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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 20D37CCA47B for ; Tue, 5 Jul 2022 17:05:13 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 8DC0A6B0071; Tue, 5 Jul 2022 13:05:12 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 88B626B0073; Tue, 5 Jul 2022 13:05:12 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 753716B0074; Tue, 5 Jul 2022 13:05:12 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id 634AB6B0071 for ; Tue, 5 Jul 2022 13:05:12 -0400 (EDT) Received: from smtpin18.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay09.hostedemail.com (Postfix) with ESMTP id 3BEF934EC1 for ; Tue, 5 Jul 2022 17:05:12 +0000 (UTC) X-FDA: 79653671664.18.D025460 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by imf28.hostedemail.com (Postfix) with ESMTP id A79CAC0054 for ; Tue, 5 Jul 2022 17:05:11 +0000 (UTC) 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 30A1FB8181F; Tue, 5 Jul 2022 17:05:10 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5071BC341C7; Tue, 5 Jul 2022 17:05:05 +0000 (UTC) Date: Tue, 5 Jul 2022 18:05:01 +0100 From: Catalin Marinas To: Mike Rapoport Cc: Will Deacon , "guanghui.fgh" , 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: <20220704142313.GE31684@willie-the-truck> <6977c692-78ca-5a67-773e-0389c85f2650@linux.alibaba.com> <20220704163815.GA32177@willie-the-truck> <20220705095231.GB552@willie-the-truck> <5d044fdd-a61a-d60f-d294-89e17de37712@linux.alibaba.com> <20220705121115.GB1012@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1657040711; a=rsa-sha256; cv=none; b=50QoWKNGNdsahMCQxn3/37YFUcqdrae8zqUdsYYSMscyypSdXYctnjBBh164slV1PGHckq uuX9s3Ieu4ckuqkkq4a+30QJ5sLUBJvqmJo6aOFH+cjZtmFk7d3kqE6s62t7yKzlxovVd8 bpGH1JFgHdCcyaVO9//Rb4eIZE4C7Q0= ARC-Authentication-Results: i=1; imf28.hostedemail.com; dkim=none; dmarc=fail reason="SPF not aligned (relaxed), No valid DKIM" header.from=arm.com (policy=none); spf=pass (imf28.hostedemail.com: domain of cmarinas@kernel.org designates 145.40.68.75 as permitted sender) smtp.mailfrom=cmarinas@kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1657040711; h=from:from:sender: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: in-reply-to:in-reply-to:references:references; bh=W5mVUUdQ0mqwtqa4d2/uM8e6Nk2/bp0XzXOEo1uNU8E=; b=bCl0L2GjSdEZxv/No6ocQMGu1Tw2VliwA+vvJZvsxKBwpZJLHBRt0uN2kGYcbeuyZ54Vuh QiS8cGhdfUbXS2jZjzMwZgI1YSxyO+njt+0dOY2OMQt6zTzE1hbQr3JB+lzlZ5KEpkIegL 0ohYjXvo1I8KPOLfliVqPq7DXiF4Dag= X-Rspam-User: X-Rspamd-Server: rspam07 Authentication-Results: imf28.hostedemail.com; dkim=none; dmarc=fail reason="SPF not aligned (relaxed), No valid DKIM" header.from=arm.com (policy=none); spf=pass (imf28.hostedemail.com: domain of cmarinas@kernel.org designates 145.40.68.75 as permitted sender) smtp.mailfrom=cmarinas@kernel.org X-Stat-Signature: a4ti7rax6f79b3bdsfh6ekhozg49pnen X-Rspamd-Queue-Id: A79CAC0054 X-HE-Tag: 1657040711-161470 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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. -- Catalin