From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id B5541C4332F for ; Thu, 17 Nov 2022 04:24:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239040AbiKQEYR (ORCPT ); Wed, 16 Nov 2022 23:24:17 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47968 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234037AbiKQEYO (ORCPT ); Wed, 16 Nov 2022 23:24:14 -0500 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 622875BD75 for ; Wed, 16 Nov 2022 20:24:13 -0800 (PST) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 6872D13D5; Wed, 16 Nov 2022 20:24:19 -0800 (PST) Received: from [10.162.40.18] (unknown [10.162.40.18]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 776373F73B; Wed, 16 Nov 2022 20:24:09 -0800 (PST) Message-ID: <53c0c955-7afe-905b-468a-cd7ac81238c5@arm.com> Date: Thu, 17 Nov 2022 09:54:06 +0530 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.2.2 Subject: Re: [PATCH 2/2] arm64/mm: fix incorrect file_map_count for invalid pmd/pud Content-Language: en-US To: Mark Rutland , David Hildenbrand Cc: Liu Shixin , Catalin Marinas , Will Deacon , Denys Vlasenko , Kefeng Wang , David Hildenbrand , Rafael Aquini , Pasha Tatashin , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org References: <20221116083811.464678-1-liushixin2@huawei.com> <20221116083811.464678-3-liushixin2@huawei.com> <73189e81-08c2-bb18-dd4a-f717996ea819@redhat.com> From: Anshuman Khandual In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/16/22 21:16, Mark Rutland wrote: > On Wed, Nov 16, 2022 at 10:08:27AM +0100, David Hildenbrand wrote: >> On 16.11.22 09:38, Liu Shixin wrote: >>> The page table check trigger BUG_ON() unexpectedly when split hugepage: >>> >>> ------------[ cut here ]------------ >>> kernel BUG at mm/page_table_check.c:119! >>> Internal error: Oops - BUG: 00000000f2000800 [#1] SMP >>> Dumping ftrace buffer: >>> (ftrace buffer empty) >>> Modules linked in: >>> CPU: 7 PID: 210 Comm: transhuge-stres Not tainted 6.1.0-rc3+ #748 >>> Hardware name: linux,dummy-virt (DT) >>> pstate: 20000005 (nzCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--) >>> pc : page_table_check_set.isra.0+0x398/0x468 >>> lr : page_table_check_set.isra.0+0x1c0/0x468 >>> [...] >>> Call trace: >>> page_table_check_set.isra.0+0x398/0x468 >>> __page_table_check_pte_set+0x160/0x1c0 >>> __split_huge_pmd_locked+0x900/0x1648 >>> __split_huge_pmd+0x28c/0x3b8 >>> unmap_page_range+0x428/0x858 >>> unmap_single_vma+0xf4/0x1c8 >>> zap_page_range+0x2b0/0x410 >>> madvise_vma_behavior+0xc44/0xe78 >>> do_madvise+0x280/0x698 >>> __arm64_sys_madvise+0x90/0xe8 >>> invoke_syscall.constprop.0+0xdc/0x1d8 >>> do_el0_svc+0xf4/0x3f8 >>> el0_svc+0x58/0x120 >>> el0t_64_sync_handler+0xb8/0xc0 >>> el0t_64_sync+0x19c/0x1a0 >>> [...] >>> >>> On arm64, pmd_present() will return true even if the pmd is invalid. >> >> I assume that's because of the pmd_present_invalid() check. >> >> ... I wonder why that behavior was chosen. Sounds error-prone to me. > > That seems to be down to commit: > > b65399f6111b03df ("arm64/mm: Change THP helpers to comply with generic MM semantics") > > ... apparently because Andrea Arcangelli said this was necessary in: > > https://lore.kernel.org/lkml/20181017020930.GN30832@redhat.com/ > > ... but that does see to contradict what's said in: > > Documentation/mm/arch_pgtable_helpers.rst > > ... which just says: > > pmd_present Tests a valid mapped PMD It should be as follows instead, will update. Not sure about PUD level though, where anon THP is not supported (AFAIK). +---------------------------+--------------------------------------------------+ | pmd_present | Tests if pmd_page() points to valid memory page | +---------------------------+--------------------------------------------------+ > > ... and it's not clear to me why this *only* applies to the PMD level. > > Anshuman? Because THP is supported at PMD level. As Andrea had explained earlier, pmd_present() should return positive if pmd_page() on the entry points to valid memory irrespective of whether the entry is valid/mapped or not. That is the semantics expected in generic THP during PMD split, collapse, migration etc and other memory code walking past such PMD entries. That was my understanding.