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

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Message-ID: <f36ebc15-c724-487a-8e4a-9ca95edc544a@arm.com>
Date: Mon, 24 Nov 2025 13:47:53 +0100
From: Kevin Brodsky <kevin.brodsky@....com>
To: Ryan Roberts <ryan.roberts@....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 Hildenbrand <david@...hat.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

On 10/11/2025 11:45, Kevin Brodsky wrote:
>>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>>> index b8d37eb037fc..d9c8e94f140f 100644
>>> --- a/arch/arm64/mm/mmu.c
>>> +++ b/arch/arm64/mm/mmu.c
>>> @@ -731,7 +731,7 @@ int split_kernel_leaf_mapping(unsigned long start, unsigned long end)
>>>  		return -EINVAL;
>>>  
>>>  	mutex_lock(&pgtable_split_lock);
>>> -	arch_enter_lazy_mmu_mode();
>>> +	lazy_mmu_mode_enable();
>>>  
>>>  	/*
>>>  	 * The split_kernel_leaf_mapping_locked() may sleep, it is not a
>> This is a bit unfortunate, IMHO. The rest of this comment explains that although
>> you're not supposed to sleep inside lazy mmu mode, it's fine for arm64's
>> implementation. But we are no longer calling arm64's implementation; we are
>> calling a generic function, which does who knows what.
>>
>> I think it all still works, but we are no longer containing our assumptions in
>> arm64 code. We are relying on implementation details of generic code.
> I see your point. The change itself is still correct (and required
> considering patch 8), but maybe the documentation of the generic
> interface should be clarified to guarantee that the generic layer can
> itself cope with sleeping - without any guarantee regarding the
> behaviour of arch_*_lazy_mmu_mode.

Re-reading the existing comment in <linux/pgtable.h>, I think it already
makes clear that sleeping is not forbidden by design:

>  * In the general case, no lock is guaranteed to be held between entry
> and exit
>  * of the lazy mode. So the implementation must assume preemption may
> be enabled
>  * and cpu migration is possible; it must take steps to be robust
> against this.

The arch implementation may disable preemption, but arm64 code can rely
on the arm64 implementation allowing sleeping.

- Kevin