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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a53243661csm12420074f8f.53.2025.06.10.06.31.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 10 Jun 2025 06:31:57 -0700 (PDT) Message-ID: Date: Tue, 10 Jun 2025 15:31:56 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/2] mm: Allow lockless kernel pagetable walking To: Lorenzo Stoakes Cc: Dev Jain , 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> <359668fb-df61-4135-89ee-2675a06a5c1b@lucifer.local> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: ra8zbfq4nn7h83zrbcn39qc5ya3pw88u X-Rspamd-Queue-Id: E2F8740019 X-Rspamd-Server: rspam11 X-HE-Tag: 1749562323-661231 X-HE-Meta: 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 8HhWKOFE D4n3zBCl5MHgXYgFGiBuzCG6RVZ3MI2E7KFilL1qHC/eBGhjNplQIFFRQfsqycSFi33KvQuRJTwMVUQT59tr+Icf7TKYj4HuglEw0itp9eybooiysPp01oF6a2pa42uqCuIWQDNJMTUDkLuji+FXLpT0F0YORQqvZS1dk8cZkGoBbeMCbSi4Bc7y2NjTju00gchc5vqYYiQCnvqPAashAadhXFba9HQ4OklVoQMvu8Z2Jemd/6/sQfhCMUtYQRGwjhVJ7IdVWrA7A5PsW/ou/kSgEbpItX3fqFkjIEu+GaIq1id6ktCOsqLHVRkP/n+p+oFj524OPGX4YSYWBX1xEXRZ3EQh/z1Etm1x2jLuBmx+ErARYTRFfzDpGjb3hNS2UdkDPPPnkP9g1kOZFk36xlOM5Mnawc3tuTfLFmV731AvuB6q2Oi66cuOLjFuEk/+dDqTyo6htloSW1DtEWGhFu70UEA== 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 15:27, Lorenzo Stoakes wrote: > On Tue, Jun 10, 2025 at 03:24:16PM +0200, 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. > > Agree completely, Dev - let's definitely do this. > >> >> So, I am wondering if we should further start splitting the >> kernel-page-table walker up from the mm walker, at least on the "entry" >> function for now. > > How do you mean? In particular, "struct mm_walk_ops" does not quite make sense when not actually walking a "real" mm . So maybe we should start having a separate structure where *vma, install_pte, walk_lock, hugetlb* does not even exist. It might be a bit of churn, though ... not sure if there could be an easy translation layer for now. -- Cheers, David / dhildenb