linux-kernel - Re: [PATCH] arm64/mm: Permit lazy_mmu

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Message-Id: <174722572203.77262.3085803317219491344.b4-ty@kernel.org>
Date: Wed, 14 May 2025 16:14:12 +0100
From: Will Deacon <will@...nel.org>
To: Catalin Marinas <catalin.marinas@....com>,
	Pasha Tatashin <pasha.tatashin@...een.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Uladzislau Rezki <urezki@...il.com>,
	Christoph Hellwig <hch@...radead.org>,
	David Hildenbrand <david@...hat.com>,
	"Matthew Wilcox (Oracle)" <willy@...radead.org>,
	Mark Rutland <mark.rutland@....com>,
	Anshuman Khandual <anshuman.khandual@....com>,
	Alexandre Ghiti <alexghiti@...osinc.com>,
	Kevin Brodsky <kevin.brodsky@....com>,
	Ryan Roberts <ryan.roberts@....com>
Cc: kernel-team@...roid.com,
	Will Deacon <will@...nel.org>,
	linux-arm-kernel@...ts.infradead.org,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64/mm: Permit lazy_mmu_mode to be nested

On Mon, 12 May 2025 16:03:31 +0100, Ryan Roberts wrote:
> 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 (see stack trace below).
> 
> The warning checking that nesting was not happening was previously being
> triggered due to this. So let's relax by removing the warning and
> tolerate nesting in the arm64 implementation. The first (inner) 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.
> 
> [...]

Applied to arm64 (for-next/mm), thanks!

[1/1] arm64/mm: Permit lazy_mmu_mode to be nested
      https://git.kernel.org/arm64/c/1ef3095b1405

Cheers,
-- 
Will

https://fixes.arm64.dev
https://next.arm64.dev
https://will.arm64.dev