linux-kernel - Re: [PATCH v3 08/22] mm: Allow page table accessors to be non-idempotent

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Message-ID: <c2597d43-d909-4259-bb5b-9294e4069385@kernel.org>
Date: Thu, 27 Nov 2025 18:47:38 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Ryan Roberts <ryan.roberts@....com>,
 Samuel Holland <samuel.holland@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>, Paul Walmsley <pjw@...nel.org>,
 linux-riscv@...ts.infradead.org, Andrew Morton <akpm@...ux-foundation.org>,
 linux-mm@...ck.org
Cc: devicetree@...r.kernel.org, Suren Baghdasaryan <surenb@...gle.com>,
 linux-kernel@...r.kernel.org, Mike Rapoport <rppt@...nel.org>,
 Michal Hocko <mhocko@...e.com>, Conor Dooley <conor@...nel.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Alexandre Ghiti <alex@...ti.fr>,
 Emil Renner Berthing <kernel@...il.dk>, Rob Herring <robh+dt@...nel.org>,
 Vlastimil Babka <vbabka@...e.cz>, "Liam R . Howlett"
 <Liam.Howlett@...cle.com>
Subject: Re: [PATCH v3 08/22] mm: Allow page table accessors to be
 non-idempotent

On 11/27/25 17:57, Ryan Roberts wrote:
> On 13/11/2025 01:45, Samuel Holland wrote:
>> Currently, some functions such as pte_offset_map() are passed both
>> pointers to hardware page tables, and pointers to previously-read PMD
>> entries on the stack. To ensure correctness in the first case, these
>> functions must use the page table accessor function (pmdp_get()) to
>> dereference the supplied pointer. However, this means pmdp_get() is
>> called twice in the second case. This double call must be avoided if
>> pmdp_get() applies some non-idempotent transformation to the value.
>>
>> Avoid the double transformation by calling set_pmd() on the stack
>> variables where necessary to keep set_pmd()/pmdp_get() calls balanced.
> 
> I don't think this is a good solution.

Agreed,

	set_pmd(&pmd, pmd);

is rather horrible.

-- 
Cheers

David