From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752132AbdJDMle (ORCPT ); Wed, 4 Oct 2017 08:41:34 -0400 Received: from aserp1040.oracle.com ([141.146.126.69]:30455 "EHLO aserp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751699AbdJDMlc (ORCPT ); Wed, 4 Oct 2017 08:41:32 -0400 Subject: Re: [PATCH v9 08/12] mm: zero reserved and unavailable struct pages To: Michal Hocko Cc: linux-kernel@vger.kernel.org, sparclinux@vger.kernel.org, linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-arm-kernel@lists.infradead.org, x86@kernel.org, kasan-dev@googlegroups.com, borntraeger@de.ibm.com, heiko.carstens@de.ibm.com, davem@davemloft.net, willy@infradead.org, ard.biesheuvel@linaro.org, mark.rutland@arm.com, will.deacon@arm.com, catalin.marinas@arm.com, sam@ravnborg.org, mgorman@techsingularity.net, steven.sistare@oracle.com, daniel.m.jordan@oracle.com, bob.picco@oracle.com References: <20170920201714.19817-1-pasha.tatashin@oracle.com> <20170920201714.19817-9-pasha.tatashin@oracle.com> <20171003131817.omzbam3js67edp3s@dhcp22.suse.cz> <691dba28-718c-e9a9-d006-88505eb5cd7e@oracle.com> <20171004085636.w2rnwf5xxhahzuy7@dhcp22.suse.cz> From: Pasha Tatashin Message-ID: <9198a33d-cd40-dd70-4823-7f70c57ef9a2@oracle.com> Date: Wed, 4 Oct 2017 08:40:11 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.3.0 MIME-Version: 1.0 In-Reply-To: <20171004085636.w2rnwf5xxhahzuy7@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Source-IP: aserv0021.oracle.com [141.146.126.233] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org >>> Could you be more specific where is such a memory reserved? >>> >> >> I know of one example: trim_low_memory_range() unconditionally reserves from >> pfn 0, but e820__memblock_setup() might provide the exiting memory from pfn >> 1 (i.e. KVM). > > Then just initialize struct pages for that mapping rigth there where a > special API is used. > >> But, there could be more based on this comment from linux/page-flags.h: >> >> 19 * PG_reserved is set for special pages, which can never be swapped out. >> Some >> 20 * of them might not even exist (eg empty_bad_page)... > > I have no idea wht empty_bad_page is but a quick grep shows that this is > never used. I might be wrong here but if somebody is reserving a memory > in a special way then we should handle the initialization right there. > E.g. create an API for special memblock reservations. > Hi Michal, The reservations happen before struct pages are allocated and mapped. So, it is not always possible to do it at call sites. Previously, I have solved this problem like this: https://patchwork.kernel.org/patch/9886163 But, I was not too happy with that approach, so I replaced it with the current approach as it is more generic, and solves similar issues if they happen in other places. Also, the comment in page-flags got me scared that there are probably other places perhaps on other architectures that can have the similar issue. In addition, I did not like my solution, I was simply shrinking the low reservation from: [0 - reserve_low) to [min_pfn - reserve_low), but if min_pfn > reserve_low can we skip low reservation entirely? I was not sure. The current approach notifies us if there are such pages, and we can fix/remove them in the future without crashing kernel in the meantime. Pasha