From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from aserp1040.oracle.com (aserp1040.oracle.com [141.146.126.69]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 3y6b9q5RS6zDqqQ for ; Wed, 4 Oct 2017 23:41:19 +1100 (AEDT) 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 MIME-Version: 1.0 In-Reply-To: <20171004085636.w2rnwf5xxhahzuy7@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8; format=flowed List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , >>> 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