linux-kernel - Re: [PATCH 2/7] mm: introduce local state for lazy

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Message-ID: <aLrHL-C_UXfUbCd9@kernel.org>
Date: Fri, 5 Sep 2025 14:19:11 +0300
From: Mike Rapoport <rppt@...nel.org>
To: Kevin Brodsky <kevin.brodsky@....com>
Cc: linux-mm@...ck.org, 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>,
	"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>, 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 2/7] mm: introduce local state for lazy_mmu sections

On Thu, Sep 04, 2025 at 01:57:31PM +0100, Kevin Brodsky wrote:
> arch_{enter,leave}_lazy_mmu_mode() currently have a stateless API
> (taking and returning no value). This is proving problematic in
> situations where leave() needs to restore some context back to its
> original state (before enter() was called). In particular, this
> makes it difficult to support the nesting of lazy_mmu sections -
> leave() does not know whether the matching enter() call occurred
> while lazy_mmu was already enabled, and whether to disable it or
> not.
> 
> This patch gives all architectures the chance to store local state
> while inside a lazy_mmu section by making enter() return some value,
> storing it in a local variable, and having leave() take that value.
> That value is typed lazy_mmu_state_t - each architecture defining
> __HAVE_ARCH_ENTER_LAZY_MMU_MODE is free to define it as it sees fit.
> For now we define it as int everywhere, which is sufficient to
> support nesting.
> 
> The diff is unfortunately rather large as all the API changes need
> to be done atomically. Main parts:
> 
> * Changing the prototypes of arch_{enter,leave}_lazy_mmu_mode()
>   in generic and arch code, and introducing lazy_mmu_state_t.
> 
> * Introducing LAZY_MMU_{DEFAULT,NESTED} for future support of
>   nesting. enter() always returns LAZY_MMU_DEFAULT for now.
>   (linux/mm_types.h is not the most natural location for defining
>   those constants, but there is no other obvious header that is
>   accessible where arch's implement the helpers.)
> 
> * Changing all lazy_mmu sections to introduce a lazy_mmu_state
>   local variable, having enter() set it and leave() take it. Most of
>   these changes were generated using the Coccinelle script below.
> 
> @@
> @@
> {
> + lazy_mmu_state_t lazy_mmu_state;
> ...
> - arch_enter_lazy_mmu_mode();
> + lazy_mmu_state = arch_enter_lazy_mmu_mode();
> ...
> - arch_leave_lazy_mmu_mode();
> + arch_leave_lazy_mmu_mode(lazy_mmu_state);
> ...
> }
> 
> Note: it is difficult to provide a default definition of
> lazy_mmu_state_t for architectures implementing lazy_mmu, because
> that definition would need to be available in
> arch/x86/include/asm/paravirt_types.h and adding a new generic
>  #include there is very tricky due to the existing header soup.
> 
> Signed-off-by: Kevin Brodsky <kevin.brodsky@....com>

Acked-by: Mike Rapoport (Microsoft) <rppt@...nel.org>

-- 
Sincerely yours,
Mike.