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[2003:cb:c734:9600:af27:4326:a216:2bfb]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-390e485db6csm15635940f8f.91.2025.03.03.12.49.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 03 Mar 2025 12:49:49 -0800 (PST) Message-ID: <8cae1e56-239f-4f67-a18c-b4f4d09f40d0@redhat.com> Date: Mon, 3 Mar 2025 21:49:48 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 0/2] SKSM: Synchronous Kernel Samepage Merging To: Mathieu Desnoyers , Peter Xu Cc: Linus Torvalds , Andrew Morton , linux-kernel@vger.kernel.org, Matthew Wilcox , Olivier Dion , linux-mm@kvack.org References: <20250228023043.83726-1-mathieu.desnoyers@efficios.com> <8524caa9-e1f6-4411-b86b-d9457ddb8007@efficios.com> <60f148db-7586-4154-a909-d433bad39794@efficios.com> <72810548-b917-49b7-b7ef-043c6b395d31@efficios.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: <72810548-b917-49b7-b7ef-043c6b395d31@efficios.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: pCJ308nQ3xCtK_6EgKTOgHT3oCtrG1mhc67epqU9zcs_1741034990 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: BDCB54000B X-Rspamd-Server: rspam09 X-Stat-Signature: g81odety7ix7yzbpgfggbezcxacn8wex X-HE-Tag: 1741034998-764840 X-HE-Meta: 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 FzUzdUCp gFFsXnRfs3Zg6UAG7IxOv8fqFCLOfV+jqcGAwoUwmQCogruZVV7b9wdUmldwKxQQFdibCV1TwWkXZQSh0smoPh50tF43agbqlDURp6SqutLQlPou6Pp0EzE/QrjFEz3Z50zbcUvjzQ+ZLRvZYZGf7NtLyF+yEHoo//Y2iNC8t+FIYrgYX5LPBh34PEj2yOH+sGdE3QGQMhChR3MZcMggZk+0wWdwXbTUzi6B+LkgMHs/5GZToaXDZCfGyeg0PtyzBxY2GYo6wkIEYs464R6/gG1sfBI2SMRfq9s6JgsSsuHU0QuJqFzR7l5ZH57I0uY36qZXeEL57f81KtZaHiVewr4h8X5TDh87rPe4Xbc0cHsJY9XYwqlz+nbLY0fraXhzd+82y2OldOAEwwzGTxtQPc72p8n6JDYef5/ANpoDisead92XT9fxsQwk+vi8H/z9tM/TB16qpB2ZYF1OSuLdZ5pe6r1CNyq0EiLwg 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 03.03.25 21:01, Mathieu Desnoyers wrote: > On 2025-02-28 17:32, Peter Xu wrote: >> On Fri, Feb 28, 2025 at 12:53:02PM -0500, Mathieu Desnoyers wrote: >>> On 2025-02-28 11:32, Peter Xu wrote: >>>> 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 >>> >>> userfaultfd UFFDIO_REGISTER only seems to work if I pass an address >>> resulting from a mmap mapping, but returns EINVAL if I pass a >>> page-aligned address which sits within a private file mapping >>> (e.g. executable data). >> >> Yes, so far sync traps only supports RAM-based file systems, or anonymous. >> Generic private file mappings (that stores executables and libraries) are >> not yet supported. >> >>> >>> Also, I notice that do_wp_page() only calls handle_userfault >>> VM_UFFD_WP when vm_fault flags does not have FAULT_FLAG_UNSHARE >>> set. >> >> AFAICT that's expected, unshare should only be set on reads, never writes. >> So uffd-wp shouldn't trap any of those. >> >>> >>> AFAIU, as it stands now userfaultfd would not help tracking COW faults >>> caused by stores to private file mappings. Am I missing something ? >> >> I think you're right. So we have UFFD_FEATURE_WP_ASYNC that should work on >> most mappings. That one is async, though, so more like soft-dirty. It >> might be doable to try making it sync too without a lot of changes based on >> how async tracking works. > > I'm looking more closely at admin-guide/mm/pagemap.rst and it appears to > be a good fit. Here is what I have in mind to replace the ksmd scanning > thread for the VM use-case by a purely user-space driven scanning: > > Within qemu or similar user-space process: > > 1) Track guest memory with the userfaultfd UFFD_FEATURE_WP_ASYNC feature and > UFFDIO_REGISTER_MODE_WP mode. > > 2) Protect user-space memory with the PAGEMAP_SCAN ioctl PM_SCAN_WP_MATCHING flag > to detect memory which stays invariant for a long time. > > 3) Use the PAGEMAP_SCAN ioctl with PAGE_IS_WRITTEN to detect which pages are written to. > Keep track of memory which is frequently modified, so it can be left alone and > not write-protected nor merged anymore. > > 4) Whenever pages stay invariant for a given lapse of time, merge them with the new > madvise(2) KSM_MERGE behavior. > > Let me know if that makes sense. Note that one of the strengths of ksm in the kernel right now is that we write-protect + try-deduplicate only when we are fairly sure that we can deduplicate (unstable tree), and that the interaction with THPs / large folios is fairly well thought-through. Also note that, just because data hasn't been written in some time interval, doesn't mean that it should be deduplicated and result in CoW on next write access. One probably would have to mimic what the KSM implementation in the kernel does, and built something like the unstable tree, to find candidates where we can actually deduplciate. Then, have a way to not-deduplicate if the content changed. -- Cheers, David / dhildenb