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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a5323bee86sm12051355f8f.43.2025.06.10.06.25.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 10 Jun 2025 06:25:56 -0700 (PDT) Message-ID: Date: Tue, 10 Jun 2025 15:25:55 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/2] mm: Allow lockless kernel pagetable walking From: David Hildenbrand To: Lorenzo Stoakes , Dev Jain Cc: akpm@linux-foundation.org, catalin.marinas@arm.com, will@kernel.org, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, suzuki.poulose@arm.com, steven.price@arm.com, gshan@redhat.com, linux-arm-kernel@lists.infradead.org, yang@os.amperecomputing.com, ryan.roberts@arm.com, anshuman.khandual@arm.com References: <20250610114401.7097-1-dev.jain@arm.com> <20250610114401.7097-2-dev.jain@arm.com> <970e1b84-7f47-4a51-ad99-4df9026b8f7d@redhat.com> Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 07C6E40004 X-Stat-Signature: eij8y4886qzjw3ucpzz6fsza1nxur8sy X-Rspam-User: X-HE-Tag: 1749561959-579312 X-HE-Meta: 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 BZL7RZSf zKrQspCETXGQbozVIY0cGTGJDhyC2DgJBLCS9a+ZT1qZ/IgqqrPBhvKdoB4wO62mnP0SKMKbnLM+ZyNR34K8AM6bxSemzOdbNM468jeYJ5XrZB1y1icNvnhmbFy5T1NgebBciO8UFulTXvRjnuJZ+GPBmCdVUaf+RLtWcllSRUzw0rVQ16zzMJaS9rBmwPVNfDTwdSjeUs0BH/7F+v92sih5efUGgaVLMjKy7Sf2H0bHRYMoxGisXNabkFI3vH7FPwwcTd8WbZo3QfHY3NiskJaUnG89Y99hld2Io48fWEeikq/JXKlrcnF85zOe70IAmRc6IFrYdEws2MEdXTnDEj0VJB7fmRM0+vjxZ1M7CNs7XvHFkGGWT43WCtwHg99KeRBrIzdo+zJ3NZYOkotZuf6YeTR1ZyHi2S2wg1/nvFfkHtKCUozneKmKfOYTd+qoxm/pdp5aseTq8I/5cYo02VO+x5g== 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 10.06.25 15:24, David Hildenbrand wrote: > On 10.06.25 14:07, Lorenzo Stoakes wrote: >> OK so I think the best solution here is to just update check_ops_valid(), which >> was kind of sucky anyway (we check everywhere but walk_page_range_mm() to >> enforce the install pte thing). >> >> Let's do something like: >> >> #define OPS_MAY_INSTALL_PTE (1<<0) >> #define OPS_MAY_AVOID_LOCK (1<<1) >> >> and update check_ops_valid() to take a flags or maybe 'capabilities' field. >> >> Then check based on this e.g.: >> >> if (ops->install_pte && !(capabilities & OPS_MAY_INSTALL_PTE)) >> return false; >> >> if (ops->walk_lock == PGWALK_NOLOCK && !(capabilities & OPS_MAY_AVOID_LOCK)) >> return false; >> > > Hm. I mean, we really only want to allow this lockless check for > walk_kernel_page_table_range(), right? > > Having a walk_kernel_page_table_range_lockeless() might (or might not) > be better, to really only special-case this specific path. Note that I am also not quite happy bout that function name, but I think we should add a proper interface that documents clearly when it is even okay to call that function ... -- Cheers, David / dhildenb