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s=arc-20220608; d=hostedemail.com; t=1758644102; a=rsa-sha256; cv=none; b=bL6z8QXmVYWylneXZ9a1WKdDwMs3I6oi1sEsShsIVASjHbf32XkgeDjA/Tmyvruz92zFNo z6/6K8K/I13KkXOb8gAvpRJavACH4GZtL6lQgVc5Vjo3RYwC4thiVIeS4gOxU34VOXjsTM gW3uJLxl354ozPjkaLmBQadTrc/M/+I= ARC-Authentication-Results: i=1; imf30.hostedemail.com; dkim=none; dmarc=fail reason="SPF not aligned (relaxed), No valid DKIM" header.from=arm.com (policy=none); spf=pass (imf30.hostedemail.com: domain of cmarinas@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=cmarinas@kernel.org Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id 9747844A33; Tue, 23 Sep 2025 16:15:00 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id B4DEEC4CEF5; Tue, 23 Sep 2025 16:14:53 +0000 (UTC) Date: Tue, 23 Sep 2025 17:14:51 +0100 From: Catalin Marinas To: David Hildenbrand Cc: Lance Yang , akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, usamaarif642@gmail.com, yuzhao@google.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, baohua@kernel.org, voidice@gmail.com, Liam.Howlett@oracle.com, cerasuolodomenico@gmail.com, hannes@cmpxchg.org, kaleshsingh@google.com, npache@redhat.com, riel@surriel.com, roman.gushchin@linux.dev, rppt@kernel.org, ryan.roberts@arm.com, dev.jain@arm.com, ryncsn@gmail.com, shakeel.butt@linux.dev, surenb@google.com, hughd@google.com, willy@infradead.org, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, qun-wei.lin@mediatek.com, Andrew.Yang@mediatek.com, casper.li@mediatek.com, chinwen.chang@mediatek.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mediatek@lists.infradead.org, linux-mm@kvack.org, ioworker0@gmail.com, stable@vger.kernel.org Subject: Re: [PATCH 1/1] mm/thp: fix MTE tag mismatch when replacing zero-filled subpages Message-ID: References: <20250922021458.68123-1-lance.yang@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: 6apa4tfb9ez9rd36cmcwkzreipwgp3mp X-Rspam-User: X-Rspamd-Queue-Id: 39BD98000C X-Rspamd-Server: rspam10 X-HE-Tag: 1758644102-936611 X-HE-Meta: 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 W89NHqOw 9Fbx7pJYOoaY3dO5rPYvMQDufsuHlMWrTbYwmKKbr3xjCL9WD++eqVJxiSEfPuNhGJAqfkK+3Q2iV/tfPbIO2VW6Ekh93HrC1KKpDR3+O+KIK51vlY4wt8BU27XxE/zkqynmZHGRJMaorJVJ0ubbqt3wSQV6Wqlqf9/vVhfFu5aFJA1lPySCZz8V3YaV3DTH616ZSCO3dBVFuV8HultMxBI92VRn9CNIMxeZ86OZTax4i6l8IvuDBR4s466QdfbtddDwuOCSNuIwWHSj0lzGtya8uJJeND4G3pVLbbuawR+fmsplWL9sMrgW279MPNH9Eg62Xlu0DqLhDhvDRd4s4ifIANgBbWgWSbmsk2A2pZG9Wxs9rqW01Bc8nNs4oocclWK+rvjR80d6M/ZD//SWxzmVnRk3Ts6Pn/90O 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 Tue, Sep 23, 2025 at 12:52:06PM +0100, Catalin Marinas wrote: > I just realised that on arm64 with MTE we won't get any merging with the > zero page even if the user page isn't mapped with PROT_MTE. In > cpu_enable_mte() we zero the tags in the zero page and set > PG_mte_tagged. The reason is that we want to use the zero page with > PROT_MTE mappings (until tag setting causes CoW). Hmm, the arm64 > memcmp_pages() messed up KSM merging with the zero page even before this > patch. [...] > diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c > index e5e773844889..72a1dfc54659 100644 > --- a/arch/arm64/kernel/mte.c > +++ b/arch/arm64/kernel/mte.c > @@ -73,6 +73,8 @@ int memcmp_pages(struct page *page1, struct page *page2) > { > char *addr1, *addr2; > int ret; > + bool page1_tagged = page_mte_tagged(page1) && !is_zero_page(page1); > + bool page2_tagged = page_mte_tagged(page2) && !is_zero_page(page2); > > addr1 = page_address(page1); > addr2 = page_address(page2); > @@ -83,11 +85,10 @@ int memcmp_pages(struct page *page1, struct page *page2) > > /* > * If the page content is identical but at least one of the pages is > - * tagged, return non-zero to avoid KSM merging. If only one of the > - * pages is tagged, __set_ptes() may zero or change the tags of the > - * other page via mte_sync_tags(). > + * tagged, return non-zero to avoid KSM merging. Ignore the zero page > + * since it is always tagged with the tags cleared. > */ > - if (page_mte_tagged(page1) || page_mte_tagged(page2)) > + if (page1_tagged || page2_tagged) > return addr1 != addr2; > > return ret; Unrelated to this discussion, I got an internal report that Linux hangs during boot with CONFIG_DEFERRED_STRUCT_PAGE_INIT because try_page_mte_tagging() locks up on uninitialised page flags. Since we (always?) map the zero page as pte_special(), set_pte_at() won't check if the tags have to be initialised, so we can skip the PG_mte_tagged altogether. We actually had this code for some time until we introduced the pte_special() check in set_pte_at(). So alternative patch that also fixes the deferred struct page init (on the assumptions that the zero page is always mapped as pte_special(): diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c index 7b78c95a9017..e325ba34f45c 100644 --- a/arch/arm64/kernel/cpufeature.c +++ b/arch/arm64/kernel/cpufeature.c @@ -2419,17 +2419,21 @@ static void bti_enable(const struct arm64_cpu_capabilities *__unused) #ifdef CONFIG_ARM64_MTE static void cpu_enable_mte(struct arm64_cpu_capabilities const *cap) { + static bool cleared_zero_page = false; + sysreg_clear_set(sctlr_el1, 0, SCTLR_ELx_ATA | SCTLR_EL1_ATA0); mte_cpu_setup(); /* * Clear the tags in the zero page. This needs to be done via the - * linear map which has the Tagged attribute. + * linear map which has the Tagged attribute. Since this page is + * always mapped as pte_special(), set_pte_at() will not attempt to + * clear the tags or set PG_mte_tagged. */ - if (try_page_mte_tagging(ZERO_PAGE(0))) { + if (!cleared_zero_page) { + cleared_zero_page = true; mte_clear_page_tags(lm_alias(empty_zero_page)); - set_page_mte_tagged(ZERO_PAGE(0)); } kasan_init_hw_tags_cpu(); -- Catalin