linux-kernel - Re: [PATCH v5 08/12] mm: enable lazy

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Message-ID: <4d0eab1a-e1dc-49cf-8640-727c9734233d-agordeev@linux.ibm.com>
Date: Thu, 27 Nov 2025 13:33:04 +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 08/12] mm: enable lazy_mmu sections to nest

On Mon, Nov 24, 2025 at 01:22:24PM +0000, Kevin Brodsky wrote:

Hi Kevin,

...
> +/**
> + * lazy_mmu_mode_pause() - Pause the lazy MMU mode.
> + *
> + * Pauses the lazy MMU mode; if it is currently active, disables it and calls
> + * arch_leave_lazy_mmu_mode().
> + *
> + * Must be paired with a call to lazy_mmu_mode_resume(). Calls to the
> + * lazy_mmu_mode_* API have no effect until the matching resume() call.

Sorry if it was discussed already - if yes, I somehow missed it. If I read
the whole thing correctly enter()/pause() interleaving is not forbidden?

lazy_mmu_mode_enable()
	lazy_mmu_mode_pause()
		lazy_mmu_mode_enable()
		...
		lazy_mmu_mode_disable()
	lazy_mmu_mode_resume()
lazy_mmu_mode_disable()

> + *
> + * Has no effect if called:
> + * - While paused (inside another pause()/resume() pair)
> + * - In interrupt context
> + */
>  static inline void lazy_mmu_mode_pause(void)
>  {
> +	struct lazy_mmu_state *state = &current->lazy_mmu_state;
> +
>  	if (in_interrupt())
>  		return;
>  
> -	arch_leave_lazy_mmu_mode();
> +	VM_WARN_ON_ONCE(state->pause_count == U8_MAX);
> +
> +	if (state->pause_count++ == 0 && state->enable_count > 0)
> +		arch_leave_lazy_mmu_mode();
>  }
>  
> +/**
> + * lazy_mmu_mode_pause() - Resume the lazy MMU mode.
                    resume() ?
> + *
> + * Resumes the lazy MMU mode; if it was active at the point where the matching
> + * call to lazy_mmu_mode_pause() was made, re-enables it and calls
> + * arch_enter_lazy_mmu_mode().
> + *
> + * Must match a call to lazy_mmu_mode_pause().
> + *
> + * Has no effect if called:
> + * - While paused (inside another pause()/resume() pair)
> + * - In interrupt context
> + */
>  static inline void lazy_mmu_mode_resume(void)
>  {
> +	struct lazy_mmu_state *state = &current->lazy_mmu_state;
> +
>  	if (in_interrupt())
>  		return;
>  
> -	arch_enter_lazy_mmu_mode();
> +	VM_WARN_ON_ONCE(state->pause_count == 0);
> +
> +	if (--state->pause_count == 0 && state->enable_count > 0)
> +		arch_enter_lazy_mmu_mode();
>  }
...
Thanks!