linux-kernel - Re: [PATCH v4 08/12] arm64: mm: replace TIF_LAZY_MMU with in_lazy_mmu

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Message-ID: <06026ad7-fb85-4a78-ba70-7fa1f5bca0cd@arm.com>
Date: Fri, 7 Nov 2025 15:28:53 +0000
From: Ryan Roberts <ryan.roberts@....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>, 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, x86@...nel.org
Subject: Re: [PATCH v4 08/12] arm64: mm: replace TIF_LAZY_MMU with
 in_lazy_mmu_mode()

On 29/10/2025 10:09, Kevin Brodsky wrote:
> The generic lazy_mmu layer now tracks whether a task is in lazy MMU
> mode. As a result we no longer need a TIF flag for that purpose -
> let's use the new in_lazy_mmu_mode() helper instead.
> 
> Signed-off-by: Kevin Brodsky <kevin.brodsky@....com>
> ---
>  arch/arm64/include/asm/pgtable.h     | 16 +++-------------
>  arch/arm64/include/asm/thread_info.h |  3 +--
>  2 files changed, 4 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 535435248923..61ca88f94551 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -62,30 +62,21 @@ static inline void emit_pte_barriers(void)
>  
>  static inline void queue_pte_barriers(void)
>  {
> -	unsigned long flags;
> -
>  	if (in_interrupt()) {
>  		emit_pte_barriers();
>  		return;
>  	}
>  
> -	flags = read_thread_flags();
> -
> -	if (flags & BIT(TIF_LAZY_MMU)) {
> -		/* Avoid the atomic op if already set. */
> -		if (!(flags & BIT(TIF_LAZY_MMU_PENDING)))
> -			set_thread_flag(TIF_LAZY_MMU_PENDING);
> -	} else {
> +	if (in_lazy_mmu_mode())
> +		test_and_set_thread_flag(TIF_LAZY_MMU_PENDING);

This removes the optimization to only do the atomic set operation if the bit is
not already set. I think that should remain.

> +	else
>  		emit_pte_barriers();
> -	}
>  }
>  
>  static inline void arch_enter_lazy_mmu_mode(void)
>  {
>  	if (in_interrupt())
>  		return;

Why are you keeping this test? Surely it can go?

> -
> -	set_thread_flag(TIF_LAZY_MMU);
>  }
>  
>  static inline void arch_flush_lazy_mmu_mode(void)
> @@ -103,7 +94,6 @@ static inline void arch_leave_lazy_mmu_mode(void)
>  		return;
>  
>  	arch_flush_lazy_mmu_mode();
> -	clear_thread_flag(TIF_LAZY_MMU);
>  }
>  
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
> index f241b8601ebd..4ff8da0767d9 100644
> --- a/arch/arm64/include/asm/thread_info.h
> +++ b/arch/arm64/include/asm/thread_info.h
> @@ -84,8 +84,7 @@ void arch_setup_new_exec(void);
>  #define TIF_SME_VL_INHERIT	28	/* Inherit SME vl_onexec across exec */
>  #define TIF_KERNEL_FPSTATE	29	/* Task is in a kernel mode FPSIMD section */
>  #define TIF_TSC_SIGSEGV		30	/* SIGSEGV on counter-timer access */
> -#define TIF_LAZY_MMU		31	/* Task in lazy mmu mode */
> -#define TIF_LAZY_MMU_PENDING	32	/* Ops pending for lazy mmu mode exit */
> +#define TIF_LAZY_MMU_PENDING	31	/* Ops pending for lazy mmu mode exit */
>  
>  #define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
>  #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)