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=-15.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 E5D23C433B4 for ; Thu, 20 May 2021 12:00:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C347B6108C for ; Thu, 20 May 2021 12:00:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S242151AbhETMB7 (ORCPT ); Thu, 20 May 2021 08:01:59 -0400 Received: from foss.arm.com ([217.140.110.172]:49020 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237411AbhETMBP (ORCPT ); Thu, 20 May 2021 08:01:15 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 25615101E; Thu, 20 May 2021 04:59:54 -0700 (PDT) Received: from [10.163.80.140] (unknown [10.163.80.140]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id D8D143F73D; Thu, 20 May 2021 04:59:49 -0700 (PDT) Subject: Re: [External] Re: [PATCH] arm64: mm: hugetlb: add support for free vmemmap pages of HugeTLB To: Muchun Song Cc: Will Deacon , Andrew Morton , David Hildenbrand , "Bodeddula, Balasubramaniam" , Oscar Salvador , Mike Kravetz , David Rientjes , linux-arm-kernel@lists.infradead.org, LKML , Linux Memory Management List , Xiongchun duan , fam.zheng@bytedance.com, zhengqi.arch@bytedance.com References: <20210518091826.36937-1-songmuchun@bytedance.com> <1b9d008a-7544-cc85-5c2f-532b984eb5b5@arm.com> From: Anshuman Khandual Message-ID: Date: Thu, 20 May 2021 17:30:34 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 5/19/21 6:19 PM, Muchun Song wrote: > On Wed, May 19, 2021 at 7:44 PM Anshuman Khandual > wrote: >> On 5/18/21 2:48 PM, Muchun Song wrote: >>> The preparation of supporting freeing vmemmap associated with each >>> HugeTLB page is ready, so we can support this feature for arm64. >>> >>> Signed-off-by: Muchun Song >>> --- >>> arch/arm64/mm/mmu.c | 5 +++++ >>> fs/Kconfig | 2 +- >>> 2 files changed, 6 insertions(+), 1 deletion(-) >>> >>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c >>> index 5d37e461c41f..967b01ce468d 100644 >>> --- a/arch/arm64/mm/mmu.c >>> +++ b/arch/arm64/mm/mmu.c >>> @@ -23,6 +23,7 @@ >>> #include >>> #include >>> #include >>> +#include >>> >>> #include >>> #include >>> @@ -1134,6 +1135,10 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node, >>> pmd_t *pmdp; >>> >>> WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END)); >>> + >>> + if (is_hugetlb_free_vmemmap_enabled() && !altmap) >>> + return vmemmap_populate_basepages(start, end, node, altmap); >> >> Not considering the fact that this will force the kernel to have only >> base page size mapping for vmemmap (unless altmap is also requested) >> which might reduce the performance, it also enables vmemmap mapping to >> be teared down or build up at runtime which could potentially collide >> with other kernel page table walkers like ptdump or memory hotremove >> operation ! How those possible collisions are protected right now ? > > For the ptdump, there seems no problem. The change of pte seems > not to affect the ptdump unless I miss something. The worst case scenario for ptdump would be an wrong information being dumped. But as mentioned earlier, this must be protected against memory hot remove operation which could free up entries for vmemmap areas in the kernel page table.