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 X-Spam-Level: X-Spam-Status: No, score=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6983CC4360F for ; Thu, 4 Apr 2019 08:23:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3BDD92075E for ; Thu, 4 Apr 2019 08:23:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728632AbfDDIXz (ORCPT ); Thu, 4 Apr 2019 04:23:55 -0400 Received: from foss.arm.com ([217.140.101.70]:54228 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726790AbfDDIXy (ORCPT ); Thu, 4 Apr 2019 04:23:54 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id E851C80D; Thu, 4 Apr 2019 01:23:53 -0700 (PDT) Received: from [10.162.40.100] (p8cg001049571a15.blr.arm.com [10.162.40.100]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 0FD773F557; Thu, 4 Apr 2019 01:23:47 -0700 (PDT) Subject: Re: [PATCH 2/6] arm64/mm: Enable memory hot remove To: Robin Murphy , Logan Gunthorpe , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, akpm@linux-foundation.org, will.deacon@arm.com, catalin.marinas@arm.com Cc: mark.rutland@arm.com, mhocko@suse.com, david@redhat.com, cai@lca.pw, pasha.tatashin@oracle.com, Stephen Bates , james.morse@arm.com, cpandya@codeaurora.org, arunks@codeaurora.org, dan.j.williams@intel.com, mgorman@techsingularity.net, osalvador@suse.de References: <1554265806-11501-1-git-send-email-anshuman.khandual@arm.com> <1554265806-11501-3-git-send-email-anshuman.khandual@arm.com> <85fbfe49-d49e-fd6e-21dd-ff4d9808610b@arm.com> From: Anshuman Khandual Message-ID: <755a1c1a-12ac-e081-c315-117de53f7a4b@arm.com> Date: Thu, 4 Apr 2019 13:53:49 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <85fbfe49-d49e-fd6e-21dd-ff4d9808610b@arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/03/2019 11:27 PM, Robin Murphy wrote: > On 03/04/2019 18:32, Logan Gunthorpe wrote: >> >> >> On 2019-04-02 10:30 p.m., Anshuman Khandual wrote: >>> Memory removal from an arch perspective involves tearing down two different >>> kernel based mappings i.e vmemmap and linear while releasing related page >>> table pages allocated for the physical memory range to be removed. >>> >>> Define a common kernel page table tear down helper remove_pagetable() which >>> can be used to unmap given kernel virtual address range. In effect it can >>> tear down both vmemap or kernel linear mappings. This new helper is called >>> from both vmemamp_free() and ___remove_pgd_mapping() during memory removal. >>> The argument 'direct' here identifies kernel linear mappings. >>> >>> Vmemmap mappings page table pages are allocated through sparse mem helper >>> functions like vmemmap_alloc_block() which does not cycle the pages through >>> pgtable_page_ctor() constructs. Hence while removing it skips corresponding >>> destructor construct pgtable_page_dtor(). >>> >>> While here update arch_add_mempory() to handle __add_pages() failures by >>> just unmapping recently added kernel linear mapping. Now enable memory hot >>> remove on arm64 platforms by default with ARCH_ENABLE_MEMORY_HOTREMOVE. >>> >>> This implementation is overall inspired from kernel page table tear down >>> procedure on X86 architecture. >> >> I've been working on very similar things for RISC-V. In fact, I'm >> currently in progress on a very similar stripped down version of >> remove_pagetable(). (Though I'm fairly certain I've done a bunch of >> stuff wrong.) >> >> Would it be possible to move this work into common code that can be used >> by all arches? Seems like, to start, we should be able to support both >> arm64 and RISC-V... and maybe even x86 too. >> >> I'd be happy to help integrate and test such functions in RISC-V. I am more inclined towards consolidating remove_pagetable() across platforms like arm64 and RISC-V (probably others). But there are clear distinctions between user page table and kernel page table tear down process. > > Indeed, I had hoped we might be able to piggyback off generic code for this anyway, > given that we have generic pagetable code which knows how to free process pagetables, > and kernel pagetables are also pagetables. But there are differences. To list some * Freeing mapped and pagetable pages - Memory hot remove deals with both vmemmap and linear mappings - Selectively call pgtable_page_dtor() for linear mappings (arch specific) - Not actually freeing PTE|PMD|PUD mapped pages for linear mappings - Freeing mapped pages for vmemap mappings * TLB shootdown - User page table process uses mmu_gather mechanism for TLB flush - Kernel page table tear down can do with less TLB flush invocations - Dont have to care about flush deferral etc * THP and HugeTLB - Kernel page table tear down procedure does not have to understand THP or HugeTLB - Though it has to understand possible arch specific special block mappings - Specific kernel linear mappings on arm64 - PUD|PMD|CONT_PMD|CONT_PTE large page mappings - Specific vmemmap mappings on arm64 - PMD large or PTE mappings -User page table tear down procedure needs to understand THP and HugeTLB * Page table locking - Kernel procedure locks init_mm.page_table_lock while clearing an individual entry - Kernel procedure does not have to worry about mmap_sem * ZONE_DEVICE struct vmem_altmap - Kernel page table tear down procedure needs to accommodate 'struct vmem_altmap' when vmemmap mappings are created with pages allocated from 'struct vmem_altmap' (ZONE_DEVICE) rather than buddy allocator or memblock.