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=-4.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY autolearn=no 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 BF24FC433DF for ; Wed, 24 Jun 2020 22:44:16 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 845A12065D for ; Wed, 24 Jun 2020 22:44:16 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 845A12065D Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 181DF6B0002; Wed, 24 Jun 2020 18:44:16 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 10BA46B0003; Wed, 24 Jun 2020 18:44:16 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id F14C46B0005; Wed, 24 Jun 2020 18:44:15 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0183.hostedemail.com [216.40.44.183]) by kanga.kvack.org (Postfix) with ESMTP id D2BAF6B0002 for ; Wed, 24 Jun 2020 18:44:15 -0400 (EDT) Received: from smtpin18.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id 7B9122C6D for ; Wed, 24 Jun 2020 22:44:15 +0000 (UTC) X-FDA: 76965585270.18.grain28_04182c426e47 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin18.hostedemail.com (Postfix) with ESMTP id 524DF100ED9DD for ; Wed, 24 Jun 2020 22:44:15 +0000 (UTC) X-HE-Tag: grain28_04182c426e47 X-Filterd-Recvd-Size: 4837 Received: from out30-57.freemail.mail.aliyun.com (out30-57.freemail.mail.aliyun.com [115.124.30.57]) by imf37.hostedemail.com (Postfix) with ESMTP for ; Wed, 24 Jun 2020 22:44:14 +0000 (UTC) X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R231e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04357;MF=richard.weiyang@linux.alibaba.com;NM=1;PH=DS;RN=9;SR=0;TI=SMTPD_---0U0dYaOT_1593038651; Received: from localhost(mailfrom:richard.weiyang@linux.alibaba.com fp:SMTPD_---0U0dYaOT_1593038651) by smtp.aliyun-inc.com(127.0.0.1); Thu, 25 Jun 2020 06:44:11 +0800 Date: Thu, 25 Jun 2020 06:44:10 +0800 From: Wei Yang To: Dan Williams Cc: Wei Yang , Michal Hocko , Andrew Morton , Oscar Salvador , Linux MM , Baoquan He , Linux Kernel Mailing List , David Hildenbrand Subject: Re: [PATCH] mm/spase: never partially remove memmap for early section Message-ID: <20200624224410.GD15016@L-31X9LVDL-1304.local> Reply-To: Wei Yang References: <20200623094258.6705-1-richard.weiyang@linux.alibaba.com> <20200623151828.GA31426@dhcp22.suse.cz> <20200624061340.GA11552@L-31X9LVDL-1304.local> <20200624220552.GA15016@L-31X9LVDL-1304.local> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 524DF100ED9DD X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam04 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 Wed, Jun 24, 2020 at 03:20:59PM -0700, Dan Williams wrote: >On Wed, Jun 24, 2020 at 3:06 PM Wei Yang > wrote: >> >> On Wed, Jun 24, 2020 at 09:10:09AM -0700, Dan Williams wrote: >> >On Tue, Jun 23, 2020 at 11:14 PM Wei Yang >> > wrote: >> >> >> >> On Tue, Jun 23, 2020 at 05:18:28PM +0200, Michal Hocko wrote: >> >> >On Tue 23-06-20 17:42:58, Wei Yang wrote: >> >> >> For early sections, we assumes its memmap will never be partially >> >> >> removed. But current behavior breaks this. >> >> >> >> >> >> Let's correct it. >> >> >> >> >> >> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug") >> >> >> Signed-off-by: Wei Yang >> >> > >> >> >Can a user trigger this or is this a theoretical bug? >> >> >> >> Let me rewrite the changelog a little. Look forward any comments. >> >> >> >> For early sections, its memmap is handled specially even sub-section is >> >> enabled. The memmap could only be populated as a whole. >> >> >> >> Quoted from the comment of section_activate(): >> >> >> >> * The early init code does not consider partially populated >> >> * initial sections, it simply assumes that memory will never be >> >> * referenced. If we hot-add memory into such a section then we >> >> * do not need to populate the memmap and can simply reuse what >> >> * is already there. >> >> >> >> While current section_deactivate() breaks this rule. When hot-remove a >> >> sub-section, section_deactivate() would depopulate its memmap. The >> >> consequence is if we hot-add this subsection again, its memmap never get >> >> proper populated. >> > >> >Ok, forgive the latency as re-fetched this logic into my mental cache. >> >So what I was remembering was the initial state of the code that >> >special cased early sections, and that still seems to be the case in >> >pfn_valid(). IIRC early_sections / bootmem are blocked from being >> >removed entirely. Partial / subsection removals are ok. >> >> Would you mind giving more words? Partial subsection removal is ok, so no need >> to fix this? > >Early sections establish a memmap for the full section. There's >conceptually nothing wrong with unplugging the non-system-RAM portion >of the memmap, but it would need to be careful, at least on x86, to >map the partial section with PTEs instead of PMDs. > >So, you are right that there is a mismatch here, but I think the >comprehensive fix is to allow early sections to be partially >depopulated/repopulated rather than have section_activate() and >section_deacticate() special case early sections. The special casing >is problematic in retrospect as section_deactivate() can't be >maintained without understand special rules in section_activate(). Hmm... This means we need to adjust pfn_valid() too, which always return true for early sections. -- Wei Yang Help you, Help me