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Message-ID: <60739cf6-42ff-44c7-8e33-6c42eed71a66@arm.com>
Date: Thu, 2 May 2024 14:47:05 +0100
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Subject: Re: [PATCH v3] mm: Fix race between __split_huge_pmd_locked() and
 GUP-fast
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To: David Hildenbrand <david@redhat.com>,
 Andrew Morton <akpm@linux-foundation.org>,
 Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>,
 Mark Rutland <mark.rutland@arm.com>,
 Anshuman Khandual <anshuman.khandual@arm.com>, Zi Yan
 <zi.yan@cs.rutgers.edu>, "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>,
 Jonathan Corbet <corbet@lwn.net>, Nicholas Piggin <npiggin@gmail.com>,
 Christophe Leroy <christophe.leroy@csgroup.eu>,
 "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>,
 Christian Borntraeger <borntraeger@linux.ibm.com>,
 Sven Schnelle <svens@linux.ibm.com>, "David S. Miller"
 <davem@davemloft.net>, Andreas Larsson <andreas@gaisler.com>,
 Dave Hansen <dave.hansen@linux.intel.com>, Andy Lutomirski
 <luto@kernel.org>, Peter Zijlstra <peterz@infradead.org>,
 Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
 Borislav Petkov <bp@alien8.de>
Cc: linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org, stable@vger.kernel.org,
 Felix Kuehling <Felix.Kuehling@amd.com>
References: <20240501143310.1381675-1-ryan.roberts@arm.com>
 <3d88297b-ce6f-4b97-8a25-75f0987af6fd@redhat.com>
From: Ryan Roberts <ryan.roberts@arm.com>
In-Reply-To: <3d88297b-ce6f-4b97-8a25-75f0987af6fd@redhat.com>
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On 02/05/2024 14:08, David Hildenbrand wrote:
> On 01.05.24 16:33, 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. This is a problem for the migration entry
>> case because pmd_mkinvalid(), called by pmdp_invalidate() must only be
>> called for a present pmd.
>>
>> On arm64 at least, pmd_mkinvalid() will mark the pmd such that any
>> future call to pmd_present() will return true. And therefore any
>> lockless pgtable walker could see the migration entry pmd in this state
>> and start interpretting the fields as if it were present, leading to
>> BadThings (TM). GUP-fast appears to be one such lockless pgtable walker.
>>
>> x86 does not suffer the above problem, but instead pmd_mkinvalid() will
>> corrupt the offset field of the swap entry within the swap pte. See link
>> below for discussion of that problem.
> 
> Could that explain:
> 
> https://lore.kernel.org/all/YjoGbhreg8lGCGIJ@linutronix.de/
> 
> Where the PFN of a migration entry might have been corrupted?

Ahh interesting! Yes, it seems to fit...

> 
> Ccing Felix

Are you able to reliably reproduce the bug, Felix? If so, would you mind trying
with this patch to see if it goes away?

> 
> 
> Patch itself looks good to me
> 
> Acked-by: David Hildenbrand <david@redhat.com>

Thanks!