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Wed, 10 Dec 2025 16:33:46 -0800 (PST) Message-ID: Date: Thu, 11 Dec 2025 09:33:41 +0900 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 08/22] mm: Allow page table accessors to be non-idempotent To: "David Hildenbrand (Red Hat)" , Ryan Roberts , Palmer Dabbelt , Paul Walmsley , linux-riscv@lists.infradead.org, Andrew Morton , linux-mm@kvack.org Cc: devicetree@vger.kernel.org, Suren Baghdasaryan , linux-kernel@vger.kernel.org, Mike Rapoport , Michal Hocko , Conor Dooley , Lorenzo Stoakes , Krzysztof Kozlowski , Alexandre Ghiti , Emil Renner Berthing , Rob Herring , Vlastimil Babka , "Liam R . Howlett" References: <20251113014656.2605447-1-samuel.holland@sifive.com> <20251113014656.2605447-9-samuel.holland@sifive.com> From: Samuel Holland Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Stat-Signature: 4zrb6irz5qy5jrd4ypw8txbkzz9g4sqj X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: E5CB0C000B X-HE-Tag: 1765413227-680616 X-HE-Meta: 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 006U7Vt/ HlaITsd8ZMzwuAJe71+2GBjGiHaclWyPtjIrr5MOPiVnwY7LS/fbeikI7lAe3oK3vCV7XoCDgKzZqw8cPuXILbplZCfLgOPqTtRjcetnnuXJD7jtYIOGpXR1JAUeOkKq+35wlg34wKxBp6oaQPn46DKqw/z2UYL0UFtWtxKmSLVaMrfR01YEjy96kM2UtmsVP7MOpbxnoSGq4uc/z3FNlqGvRuAH67jqB3V7VKfRNoMZQVniOnEWapmiFcqD7ZkSTGtNxS5wSg/J/Zs/r/sGkYtT3btFV5KZc/Hv9ccUwpBhJFOWT6YygrTnXM3Ko2+nvFJDZmPf1VtqCqvyB+3kfu+n1vzFj26sW9OnPoGMsf7CjGF9+C9sn664ElhLa8ThTNAfeR3HJLO/qIFhyalzaw/fVwxfZaIODcudB X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2025-11-28 2:47 AM, David Hildenbrand (Red Hat) wrote: > 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. I agree that this patch is ugly. The only way I see to avoid code like this is to refactor (or duplicate) the functions so no function takes pointers to both hardware page tables and on-stack page table entries. Is that sort of refactoring the right direction to go for v4? Regards, Samuel