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 F2810C43334 for ; Wed, 6 Jul 2022 10:04:34 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 94A1A6B0073; Wed, 6 Jul 2022 06:04:34 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 8F8BC6B0074; Wed, 6 Jul 2022 06:04:34 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 80F626B0075; Wed, 6 Jul 2022 06:04:34 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0011.hostedemail.com [216.40.44.11]) by kanga.kvack.org (Postfix) with ESMTP id 73A806B0073 for ; Wed, 6 Jul 2022 06:04:34 -0400 (EDT) Received: from smtpin23.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay09.hostedemail.com (Postfix) with ESMTP id 343A8341D5 for ; Wed, 6 Jul 2022 10:04:34 +0000 (UTC) X-FDA: 79656240468.23.D776B5E Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by imf26.hostedemail.com (Postfix) with ESMTP id BC0A9140004 for ; Wed, 6 Jul 2022 10:04:33 +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 dfw.source.kernel.org (Postfix) with ESMTPS id B65E761D82; Wed, 6 Jul 2022 10:04:32 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7825BC3411C; Wed, 6 Jul 2022 10:04:28 +0000 (UTC) Date: Wed, 6 Jul 2022 11:04:24 +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: <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=1657101873; a=rsa-sha256; cv=none; b=G4pa57s9GwEOR0rVw1UuKBXPadMatxvmiD/mhtOsEdQ8j1cMnzGTcTy1VDohaJA/+iGXgD XF2kC7xpyM949U75zrVMafABcK13BJg7waMzFOt6jdeGPdjobW0nSCiHNUWHsZveZcs0j1 ds8QKo+z5NWJZWIY61lbnqLD5Ty9dVw= ARC-Authentication-Results: i=1; imf26.hostedemail.com; dkim=none; dmarc=fail reason="SPF not aligned (relaxed), No valid DKIM" header.from=arm.com (policy=none); spf=pass (imf26.hostedemail.com: domain of cmarinas@kernel.org designates 139.178.84.217 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=1657101873; 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=mv9A0IW2kN5FcEckZcCcy3Wd/QXqPZPFVYOZ0p1FJXI=; b=cCUph79JXpwhuolFpmAdWxrKKpfEdP7u64q3/0ht0Ria5lH9UEeZnr4/ePzktbQZ3ujchk iOq3jW5VfaGI3ohP0gAecRdJuLapueG/7CpSWoJKux3eWqmoA7bngFDsNnOU4Gd6bp/e0M 2mmMepFgff52YYf3AqlMDUIZUUhr1gc= X-Rspamd-Server: rspam04 X-Rspam-User: Authentication-Results: imf26.hostedemail.com; dkim=none; dmarc=fail reason="SPF not aligned (relaxed), No valid DKIM" header.from=arm.com (policy=none); spf=pass (imf26.hostedemail.com: domain of cmarinas@kernel.org designates 139.178.84.217 as permitted sender) smtp.mailfrom=cmarinas@kernel.org X-Stat-Signature: hohiuu9ts3p6kr1kp6nbwhjejmaz6fiz X-Rspamd-Queue-Id: BC0A9140004 X-HE-Tag: 1657101873-27478 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 11:45:40PM +0300, Mike Rapoport wrote: > 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. We can reserve the region just before unmapping to avoid new allocations for the page tables but we can't do much about pages already allocated prior to calling remap_crashkernel(). -- Catalin