From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-186.mta0.migadu.com (out-186.mta0.migadu.com [91.218.175.186]) (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 C2BA928F4; Wed, 24 Sep 2025 02:49:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.186 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758682193; cv=none; b=cf/KIhMsXIQug97+oZLLLyf4e4FK1NwmaWlp61eUx7sR57dDxkqU49b67deW7SejDYihJHt5xbAvvbP8aRsRYspvWAmIbwh04sSKKKh2Aae6kaPH2Wnz+3Zb5vERpkQXMN2P/Y08L5oYzsz+Gonxd2E6nCyBnyCg06R1Bk33aqE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758682193; c=relaxed/simple; bh=64CFxy1leSLiK9hVFUzlUCskiEaFPpH/XPEBG1eyBfM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=WT43raC/hikCY6NzWR2C7rB+mm/MWXU4yZNXSL93NB+0i5rvvAYTOJfaZbJf8HhxRTn74jmnUCkeQ/veBvW3nlPbzXkKfBOghjMkYO6zeMENz6Tkii9i0ZIbUpSm6kxdQOGTJi9gCyZwxybgZgktV0KwuuHorx5FSwZ7iAHyIZA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=D/w43Tn3; arc=none smtp.client-ip=91.218.175.186 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="D/w43Tn3" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1758682186; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=gS3UMQYfDRiVXfktUZqtEyuly+0oheyOGtsfiL3CXxU=; b=D/w43Tn3x7CWbbsMju6wP2WmE63sMFT1L5kwMs0zBjv7gSY8AEfER8sfxukJxuwIgzMoyb HkO0RKC4KmgarsNHiuI71MwRgtTRzO2yF3PnzTH3CFUBqrRPSCu3rabdUvARAHDsykafy8 GMLfw99XCOHABMYrqi82Sa7ykFYURTI= Date: Wed, 24 Sep 2025 10:49:27 +0800 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH 1/1] mm/thp: fix MTE tag mismatch when replacing zero-filled subpages Content-Language: en-US To: Catalin Marinas , David Hildenbrand Cc: 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 References: <20250922021458.68123-1-lance.yang@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT On 2025/9/24 00:14, Catalin Marinas wrote: > 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(): I can confirm that this alternative patch also works correctly; my tests for MTE all pass ;) This looks like a better fix since it solves the boot hang issue too. > > 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(); >