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

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Message-ID: <07ffb66d-1e74-4634-bccb-75575b3862af-agordeev@linux.ibm.com>
Date: Fri, 28 Nov 2025 14:50:44 +0100
From: Alexander Gordeev <agordeev@...ux.ibm.com>
To: Kevin Brodsky <kevin.brodsky@....com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        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 Mon, Nov 24, 2025 at 01:22:22PM +0000, 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();
> ...
> }
> 
> 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.

Would it make sense to explicitly describe the policy wrt sleeping while
in lazy MMU mode? If I understand the conclusion of conversation right:

* An arch implementation may disable preemption, but then it is arch
  responsibility not to call any arch-specific code that might sleep;
* As result, while in lazy MMU mode the generic code should never
  call a code that might sleep;

1. https://lore.kernel.org/linux-mm/b52726c7-ea9c-4743-a68d-3eafce4e5c61@arm.com/

> Acked-by: David Hildenbrand <david@...hat.com>
> Signed-off-by: Kevin Brodsky <kevin.brodsky@....com>
...