linux-kernel - Re: [PATCH v6 13/18] arm64/mm: Implement new wrprotect

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Message-ID: <Zc9VVC4cSFcYzgCZ@arm.com>
Date: Fri, 16 Feb 2024 12:30:12 +0000
From: Catalin Marinas <catalin.marinas@....com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Will Deacon <will@...nel.org>, Ard Biesheuvel <ardb@...nel.org>,
	Marc Zyngier <maz@...nel.org>, James Morse <james.morse@....com>,
	Andrey Ryabinin <ryabinin.a.a@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Matthew Wilcox <willy@...radead.org>,
	Mark Rutland <mark.rutland@....com>,
	David Hildenbrand <david@...hat.com>,
	Kefeng Wang <wangkefeng.wang@...wei.com>,
	John Hubbard <jhubbard@...dia.com>, Zi Yan <ziy@...dia.com>,
	Barry Song <21cnbao@...il.com>,
	Alistair Popple <apopple@...dia.com>,
	Yang Shi <shy828301@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	linux-arm-kernel@...ts.infradead.org, x86@...nel.org,
	linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 13/18] arm64/mm: Implement new wrprotect_ptes() batch
 API

On Thu, Feb 15, 2024 at 10:32:00AM +0000, Ryan Roberts wrote:
> Optimize the contpte implementation to fix some of the fork performance
> regression introduced by the initial contpte commit. Subsequent patches
> will solve it entirely.
> 
> During fork(), any private memory in the parent must be write-protected.
> Previously this was done 1 PTE at a time. But the core-mm supports
> batched wrprotect via the new wrprotect_ptes() API. So let's implement
> that API and for fully covered contpte mappings, we no longer need to
> unfold the contpte. This has 2 benefits:
> 
>   - reduced unfolding, reduces the number of tlbis that must be issued.
>   - The memory remains contpte-mapped ("folded") in the parent, so it
>     continues to benefit from the more efficient use of the TLB after
>     the fork.
> 
> The optimization to wrprotect a whole contpte block without unfolding is
> possible thanks to the tightening of the Arm ARM in respect to the
> definition and behaviour when 'Misprogramming the Contiguous bit'. See
> section D21194 at https://developer.arm.com/documentation/102105/ja-07/
> 
> Tested-by: John Hubbard <jhubbard@...dia.com>
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>

Acked-by: Catalin Marinas <catalin.marinas@....com>