From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E277330E84A; Tue, 23 Sep 2025 16:15:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758644100; cv=none; b=Tnmpy3F5g4DI0aY4mwPSyFIydFN6b3D5K9o+DM+VVKq1L8kVcbQ1HNrcoEiv8m+XysF306c95XeSZZ/4c1tPbsb2tctKQ6zZbATM6C1XWragy+RNs69aHid/Y2Grly4fMFepTdq5XU/mWvkkSJQNciu2Xn4G3UfAiJfqBIpijX0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758644100; c=relaxed/simple; bh=GLX+Bs10++gMcM1RHPq8kj4gcq/8AhAuZIuYnjJmuGU=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=GcuHPIhKMzKaX8GtM7lqN3ijoKW4Sp9RJXhGdmKpZJP1ZVSHsqWqK9EQJvG4PgjQd75R6TeSR2/QiufeD96nD8k+UBwGPL/oAiwrorKbhyeAWfSCM6s7JYQ4bUMZsaKDRQ6zxDQ2jh2uhKk0YJcjZwfyg/2dpRzLqlE1Aa92g4Y= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 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> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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