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Fri, 28 Feb 2025 08:32:50 -0800 (PST) Date: Fri, 28 Feb 2025 11:32:47 -0500 From: Peter Xu To: Mathieu Desnoyers Cc: Linus Torvalds , Andrew Morton , linux-kernel@vger.kernel.org, Matthew Wilcox , Olivier Dion , linux-mm@kvack.org Subject: Re: [RFC PATCH 0/2] SKSM: Synchronous Kernel Samepage Merging Message-ID: References: <20250228023043.83726-1-mathieu.desnoyers@efficios.com> <8524caa9-e1f6-4411-b86b-d9457ddb8007@efficios.com> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: hl_0OlsB905Cl3dhiXjOlZVvxjmc9gQNVWwFG2_Df6s_1740760371 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Rspam-User: X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: A0BC61A0012 X-Stat-Signature: cikcotqit9se313ggfkqyb8z76h73o3m X-HE-Tag: 1740760373-64327 X-HE-Meta: U2FsdGVkX1/fbhtixkJ63ePAn4dj+49DKule/tQsiPQ2hOFExn3L+vAH2tUQaloDOlQsPRRp5pTMot69bgu4ZaGzJiJOIW4QZJP9ngebSa5zIPNdoeSnsRJsLioK1YLwC/C4KDmMgW0EN9AZ2ulcT3nudKb/RjWaHirRjZ/eRFHFYn0EvetFpqBqXZZW9CGlNzTb3TsfqNxs+GELgdyCjoMUhQd+yzlgZd2ScPX73Czak8JUT9u++8ibXzlfFwqOTmK1G0GGrKAigSTTfNkce4nsrlod+4+iLlmowoKvsZ9aLQGcbMD38eUt3rSPaEObGTpZXl0Ll5pYnyeDEohYC1mtOIOcMMgIMv+f+davkzCTluTAwTlhBNF45tZnwRIfaXAp3T9bUE5ganHHPFvEc3IOkY0BU530jCscVf5js0LrFHtMbR5QmAsfDbd4QpLZnnGYzitIBFXzTDSJfWbjxv7WzLFDFFNsWOuEXMYGoMnz+C0eKFpMCcKEDuu4YFOVnWqOx+UW/NhD9oF9EruH9i3UWehwfI2FwjN7SHzIkD6OKQm8k8yIH8dcIV/IF9nGofFjY5DstBXFIUKpg+f9K6MX2frIPPs94gipy4FdvK4TiP4Ar1ZoVVYEqxYb61a6KkFfUbdsFURGIaiMxpytFff5cMj3wctv3jXKKNmHMRdcGEygqtZecsOJ04VJVhtPKXwWXiAe4ojXNfQ6QgQOcAUD35Hr9WJYw1YSmJWfyJ9rgQP1BzDdV/8hFAKkAjeJKH9cLo/Fg82bYZ+oJq18fub2tqpl5VYt2kJdD4bZCgE8VBie1dWtKE4DhhgtXltGSLrnzRKxMLlcJX5nS90rmELz84a1yq9s7EuG7Vb4x8K/OBlXTFRvkPblPgEA5fh9uHII4hhmjeGNgfQtrlXTno3oX9dxQ2b/i77pmIcQuqn7XxweMzqkRFul5/nSAdw057Nr+S+r0Xfb/GHgTQT pC+PyBZW 7aaNvkIKedowm6VbxGq8Z0jq+BpFLzPQ0VR9NdLHBcp+s0U/jVTq5UFIKpcybFguc3vqwBfyBRp7Yhw0e/zqDVj0JOLiHytOvip7b7n/B+Vgrb5ezwgiV2rUScvcGCVFf9cjLL1y6VkP+9DMOpb6gEw4OV6vTP5JrsMFcWk1v1+fJqvBJFaXhiLlDF1guEEe/XtuJxZiRkYD5WpMOaLzEF03z0r+EqXDEZQ72JtuI4WycUI1DuZVMYAloYihaoPauW+9sg0BtI4bAx6LEyFs3QxZvO8Vd4OY192Cfr6ioLrmNiATui5dcEqOePQGHB2nzCTOkeXW9k1vs6E/grTyExtCwlS0tDh/cKnZlMt6Ek1fHIG1hzXHPaUuIj/79yBYKuS7OylYL8LHjIBYS9qfZ7ogq9yc9UTlQeMez 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 Fri, Feb 28, 2025 at 09:59:00AM -0500, Mathieu Desnoyers wrote: > For the VM use-case, I wonder if we could just add a userfaultfd > "COW" event that would notify userspace when a COW happens ? I don't know what's the best for KSM and how well this will work, but we have such event for years.. See UFFDIO_REGISTER_MODE_WP: https://man7.org/linux/man-pages/man2/userfaultfd.2.html > > This would allow userspace to replace ksmd by tracking the age of > those anonymous pages, and issue madvise MADV_MERGE on them to > write-protect+merge them when it is deemed useful. > > With both a new userfaultfd COW event and madvise MADV_MERGE, > is there anything else that is fundamentally missing to move > all the scanning complexity of KSM to userspace for the VM > deduplication use-case ? Thanks, -- Peter Xu