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b=CQuxmZUJFoHe/jVdB/Td1vVaISzkT31yz4kuoP+9Yf6XgCaMbl6F5B4zTdm8amOx6 Z2zDvTa7sws8ye+cXb9w90RQJ1xrpGccTMU/inaNNbT9EOS+7YKzBDHxrl8TVHJpKi jQVOUH5FOREAqLhZHRorelHQKb4HpsMDpXypnX13JNXddnVaAyripGFIM3k/BAYi3t qhee9qBa0Muu2zQtovUANgP48Gz45LaFAzSypGrjypZDV75SDXJOWSKQO9cLhgq8IV aahuPEKJro72FUipgua8rXcW+Tz43i0i+D+bXeiJ5dnHRDltvwZPwpMrkGuVDew2YA VT7p4t6xKB7Qw== Message-ID: Date: Mon, 26 Oct 2026 18:45:48 +0100 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCHv7 13/18] mm: Drop fake head checks To: "Kiryl Shutsemau (Meta)" , 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: <20260227194302.274384-1-kas@kernel.org> <20260227194302.274384-14-kas@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: AFE1B160006 X-Stat-Signature: n9c6xge79gimoz4muguynxp4ad59pprp X-Rspamd-Server: rspam04 X-Spam-Status: No, score=-6.0 required=3.0 tests=BAYES_00,DATE_IN_PAST_03_06, DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=4.0.1 X-Rspam-User: X-HE-Tag: 1793048906-856729 X-HE-Meta: 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 cvmnn8qV M6B+DZ6oz/uB/RRLQxHq+Qkp7vowqj0xm8Fo1GkzIAL8NV4NhLneughNrABSd06nMxUuFQv4GT6YLJOXFHIyDAm6y4RPIvp4/cUNiIwnOUhson85Xln5QuEe5VFGsjG6ifCO90061NnKiJGk4pGN38X8Z7RG+Vl2N0GE6aEOgCIUTG9MBJYI6BnUBVchnxVuOyvvrFPdW0OMAx3qN3iXgxG4G+B4b3hmoZO7clkjnftkidgx5UOb1AvmwUPYSjp+KsCZ05wJRQe7iIDrIq+IORw8F53WuFBzBMKw1 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On 2/27/26 20:42, Kiryl Shutsemau (Meta) wrote: > From: Kiryl Shutsemau > > 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; I have an AI report that removing RCU read-lock was wrong. The sequence is trivial: 1) read() -> .... page_ref_add_unless_zero() -> atomic_add_unless() int c = raw_atomic_read(v); do { if (unlikely(c == u)) break; We pass that check. Then we sleep (rescheduled, vcpu rescheduled etc) 2) Page gets freed 3) Huge page gets allocated and HVO makes the struct page read-only. 4) 1) continues } while (!raw_atomic_try_cmpxchg(v, &c, c + a)); Then we try to write to a memmap that has been optimized by HVO and is read-only. Apparently easy to reproduce by adding a delay before the cmpxchg(). The AI fix I received alongside the report is obviously complete garbage (as it causes severe overhead on all fronts) and I am going to spare you from it. Can you look into it? -- Cheers, David