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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=CQuxmZUJ; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1A2E1606A31B; Mon, 26 Oct 2026 21:08:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=CQuxmZUJ Received: by smtp.kernel.org (Postfix) with ESMTPSA id BCCDF1F01825; Mon, 26 Oct 2026 21:08:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793048905; bh=PC1CXbiHzhQLku5W1cl0FjvjhWJ5KEZGUIcph1ecHwY=; h=Date:Subject:To:Cc:References:From:In-Reply-To; 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-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list 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 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