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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=S0xegcMv; 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 2BD5B606A31E; Tue, 27 Oct 2026 12:58:11 +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=S0xegcMv Received: by smtp.kernel.org (Postfix) with ESMTPSA id D5C2C1F01828; Tue, 27 Oct 2026 12:58:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793105890; bh=2qo0Fds2+mYFVrZRAJ8lmAwaqUsuanIMWx5DL5WUc/I=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=S0xegcMvB3vaMSGrnrPh/wkamJQSwPrkkfuF2Kw75WfR03KJfD4IOUdA+PcoQs7yi bnTII5sJUgEX6cD/0Aj25/PTp9/yyvKS0Ycgs2qbKm9Ndj4Nwyod4OK8wlt7Q0ftYj JvsWQ7XWCCRARuJNEOMACCfJiswERXgOBVaIGZMWK8T8qkv3gDmIAICof/BPyrgomE LXZUoTVJ06rcHporAuBkxK1n42uq83vfPCfEvEqzKSfnh6hdPnVcYlmqeovXH1uj2m IvCBH8EheSB3VwaRbprItF7Bnn2KsfKJ3u4dUXd1KrXv+NDN64O5UXb03eEZ9MfWbd 85NfjscKJ2WlA== Message-ID: <1de9a768-5cb3-435b-8a36-d8b590c8a33e@kernel.org> Date: Tue, 27 Oct 2026 13:58:01 +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: Peter Zijlstra , Kiryl Shutsemau Cc: Andrew Morton , Muchun Song , Matthew Wilcox , Usama Arif , Frank van der Linden , 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, Dave Hansen , Andy Lutomirski References: <20260227194302.274384-1-kas@kernel.org> <20260227194302.274384-14-kas@kernel.org> <20261027110028.GH2668564@noisy.programming.kicks-ass.net> <20261027124716.GC2668804@noisy.programming.kicks-ass.net> 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 10/27/26 13:47, Peter Zijlstra wrote: > On Tue, Oct 27, 2026 at 12:31:09PM +0000, Kiryl Shutsemau wrote: >> On Tue, Oct 27, 2026 at 01:05:58PM +0100, David Hildenbrand (Arm) wrote: >>> >>> Right. Without HVO we would just corrupt pages in the single folio (which is >>> also not nice). >>> >>> I'm also questioning whether we need this RO protection all the time active. >> >> What do you have in mind? RO only under a debug option, or switching the >> tail page to RO some time after the remap? >> >> If it is the delay, a grace period alone does not close the race. We >> would need rcu_read_lock() back around the get, and then it is the old >> scheme with the wait taken off the allocation path. > > So it used to be; back when Nick introduced try_get_page, that it was > only allowed with IRQs disabled. > > And looking at gup.c now, I see gup_fast_pgd_range() still has > local_irq_save/restore around it. > > So what do you need that rcu crud for? Pagecache lookup (and other speculative page table walkers) call it as well. Some with IRQ disabled, some with RCU, others with no protection. -- Cheers, David