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

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Message-ID: <9fd076c7-f163-4b92-8201-d8a259a338c1-agordeev@linux.ibm.com>
Date: Fri, 5 Sep 2025 11:46:23 +0200
From: Alexander Gordeev <agordeev@...ux.ibm.com>
To: Kevin Brodsky <kevin.brodsky@....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 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?

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>

...

Thanks!