linux-kernel - Re: [PATCH v3] mm: Fix race between __split_huge_pmd

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Message-ID: <995e7240-5440-479b-bc90-0b20d0ebf5ab@amd.com>
Date: Wed, 28 Aug 2024 18:21:47 -0400
From: Felix Kuehling <felix.kuehling@....com>
To: Ryan Roberts <ryan.roberts@....com>, David Hildenbrand
 <david@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Mark Rutland <mark.rutland@....com>,
 Anshuman Khandual <anshuman.khandual@....com>, Zi Yan
 <zi.yan@...rutgers.edu>, "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
 Jonathan Corbet <corbet@....net>, Nicholas Piggin <npiggin@...il.com>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>, "David S. Miller"
 <davem@...emloft.net>, Andreas Larsson <andreas@...sler.com>,
 Dave Hansen <dave.hansen@...ux.intel.com>, Andy Lutomirski
 <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>
Cc: linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v3] mm: Fix race between __split_huge_pmd_locked() and
 GUP-fast

On 2024-05-02 09:47, Ryan Roberts wrote:
> 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?

Sorry, this question got lost and I found it while cleaning my inbox. 
The team that ran into this problems reported that it was a one-off 
failure. They didn't see this problem again. So I can't help with 
verifying the fix.

Regards,
   Felix

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