linux-kernel - Re: [PATCH v5 07/12] mm: bail out of lazy_mmu_mode

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Message-ID: <565ac89c-1e7a-4e2f-a866-15039fbe22f6@arm.com>
Date: Thu, 4 Dec 2025 10:04:04 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Kevin Brodsky <kevin.brodsky@....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 07/12] mm: bail out of lazy_mmu_mode_* in interrupt
 context

On 24/11/25 6:52 PM, Kevin Brodsky wrote:
> The lazy MMU mode cannot be used in interrupt context. This is
> documented in <linux/pgtable.h>, but isn't consistently handled
> across architectures.
> 
> arm64 ensures that calls to lazy_mmu_mode_* have no effect in
> interrupt context, because such calls do occur in certain
> configurations - see commit b81c688426a9 ("arm64/mm: Disable barrier
> batching in interrupt contexts"). Other architectures do not check
> this situation, most likely because it hasn't occurred so far.
> 
> Let's handle this in the new generic lazy_mmu layer, in the same
> fashion as arm64: bail out of lazy_mmu_mode_* if in_interrupt().
> Also remove the arm64 handling that is now redundant.

Ensuring a common behaviour across platforms such as this in interrupt
context bail out - can now be easily achieved now as there are generic
helpers that call the platform ones.

> > Both arm64 and x86/Xen also ensure that any lazy MMU optimisation is
> disabled while in interrupt (see queue_pte_barriers() and
> xen_get_lazy_mode() respectively). This will be handled in the
> generic layer in a subsequent patch.

Makes sense.

> 
> Signed-off-by: Kevin Brodsky <kevin.brodsky@....com>
> ---
>  arch/arm64/include/asm/pgtable.h |  9 ---------
>  include/linux/pgtable.h          | 17 +++++++++++++++--
>  2 files changed, 15 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 54f8d6bb6f22..e596899f4029 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -94,26 +94,17 @@ static inline void arch_enter_lazy_mmu_mode(void)
>  	 * keeps tracking simple.
>  	 */
>  
> -	if (in_interrupt())
> -		return;
> -
>  	set_thread_flag(TIF_LAZY_MMU);
>  }
>  
>  static inline void arch_flush_lazy_mmu_mode(void)
>  {
> -	if (in_interrupt())
> -		return;
> -
>  	if (test_and_clear_thread_flag(TIF_LAZY_MMU_PENDING))
>  		emit_pte_barriers();
>  }
>  
>  static inline void arch_leave_lazy_mmu_mode(void)
>  {
> -	if (in_interrupt())
> -		return;
> -
>  	arch_flush_lazy_mmu_mode();
>  	clear_thread_flag(TIF_LAZY_MMU);
>  }
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index c121358dba15..8ff6fdb4b13d 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -228,27 +228,40 @@ static inline int pmd_dirty(pmd_t pmd)
>   * of the lazy mode. So the implementation must assume preemption may be enabled
>   * and cpu migration is possible; it must take steps to be robust against this.
>   * (In practice, for user PTE updates, the appropriate page table lock(s) are
> - * held, but for kernel PTE updates, no lock is held). Nesting is not permitted
> - * and the mode cannot be used in interrupt context.
> + * held, but for kernel PTE updates, no lock is held). The mode is disabled in
> + * interrupt context and calls to the lazy_mmu API have no effect.
> + * Nesting is not permitted.

Right - nesting is still not not permitted.

>   */>  #ifdef CONFIG_ARCH_HAS_LAZY_MMU_MODE
>  static inline void lazy_mmu_mode_enable(void)
>  {
> +	if (in_interrupt())
> +		return;
> +
>  	arch_enter_lazy_mmu_mode();
>  }
>  
>  static inline void lazy_mmu_mode_disable(void)
>  {
> +	if (in_interrupt())
> +		return;
> +
>  	arch_leave_lazy_mmu_mode();
>  }
>  
>  static inline void lazy_mmu_mode_pause(void)
>  {
> +	if (in_interrupt())
> +		return;
> +
>  	arch_leave_lazy_mmu_mode();
>  }
>  
>  static inline void lazy_mmu_mode_resume(void)
>  {
> +	if (in_interrupt())
> +		return;
> +
>  	arch_enter_lazy_mmu_mode();
>  }
>  #else

LGTM

Reviewed-by: Anshuman Khandual <anshuman.khandual@....com>