linux-kernel - Re: [PATCH v3 12/13] mm: bail out of lazy_mmu_mode

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Message-ID: <ed949468-5425-4f82-826b-249c43a0df05@arm.com>
Date: Fri, 24 Oct 2025 14:17:04 +0200
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>, "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 v3 12/13] mm: bail out of lazy_mmu_mode_* in interrupt
 context

On 23/10/2025 22:08, David Hildenbrand wrote:
> On 15.10.25 10:27, 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.
>>
>> Both arm64 and x86/Xen also ensure that any lazy MMU optimisation is
>> disabled while in interrupt mode (see queue_pte_barriers() and
>> xen_get_lazy_mode() respectively).
>>
>> 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(), and
>> have in_lazy_mmu_mode() return false to disable any optimisation.
>> Also remove the arm64 handling that is now redundant; x86/Xen has
>> its own internal tracking so it is left unchanged.
>>
>> Signed-off-by: Kevin Brodsky <kevin.brodsky@....com>
>> ---
>>   arch/arm64/include/asm/pgtable.h | 17 +----------------
>>   include/linux/pgtable.h          | 16 ++++++++++++++--
>>   include/linux/sched.h            |  3 +++
>>   3 files changed, 18 insertions(+), 18 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/pgtable.h
>> b/arch/arm64/include/asm/pgtable.h
>> index 944e512767db..a37f417c30be 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -62,37 +62,22 @@ static inline void emit_pte_barriers(void)
>>     static inline void queue_pte_barriers(void)
>>   {
>> -    if (in_interrupt()) {
>> -        emit_pte_barriers();
>> -        return;
>> -    }
>> -
>
> That took me a while. I guess this works because in_lazy_mmu_mode() ==
> 0 in interrupt context, so we keep calling emit_pte_barriers?

Yes exactly.

- Kevin