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

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Message-ID: <b6f5b3cc-93a0-408a-b7e0-72462f3fd549@redhat.com>
Date: Mon, 3 Nov 2025 17:03:42 +0100
From: David Hildenbrand <david@...hat.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 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>, Ryan Roberts <ryan.roberts@....com>,
 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 29.10.25 11:09, 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);

You likely don't want a test_and_set here, which would do a 
test_and_set_bit() -- an atomic rmw.

You only want to avoid the atomic write if already set.

So keep the current

	/* Avoid the atomic op if already set. */
	if (!(flags & BIT(TIF_LAZY_MMU_PENDING)))
		set_thread_flag(TIF_LAZY_MMU_PENDING);

-- 
Cheers

David