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b=RUCfLDqu4SJx8/KJLBbnt2bOOruTXTFBRj3zfNkkKbLB/6TbEbz0Zv7LmeTW5kEb3 Ndti+ks1LLtKOQ5cKLBEoSYmrf26R1l/Uieb9s4ACUmCYAxAaOt6RElMgtu/NSy0hQ ty0IXApx24A3cS3dVj8/zxAYlnOkfwlb+8O3YqzK3kuc9QEJAeZJOMpnxjfjzfpGxG ZS+aq70CmWJNi8hzlzLfgMW/SPn0Yf5MY92b5kkon/PPpe4z6LDqZq1ydO0FYbYQsR vVER/vc/M3/bb1+mj8BjP6ZAph5B33SqPo6eA7qvylTuEdkb0Pk1HbEic1QMOi1sPu GivGlnw+Zi3bw== Message-ID: Date: Fri, 6 Feb 2026 10:41:45 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCHv6 12/17] mm: Drop fake head checks To: Kiryl Shutsemau , Andrew Morton , Muchun Song , Matthew Wilcox , Usama Arif , Frank van der Linden Cc: Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , Huacai Chen , WANG Xuerui , Palmer Dabbelt , Paul Walmsley , Albert Ou , Alexandre Ghiti , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, loongarch@lists.linux.dev, linux-riscv@lists.infradead.org References: <20260202155634.650837-1-kas@kernel.org> <20260202155634.650837-13-kas@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: tagzna5d1r7y498w3apn451brfirgnq3 X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 7ECDB80006 X-HE-Tag: 1770370913-466761 X-HE-Meta: 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 rFI8rIAz tfKq5REKmghw73ujs7ghmcdpMG9SwLpq2CfkHuE/A5JyzRGmcqemctwDR3GpomUiR/X0oMgN3hZYnwywiZLf+YT2/0cJ6tBcXKy4XSgtXhh8lw9qCy/3e2EjCwC/NaulzXdFn2mTpsXHOxCEo6bBFgLOHhM2wuVvNiMUequbvIjmaE7CR3vM3ZU8OhvCVB6QKDSY07w7a2CLUMJ0jjfuoUd5oNIpQ/nYIo8Hx2XKHMDbDuqoM5/zpK86vePuHx6FN9PfVLtRZMBofutsGAYvpDTsU3mlsYfireRIQ 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 2/2/26 16:56, Kiryl Shutsemau wrote: > With fake head pages eliminated in the previous commit, remove the > supporting infrastructure: > > - page_fixed_fake_head(): no longer needed to detect fake heads; > - page_is_fake_head(): no longer needed; > - page_count_writable(): no longer needed for RCU protection; > - RCU read_lock in page_ref_add_unless(): no longer needed; > > This substantially simplifies compound_head() and page_ref_add_unless(), > removing both branches and RCU overhead from these hot paths. Can you say a bit words more why RCU was required and is now no longer needed? IIRC, it's because we now no longer reuse the real head page (page 0) for a tail, and there could have been a race where we could have attempted to write to that page0 while already mapped (r/o) to page1. Also good to mention that the corresponding RCU sync will be removed separately in a following commit. Nothing jumped at me and it's a great simplification for core-mm. Acked-by: David Hildenbrand (Arm) -- Cheers, David