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 7AD26C433EF for ; Fri, 1 Jul 2022 17:24:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231372AbiGARYl (ORCPT ); Fri, 1 Jul 2022 13:24:41 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38416 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230229AbiGARYj (ORCPT ); Fri, 1 Jul 2022 13:24:39 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DA218344DC for ; Fri, 1 Jul 2022 10:24:38 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 83BFBB830FA for ; Fri, 1 Jul 2022 17:24:37 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 78BB2C3411E; Fri, 1 Jul 2022 17:24:32 +0000 (UTC) Date: Fri, 1 Jul 2022 18:24:28 +0100 From: Catalin Marinas To: Guanghui Feng Cc: baolin.wang@linux.alibaba.com, will@kernel.org, akpm@linux-foundation.org, david@redhat.com, jianyong.wu@arm.com, james.morse@arm.com, quic_qiancai@quicinc.com, christophe.leroy@csgroup.eu, jonathan@marek.ca, mark.rutland@arm.com, thunder.leizhen@huawei.com, anshuman.khandual@arm.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, rppt@kernel.org, geert+renesas@glider.be, ardb@kernel.org, linux-mm@kvack.org, yaohongbo@linux.alibaba.com, alikernel-developer@linux.alibaba.com Subject: Re: [PATCH v3] arm64: mm: fix linear mapping mem access performance degradation Message-ID: References: <1656586222-98555-1-git-send-email-guanghuifeng@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1656586222-98555-1-git-send-email-guanghuifeng@linux.alibaba.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 30, 2022 at 06:50:22PM +0800, Guanghui Feng wrote: > +static void init_pmd_remap(pud_t *pudp, unsigned long addr, unsigned long end, > + phys_addr_t phys, pgprot_t prot, > + phys_addr_t (*pgtable_alloc)(int), int flags) > +{ > + unsigned long next; > + pmd_t *pmdp; > + phys_addr_t map_offset; > + pmdval_t pmdval; > + > + pmdp = pmd_offset(pudp, addr); > + do { > + next = pmd_addr_end(addr, end); > + > + if (!pmd_none(*pmdp) && pmd_sect(*pmdp)) { > + phys_addr_t pte_phys = pgtable_alloc(PAGE_SHIFT); > + pmd_clear(pmdp); > + pmdval = PMD_TYPE_TABLE | PMD_TABLE_UXN; > + if (flags & NO_EXEC_MAPPINGS) > + pmdval |= PMD_TABLE_PXN; > + __pmd_populate(pmdp, pte_phys, pmdval); > + flush_tlb_kernel_range(addr, addr + PAGE_SIZE); This doesn't follow the architecture requirements for "break before make" when changing live page tables. While it may work, it risks triggering a TLB conflict abort. The correct sequence normally is: pmd_clear(); flush_tlb_kernel_range(); __pmd_populate(); However, do we have any guarantees that the kernel doesn't access the pmd range being unmapped temporarily? The page table itself might live in one of these sections, so set_pmd() etc. can get a translation fault. -- Catalin