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

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Message-ID: <a25ae4d3-31cc-4619-8306-f3047dd3e73b@arm.com>
Date: Wed, 1 May 2024 11:13:38 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: 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>,
 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 01/05/2024 11:04, Catalin Marinas wrote:
> On Wed, May 01, 2024 at 09:05:17AM +0100, Ryan Roberts wrote:
>> On 30/04/2024 18:57, Catalin Marinas wrote:
>>> 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.
>>
>> Thanks Catalin! I'm guessing this will turn up in today's linux-next, so if I
>> send the tests today and Andrew puts them straight in mm-unstable (which will
>> goto linux-next) there is no risk that the tests are there without the fix? Or
>> do I need to hold off until the fix is in v6.9-rc7?
> 
> It looks like we don't push for-next/fixes to linux-next, it's
> short-lived usually, it ends up upstream quickly. I can send the pull
> request later today, should turn up in mainline by tomorrow. You can add
> a note to your patch for Andrew that it will fail on arm64 until the fix
> ends up upstream. It's a matter of a couple of days anyway.

OK, I just didn't want to send it only for our CI to start failing. I'll do as
you suggest.