From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 29AC4606A33B for ; Mon, 26 Oct 2026 09:41:34 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=lists.infradead.org header.i=@lists.infradead.org header.a=rsa-sha256 header.s=bombadil.20210309 header.b=Wud449b6; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=MBQBoKup DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Content-Transfer-Encoding: Content-Type:In-Reply-To:From:References:Cc:To:Subject:MIME-Version:Date: Message-ID:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Unsubscribe-Post: List-Owner; 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Mon, 26 Oct 2026 09:41:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793007665; bh=tZqjNhLUtMM9qzCfqbSSbYtLAIMBnXc3isIanFGM8jk=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=MBQBoKupOvPboBHAGShCa2KQHrEqsjR/MrgjtQuP2sCUGgdjYOu9EegxmvwTqeS0A BhyCWDRazJkTCdvKvi93Chg2Dt9yUC3zjnKvtF8cj2a4zGLqh5ZI6f8JR+LbqZAri9 pjdttV/hbpEzrBaTLLeHdF7nRVgYcdVsz67gdzy+qo+ccnK2foO3SKTNOayx62tWdr B0iw3aidemCTPsAIvkYeMn4HjnRBFXQ7HfWuv2F6iQ02aXFnU/W0VSqZm5u3ao3vNC Zh9tR/ItbPiOGYbq8CnVXm7Zb27eTS7lH4NUtCOpOAUjhch2Ric3mQEgBHXljExawj jywrQTpbflAcw== Message-ID: <1c0141e8-3033-4f59-b247-34dee14cbb1f@kernel.org> Date: Sun, 25 Oct 2026 13:34:13 +0100 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] arm64/mm: restrict transparent contpte mappings to valid PTEs To: Zhao Gongyi , catalin.marinas@arm.com, will@kernel.org, Ryan Roberts Cc: dev.jain@arm.com, baolin.wang@linux.alibaba.com, lance.yang@linux.dev, mark.rutland@arm.com, akpm@linux-foundation.org, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20261013093600.54995-1-zhaogongyi@bytedance.com> <3b57c214-6a43-4fc7-a531-fb5eac196465@bytedance.com> 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-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" X-Spam-Status: No, score=-6.6 required=3.0 tests=BAYES_00,DATE_IN_PAST_12_24, DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=4.0.1 X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On 10/24/26 18:00, Zhao Gongyi wrote: > On 10/24/26 4:55 AM, David Hildenbrand (Arm) wrote: >> >> Hi, >> >> sorry for the late reply. >> >> If we have the spare PTE bit, why shouldn't we just use it? >> >> It will make batching over PTEs faster. And we care about that for present PTEs. >> >> So I don't quite see why the stable fix couldn't just do it right away? Do you >> have a prototype that shows that it would require significant code changes? >> > > Hi David, > > I tried a small prototype that keeps PTE_CONT on present-invalid PTEs. > It leaves contpte_set_ptes() unchanged and makes contpte_try_unfold() > recognize pte_present(pte) && pte_cont(pte), instead of checking only > pte_valid_cont(pte). > > With this change, the guard-marker reproducer passed 100 iterations > in an arm64 QEMU/KVM guest running a KASAN-enabled kernel. The fork > and PMD-split probes passed five runs each, with no warnings or KASAN > reports. In the earlier A/B test, the unmodified kernel failed on the > first guard-marker iteration. > > The prototype makes the reported reproducer pass with a small change > while retaining PTE_CONT. I therefore cannot point to evidence that > fixing this failure requires significant code changes. Can you paste the code here for easy review if that it the right approach? -- Cheers, David