linux-kernel - Re: [PATCH v4 06/12] mm: introduce generic lazy

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Message-ID: <c764489e-0626-4a50-87b5-39e15d9db733@gmail.com>
Date: Fri, 7 Nov 2025 15:34:03 +0100
From: "David Hildenbrand (Red Hat)" <davidhildenbrandkernel@...il.com>
To: Ryan Roberts <ryan.roberts@....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>, 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 06/12] mm: introduce generic lazy_mmu helpers

>>   #ifndef pte_batch_hint
>> diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
>> index 5d2a876035d6..c49b029d3593 100644
>> --- a/mm/kasan/shadow.c
>> +++ b/mm/kasan/shadow.c
>> @@ -305,7 +305,7 @@ static int kasan_populate_vmalloc_pte(pte_t *ptep, unsigned long addr,
>>   	pte_t pte;
>>   	int index;
>>   
>> -	arch_leave_lazy_mmu_mode();
>> +	lazy_mmu_mode_pause();
> 
> I wonder if there really are use cases that *require* pause/resume? I think
> these kasan cases could be correctly implemented using a new nest level instead?
> Are there cases where the effects really need to be immediate or do the effects
> just need to be visible when you get to where the resume is?
> 
> If the latter, that could just be turned into a nested disable (e.g. a flush).
> In this case, there is only 1 PTE write so no benefit, but I wonder if other
> cases may have more PTE writes that could then still be batched. It would be
> nice to simplify the API by removing pause/resume if we can?

It has clear semantics, clearer than some nest-disable IMHO.

Maybe you can elaborate how you would change ("simplify") the API in 
that regard? What would the API look like?