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 99C07C43334 for ; Wed, 6 Jul 2022 07:44:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231913AbiGFHoE (ORCPT ); Wed, 6 Jul 2022 03:44:04 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60770 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231922AbiGFHnv (ORCPT ); Wed, 6 Jul 2022 03:43:51 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 08AD623156 for ; Wed, 6 Jul 2022 00:43:46 -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 8C017B81B10 for ; Wed, 6 Jul 2022 07:43:45 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id A3F2EC3411C; Wed, 6 Jul 2022 07:43:40 +0000 (UTC) Date: Wed, 6 Jul 2022 08:43:36 +0100 From: Catalin Marinas To: "guanghui.fgh" Cc: Mike Rapoport , Will Deacon , Ard Biesheuvel , baolin.wang@linux.alibaba.com, 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, geert+renesas@glider.be, linux-mm@kvack.org, yaohongbo@linux.alibaba.com, alikernel-developer@linux.alibaba.com Subject: Re: [PATCH v4] arm64: mm: fix linear mem mapping access performance degradation Message-ID: References: <20220705095231.GB552@willie-the-truck> <5d044fdd-a61a-d60f-d294-89e17de37712@linux.alibaba.com> <20220705121115.GB1012@willie-the-truck> <7bf7c5ea-16eb-b02f-8ef5-bb94c157236d@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <7bf7c5ea-16eb-b02f-8ef5-bb94c157236d@linux.alibaba.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 06, 2022 at 10:49:43AM +0800, guanghui.fgh wrote: > 在 2022/7/6 4:45, Mike Rapoport 写道: > > On Tue, Jul 05, 2022 at 06:05:01PM +0100, Catalin Marinas wrote: > > > On Tue, Jul 05, 2022 at 06:57:53PM +0300, Mike Rapoport wrote: > > > > On Tue, Jul 05, 2022 at 04:34:09PM +0100, Catalin Marinas wrote: > > > > > On Tue, Jul 05, 2022 at 06:02:02PM +0300, Mike Rapoport wrote: > > > > > > +void __init remap_crashkernel(void) > > > > > > +{ > > > > > > +#ifdef CONFIG_KEXEC_CORE > > > > > > + phys_addr_t start, end, size; > > > > > > + phys_addr_t aligned_start, aligned_end; > > > > > > + > > > > > > + if (can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE)) > > > > > > + return; > > > > > > + > > > > > > + if (!crashk_res.end) > > > > > > + return; > > > > > > + > > > > > > + start = crashk_res.start & PAGE_MASK; > > > > > > + end = PAGE_ALIGN(crashk_res.end); > > > > > > + > > > > > > + aligned_start = ALIGN_DOWN(crashk_res.start, PUD_SIZE); > > > > > > + aligned_end = ALIGN(end, PUD_SIZE); > > > > > > + > > > > > > + /* Clear PUDs containing crash kernel memory */ > > > > > > + unmap_hotplug_range(__phys_to_virt(aligned_start), > > > > > > + __phys_to_virt(aligned_end), false, NULL); > > > > > > > > > > What I don't understand is what happens if there's valid kernel data > > > > > between aligned_start and crashk_res.start (or the other end of the > > > > > range). > > > > > > > > Data shouldn't go anywhere :) > > > > > > > > There is > > > > > > > > + /* map area from PUD start to start of crash kernel with large pages */ > > > > + size = start - aligned_start; > > > > + __create_pgd_mapping(swapper_pg_dir, aligned_start, > > > > + __phys_to_virt(aligned_start), > > > > + size, PAGE_KERNEL, early_pgtable_alloc, 0); > > > > > > > > and > > > > > > > > + /* map area from end of crash kernel to PUD end with large pages */ > > > > + size = aligned_end - end; > > > > + __create_pgd_mapping(swapper_pg_dir, end, __phys_to_virt(end), > > > > + size, PAGE_KERNEL, early_pgtable_alloc, 0); > > > > > > > > after the unmap, so after we tear down a part of a linear map we > > > > immediately recreate it, just with a different page size. > > > > > > > > This all happens before SMP, so there is no concurrency at that point. > > > > > > That brief period of unmap worries me. The kernel text, data and stack > > > are all in the vmalloc space but any other (memblock) allocation to this > > > point may be in the unmapped range before and after the crashkernel > > > reservation. The interrupts are off, so I think the only allocation and > > > potential access that may go in this range is the page table itself. But > > > it looks fragile to me. > > > > I agree there are chances there will be an allocation from the unmapped > > range. > > > > We can make sure this won't happen, though. We can cap the memblock > > allocations with memblock_set_current_limit(aligned_end) or > > memblock_reserve(algined_start, aligned_end) until the mappings are > > restored. > > I think there is no need to worry about vmalloc mem. That's not what I'm worried about. It's about memblock allocations that are accessed through the linear map. -- Catalin