linux-kernel - Re: [PATCH v6 05/14] mm: clarify lazy

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Message-ID: <0c72913e-9708-4675-a421-06ed82b7802a@kernel.org>
Date: Fri, 9 Jan 2026 16:03:45 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: 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>,
 Anshuman Khandual <anshuman.khandual@....com>,
 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>,
 "Ritesh Harjani (IBM)" <ritesh.list@...il.com>,
 Ryan Roberts <ryan.roberts@....com>, Suren Baghdasaryan <surenb@...gle.com>,
 Thomas Gleixner <tglx@...utronix.de>,
 Venkat Rao Bagalkote <venkat88@...ux.ibm.com>,
 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 v6 05/14] mm: clarify lazy_mmu sleeping constraints

On 12/15/25 16:03, Kevin Brodsky wrote:
> The lazy MMU mode documentation makes clear that an implementation
> should not assume that preemption is disabled or any lock is held
> upon entry to the mode; however it says nothing about what code
> using the lazy MMU interface should expect.
> 
> In practice sleeping is forbidden (for generic code) while the lazy
> MMU mode is active: say it explicitly.
> 
> Signed-off-by: Kevin Brodsky <kevin.brodsky@....com>
> ---
>   include/linux/pgtable.h | 14 +++++++++-----
>   1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 652f287c1ef6..1abc4a1c3d72 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -225,11 +225,15 @@ static inline int pmd_dirty(pmd_t pmd)
>    * up to date.
>    *
>    * 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.
> - * (In practice, for user PTE updates, the appropriate page table lock(s) are
> - * held, but for kernel PTE updates, no lock is held). Nesting is not permitted
> - * and the mode cannot be used in interrupt context.
> + * of the lazy mode. (In practice, for user PTE updates, the appropriate page
> + * table lock(s) are held, but for kernel PTE updates, no lock is held).
> + * The implementation must therefore assume preemption may be enabled upon
> + * entry to the mode and cpu migration is possible; it must take steps to be
> + * robust against this. An implementation may handle this by disabling
> + * preemption, as a consequence generic code may not sleep while the lazy MMU
> + * mode is active.
> + *
> + * Nesting is not permitted and the mode cannot be used in interrupt context.
>    */
>   #ifndef __HAVE_ARCH_ENTER_LAZY_MMU_MODE
>   static inline void arch_enter_lazy_mmu_mode(void) {}

Acked-by: David Hildenbrand (Red Hat) <david@...nel.org>

-- 
Cheers

David