linux-kernel - Re: [PATCH v5 09/12] arm64: mm: replace TIF_LAZY_MMU with in_lazy_mmu

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Message-ID: <b341e7fa-4382-48b5-896a-1a0d347b66ca@kernel.org>
Date: Thu, 4 Dec 2025 12:39:10 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Anshuman Khandual <anshuman.khandual@....com>,
 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>,
 "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 09/12] arm64: mm: replace TIF_LAZY_MMU with
 in_lazy_mmu_mode()

On 12/4/25 07:52, Anshuman Khandual wrote:
> On 24/11/25 6:52 PM, 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.
>>
>> The explicit check for in_interrupt() is no longer necessary either
>> as in_lazy_mmu_mode() always returns false in interrupt context.
>>
>> Signed-off-by: Kevin Brodsky <kevin.brodsky@....com>
>> ---
>>   arch/arm64/include/asm/pgtable.h     | 19 +++----------------
>>   arch/arm64/include/asm/thread_info.h |  3 +--
>>   2 files changed, 4 insertions(+), 18 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index a7d99dee3dc4..dd7ed653a20d 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -62,28 +62,16 @@ 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)) {
>> +	if (in_lazy_mmu_mode()) {
>>   		/* Avoid the atomic op if already set. */
>> -		if (!(flags & BIT(TIF_LAZY_MMU_PENDING)))
>> +		if (!test_thread_flag(TIF_LAZY_MMU_PENDING))
> 
> A small nit - will it be better not to use test_thread_flag() here and just
> keep checking flags like earlier to avoid non-related changes. Although not
> a problem TBH.

I'd assume the existing code wanted to avoid fetching the flags two 
times? So switching to test_thread_flag() should be fine now.

-- 
Cheers

David