linux-kernel - Re: [PATCH 0/7] Nesting support for lazy MMU mode

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Message-ID: <66335cf3-d49d-4b27-a37b-0a8a5e2c5d78@arm.com>
Date: Fri, 5 Sep 2025 14:11:43 +0200
From: Kevin Brodsky <kevin.brodsky@....com>
To: Alexander Gordeev <agordeev@...ux.ibm.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 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>, "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>, linux-arm-kernel@...ts.infradead.org,
 linuxppc-dev@...ts.ozlabs.org, sparclinux@...r.kernel.org,
 xen-devel@...ts.xenproject.org
Subject: Re: [PATCH 0/7] Nesting support for lazy MMU mode

On 05/09/2025 11:46, Alexander Gordeev wrote:
> On Thu, Sep 04, 2025 at 01:57:29PM +0100, Kevin Brodsky wrote:
>
> Hi Kevin,
>
>> When the lazy MMU mode was introduced eons ago, it wasn't made clear
>> whether such a sequence was legal:
>>
>> 	arch_enter_lazy_mmu_mode()
>> 	...
>> 		arch_enter_lazy_mmu_mode()
>> 		...
>> 		arch_leave_lazy_mmu_mode()
>> 	...
>> 	arch_leave_lazy_mmu_mode()
> I did not take too deep - sorry if you already answered this.
> Quick question - whether a concern Ryan expressed is addressed
> in general case?

The short answer is yes - it's good that you're asking because I failed
to clarify this in the cover letter!

> https://lore.kernel.org/all/3cad01ea-b704-4156-807e-7a83643917a8@arm.com/
>
> 	enter_lazy_mmu
> 		for_each_pte {
> 			read/modify-write pte
>
> 			alloc_page
> 				enter_lazy_mmu
> 					make page valid
> 				exit_lazy_mmu
>
> 			write_to_page
> 		}
> 	exit_lazy_mmu
>
> <quote>
> This example only works because lazy_mmu doesn't support nesting. The "make page
> valid" operation is completed by the time of the inner exit_lazy_mmu so that the
> page can be accessed in write_to_page. If nesting was supported, the inner
> exit_lazy_mmu would become a nop and write_to_page would explode.
> </quote>

Further down in the cover letter I refer to the approach Catalin
suggested [4]. This was in fact in response to this concern from Ryan.
The key point is: leave() keeps the lazy MMU mode enabled if it is
nested, but it flushes any batched state *unconditionally*, regardless
of nesting level. See patch 3-6 on the practical implementation of this;
patch 7 also spells it out in the documentation.

Hope that clarifies the situation!

- Kevin

[4] https://lore.kernel.org/all/aEhKSq0zVaUJkomX@arm.com/