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Date: Thu, 2 May 2024 15:08:20 +0200
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Subject: Re: [PATCH v3] mm: Fix race between __split_huge_pmd_locked() and
 GUP-fast
To: Ryan Roberts <ryan.roberts@arm.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>
From: David Hildenbrand <david@redhat.com>
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In-Reply-To: <20240501143310.1381675-1-ryan.roberts@arm.com>
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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?

Ccing Felix


Patch itself looks good to me

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb