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

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Message-ID: <334d6272-a1c2-4075-a956-3f41908371a6@arm.com>
Date: Mon, 3 Nov 2025 18:25:41 +0000
From: Kevin Brodsky <kevin.brodsky@....com>
To: David Hildenbrand <david@...hat.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 03/11/2025 16:03, David Hildenbrand wrote:
> 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.

Ah yes good point, the new version would do an atomic RMW in all cases.
Simpler code but also slower :/

>
> 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); 

Pretty much, since we're now only considering one flag we can simplify
it to:

if (!test_thread_flag(TIF_LAZY_MMU_PENDING))
    set_thread_flag(TIF_LAZY_MMU_PENDING);

- Kevin