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([2a02:8070:4688:86a0::a40b]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43aba5870e7sm95040575e9.35.2025.02.28.07.18.02 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 28 Feb 2025 07:18:02 -0800 (PST) Message-ID: <7c5cc18e-71e0-4ebb-bb0d-5765a9467753@redhat.com> Date: Fri, 28 Feb 2025 16:18:01 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 0/2] SKSM: Synchronous Kernel Samepage Merging To: Mathieu Desnoyers , Linus Torvalds Cc: 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> <029759d4-f7b2-4ec6-b5d0-7c8a1c0fbd80@redhat.com> <78ebaad9-c7ba-423a-a824-c2b1a499aea6@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: <78ebaad9-c7ba-423a-a824-c2b1a499aea6@efficios.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: O75vmNYXepAJcz_ir0oSF23KsBETdnnDMZKF_kphm_o_1740755887 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-Server: rspam10 X-Rspamd-Queue-Id: 1476016001A X-Stat-Signature: ia7dznackpjwfpk6c8yqjfy8em6wp74y X-HE-Tag: 1740755889-589954 X-HE-Meta: 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 e++unNE7 jHz41BAIvMhD3VlZ6RiTQ4rpjVKIERpwNiD+oNIm425LGU6hZPVkZ+M1iwT79kHjcVCNa9hJs0lAU59QQoZPoXO/XlGmFiqcHioAPl2/yY/1+DqZeRpdnDe+Ds4ZUcKs1iLgrZNBwLgR5uPFpOPeya9JvSgH/2E3TO3ZMh48w5/MDPWu2w0SdRJ7RbtDvbvSH5vFRVaQkjVqpRw7uYI0neWKUmKD4d2WXX97+S3zTdL0zIcf1VGUB/0zytrF3q0GeCHXgqa4GYJr+AxldPHMIOcga8MDHe7izWPCxvfR8M2vMJ6ENav3h1UEA0HPRWl3K+dxbzFMl0+HFbjnliOYJ0VKtd/HvckGAFUAGa+ysUYd/Qzi62HxoaeiULhLamMVMtnLTK1FXJPC+DgQG9+XC7Yg4e6sLMzeN+Y2xJgsYduMRC+jD5GG3DSW4IMBGS4tddTRnDwB7qVzIol1CL1m2IoTHSmzkUmmMogioCz+Xs0pNo99Z54vHHty1rlAde1olp0utzRCrkdG6KCpxLMQkMGlZK7ACKsA9qU9q/On5pPKTNgtCVe2vkGggrE/cZ97ztv4m2R2WjXQfZKdW+mHcRucEox40r+YKRWPVIgDjkZGbFfnLAQcrgdGTBaQJRHmxee8VQQu2xDaHMsxwq1I1OM6Lp1m2mmo+qCS6lZPlXeDialbsClPUWcrBRDwzkIMD/Iih 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 28.02.25 16:01, Mathieu Desnoyers wrote: > On 2025-02-28 08:59, David Hildenbrand wrote: >> On 28.02.25 06:17, Linus Torvalds wrote: >>> On Thu, 27 Feb 2025 at 19:03, Mathieu Desnoyers >>> wrote: >>>> >>>> I'd be fine with SKSM replacing KSM entirely. However, I don't >>>> think we should try to re-implement the existing KSM userspace ABIs >>>> over SKSM. >>> >>> No, absolutely. The only point (for me) for your new synchronous one >>> would be if it replaced the kernel thread async scanning, which would >>> make the old user space interface basically pointless. >>> >>> But I don't actually know who uses KSM right now. My reaction really >>> comes from a "it's not nice code in the kernel", not from any actual >>> knowledge of the users. >>> >>> Maybe it works really well in some cloud VM environment, and we're >>> stuck with it forever. >> >> Exactly that; and besides the VM use-case, lately people stated using it >> in the context of interpreters (IIRC inside Meta) quite successfully as >> well. >> > > I suspect that SKSM is a better fit for JIT and code patching than KSM, > because user-space knows better when a set of pages is going to become > invariant for a long time and thus benefit from merging. This removes > the background scanning from the picture. > > Does the interpreter use-case require background scanning, or does > it know when a set of pages are meant to become invariant for a long > time ? To make the JIT/interpreter use case happy, people wanted ways to *force* KSM on for *the whole process*, not just individual VMAs like the traditional VM use case would have done. I recall one of the reasons being that you don't really want to modify your JIT/interpreter to just make KSM work. See [1] "KSM at Meta" for some details, and in general, optimization work to adapt KSM to new use cases. Regarding some concerns you raised, Stefan did a lot of optimization work like "smart scanning" (slide "Optimization - Smart Scan (6.7)") to reduce the scanning overhead and make it much more efficient. So people started optimizing for that already and got pretty good results. [1] https://lpc.events/event/17/contributions/1625/attachments/1320/2649/KSM.pdf -- Cheers, David / dhildenb