linux-kernel - Re: [PATCH v2 3/7] arm64: mm: fully support nested lazy

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Message-ID: <aL6iNW8sOkSaT4KE@e129823.arm.com>
Date: Mon, 8 Sep 2025 10:30:29 +0100
From: Yeoreum Yun <yeoreum.yun@....com>
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>, 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 v2 3/7] arm64: mm: fully support nested lazy_mmu sections

Reviewed-by: Yeoreum Yun <yeoreum.yun@....com>

On Mon, Sep 08, 2025 at 08:39:27AM +0100, Kevin Brodsky wrote:
> Despite recent efforts to prevent lazy_mmu sections from nesting, it
> remains difficult to ensure that it never occurs - and in fact it
> does occur on arm64 in certain situations (CONFIG_DEBUG_PAGEALLOC).
> Commit 1ef3095b1405 ("arm64/mm: Permit lazy_mmu_mode to be nested")
> made nesting tolerable on arm64, but without truly supporting it:
> the inner leave() call clears TIF_LAZY_MMU, disabling the batching
> optimisation before the outer section ends.
>
> Now that the lazy_mmu API allows enter() to pass through a state to
> the matching leave() call, we can actually support nesting. If
> enter() is called inside an active lazy_mmu section, TIF_LAZY_MMU
> will already be set, and we can then return LAZY_MMU_NESTED to
> instruct the matching leave() call not to clear TIF_LAZY_MMU.
>
> The only effect of this patch is to ensure that TIF_LAZY_MMU (and
> therefore the batching optimisation) remains set until the outermost
> lazy_mmu section ends. leave() still emits barriers if needed,
> regardless of the nesting level, as the caller may expect any
> page table changes to become visible when leave() returns.
>
> Signed-off-by: Kevin Brodsky <kevin.brodsky@....com>
> ---
>  arch/arm64/include/asm/pgtable.h | 19 +++++--------------
>  1 file changed, 5 insertions(+), 14 deletions(-)
>
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 816197d08165..602feda97dc4 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -85,24 +85,14 @@ typedef int lazy_mmu_state_t;
>
>  static inline lazy_mmu_state_t arch_enter_lazy_mmu_mode(void)
>  {
> -	/*
> -	 * lazy_mmu_mode is not supposed to permit nesting. But in practice this
> -	 * does happen with CONFIG_DEBUG_PAGEALLOC, where a page allocation
> -	 * inside a lazy_mmu_mode section (such as zap_pte_range()) will change
> -	 * permissions on the linear map with apply_to_page_range(), which
> -	 * re-enters lazy_mmu_mode. So we tolerate nesting in our
> -	 * implementation. The first call to arch_leave_lazy_mmu_mode() will
> -	 * flush and clear the flag such that the remainder of the work in the
> -	 * outer nest behaves as if outside of lazy mmu mode. This is safe and
> -	 * keeps tracking simple.
> -	 */
> +	int lazy_mmu_nested;
>
>  	if (in_interrupt())
>  		return LAZY_MMU_DEFAULT;
>
> -	set_thread_flag(TIF_LAZY_MMU);
> +	lazy_mmu_nested = test_and_set_thread_flag(TIF_LAZY_MMU);
>
> -	return LAZY_MMU_DEFAULT;
> +	return lazy_mmu_nested ? LAZY_MMU_NESTED : LAZY_MMU_DEFAULT;
>  }
>
>  static inline void arch_leave_lazy_mmu_mode(lazy_mmu_state_t state)
> @@ -113,7 +103,8 @@ static inline void arch_leave_lazy_mmu_mode(lazy_mmu_state_t state)
>  	if (test_and_clear_thread_flag(TIF_LAZY_MMU_PENDING))
>  		emit_pte_barriers();
>
> -	clear_thread_flag(TIF_LAZY_MMU);
> +	if (state != LAZY_MMU_NESTED)
> +		clear_thread_flag(TIF_LAZY_MMU);
>  }
>
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> --
> 2.47.0
>

--
Sincerely,
Yeoreum Yun