linux-kernel - Re: [PATCH v2] arm64/mm: pmd_mkinvalid() must handle swap pmds

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Message-Id: <171449974870.639201.3165060270571039049.b4-ty@arm.com>
Date: Tue, 30 Apr 2024 18:57:52 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Will Deacon <will@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Anshuman Khandual <anshuman.khandual@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Zi Yan <zi.yan@...rutgers.edu>,
	"Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
	Ryan Roberts <ryan.roberts@....com>
Cc: linux-arm-kernel@...ts.infradead.org,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH v2] arm64/mm: pmd_mkinvalid() must handle swap pmds

On Tue, 30 Apr 2024 14:31:38 +0100, Ryan Roberts wrote:
> __split_huge_pmd_locked() can be called for a present THP, devmap or
> (non-present) migration entry. It calls pmdp_invalidate()
> unconditionally on the pmdp and only determines if it is present or not
> based on the returned old pmd.
> 
> But arm64's pmd_mkinvalid(), called by pmdp_invalidate(),
> unconditionally sets the PMD_PRESENT_INVALID flag, which causes future
> pmd_present() calls to return true - even for a swap pmd. Therefore any
> lockless pgtable walker could see the migration entry pmd in this state
> and start interpretting the fields (e.g. pmd_pfn()) as if it were
> present, leading to BadThings (TM). GUP-fast appears to be one such
> lockless pgtable walker.
> 
> [...]

Applied to arm64 (for-next/fixes), thanks! It should land in 6.9-rc7. I
removed the debug/test code, please send it as a separate patch for
6.10.

[1/1] arm64/mm: pmd_mkinvalid() must handle swap pmds
      https://git.kernel.org/arm64/c/e783331c7720

-- 
Catalin