is
>> currently identical, but nesting support will change that.
>>
>> Most of the call sites have been updated using the following
>> Coccinelle script:
>>
>> @@
>> @@
>> {
>> ...
>> - arch_enter_lazy_mmu_mode();
>> + lazy_mmu_mode_enable();
>> ...
>> - arch_leave_lazy_mmu_mode();
>> + lazy_mmu_mode_disable();
>> ...
>> }
>>
>> @@
>> @@
>> {
>> ...
>> - arch_leave_lazy_mmu_mode();
>> + lazy_mmu_mode_pause();
>> ...
>> - arch_enter_lazy_mmu_mode();
>> + lazy_mmu_mode_resume();
>> ...
>> }
> At this point arch_enter/leave_lazy_mmu_mode() helpers are still
> present on a given platform but now being called from new generic
> helpers lazy_mmu_mode_enable/disable(). Well except x86, there is
> direct call sites for those old helpers.

Indeed, see notes below regarding x86. The direct calls to arch_flush()
are specific to x86 and there shouldn't be a need for a generic abstraction.

- Kevin

> arch/arm64/include/asm/pgtable.h:static inline void arch_enter_lazy_mmu_mode(void)
> arch/arm64/include/asm/pgtable.h:static inline void arch_leave_lazy_mmu_mode(void)
>
> arch/arm64/mm/mmu.c:    lazy_mmu_mode_enable();
> arch/arm64/mm/pageattr.c:       lazy_mmu_mode_enable();
>
> arch/arm64/mm/mmu.c:    lazy_mmu_mode_disable();
> arch/arm64/mm/pageattr.c:       lazy_mmu_mode_disable();
>
>> A couple of notes regarding x86:
>>
>> * Xen is currently the only case where explicit handling is required
>>   for lazy MMU when context-switching. This is purely an
>>   implementation detail and using the generic lazy_mmu_mode_*
>>   functions would cause trouble when nesting support is introduced,
>>   because the generic functions must be called from the current task.
>>   For that reason we still use arch_leave() and arch_enter() there.
>>
>> * x86 calls arch_flush_lazy_mmu_mode() unconditionally in a few
>>   places, but only defines it if PARAVIRT_XXL is selected, and we
>>   are removing the fallback in <linux/pgtable.h>. Add a new fallback
>>   definition to <asm/pgtable.h> to keep things building.
>>
>> [...]