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[2003:cb:c709:1700:969:8e2b:e8bb:46be]) by smtp.gmail.com with ESMTPSA id t7-20020a05600c450700b003dc42d48defsm20057888wmo.6.2023.02.14.09.03.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 14 Feb 2023 09:04:00 -0800 (PST) Message-ID: <1bee97ef-7632-b1bf-f042-29b97882bfb6@redhat.com> Date: Tue, 14 Feb 2023 18:03:58 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.6.0 Subject: Re: [PATCH v4 00/14] Introduce Copy-On-Write to Page Table From: David Hildenbrand To: Chih-En Lin Cc: Pasha Tatashin , Andrew Morton , Qi Zheng , "Matthew Wilcox (Oracle)" , Christophe Leroy , John Hubbard , Nadav Amit , Barry Song , Steven Rostedt , Masami Hiramatsu , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Yang Shi , Peter Xu , Vlastimil Babka , Zach O'Keefe , Yun Zhou , Hugh Dickins , Suren Baghdasaryan , Yu Zhao , Juergen Gross , Tong Tiangen , Liu Shixin , Anshuman Khandual , Li kunyu , Minchan Kim , Miaohe Lin , Gautam Menghani , Catalin Marinas , Mark Brown , Will Deacon , Vincenzo Frascino , Thomas Gleixner , "Eric W. 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Donenfeld" , Alexey Gladkov , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, Dinglan Peng , Pedro Fonseca , Jim Huang , Huichun Feng References: <20230207035139.272707-1-shiyn.lin@gmail.com> <62c44d12-933d-ee66-ef50-467cd8d30a58@redhat.com> Organization: Red Hat In-Reply-To: X-Mimecast-Spam-Score: 0 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: rspam04 X-Rspamd-Queue-Id: CB566C0014 X-Stat-Signature: f968433iqsi8z5d3cstsmwagqujip81f X-HE-Tag: 1676394244-542186 X-HE-Meta: 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 xkXEu5+Y 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 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: On 14.02.23 17:58, David Hildenbrand wrote: > >>>> >>>> Honestly, for improving the fork(), I have an idea to skip the per-page >>>> operation without breaking the logic. However, this will introduce the >>>> complicated mechanism and may has the overhead for other features. It >>>> might not be worth it. It's hard to strike a balance between the >>>> over-complicated mechanism with (probably) better performance and data >>>> consistency with the page status. So, I would focus on the safety and >>>> stable approach at first. >>> >>> Yes, it is most probably possible, but complexity, robustness and >>> maintainability have to be considered as well. >>> >>> Thanks for implementing this approach (only deduplication without other >>> optimizations) and evaluating it accordingly. It's certainly "cleaner", such >>> that we only have to mess with unsharing and not with other >>> accounting/pinning/mapcount thingies. But it also highlights how intrusive >>> even this basic deduplication approach already is -- and that most benefits >>> of the original approach requires even more complexity on top. >>> >>> I am not quite sure if the benefit is worth the price (I am not to decide >>> and I would like to hear other options). >> >> I'm looking at the discussion of page table sharing in 2002 [1]. >> It looks like in 2002 ~ 2006, there also have some patches try to >> improve fork(). >> >> After that, I also saw one thread which is about another shared page >> table patch's benchmark. I can't find the original patch though [2]. >> But, I found the probably same patch in 2005 [3], it also mentioned >> the previous benchmark discussion: >> >> " >> For those familiar with the shared page table patch I did a couple of years >> ago, this patch does not implement copy-on-write page tables for private >> mappings. Analysis showed the cost and complexity far outweighed any >> potential benefit. >> " > > Thanks for the pointer, interesting read. And my personal opinion is > that part of that statement still hold true :) > >> >> However, it might be different right now. For example, the implemetation >> . We have split page table lock now, so we don't have to consider the >> page_table_share_lock thing. Also, presently, we have different use >> cases (shells [2] v.s. VM cloning and fuzzing) to consider. Oh, and because I stumbled over it, just as an interesting pointer on QEMU devel: "[PATCH 00/10] Retire Fork-Based Fuzzing" [1] [1] https://lore.kernel.org/all/20230205042951.3570008-1-alxndr@bu.edu/T/#u -- Thanks, David / dhildenb