From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752439AbdJDPKi (ORCPT ); Wed, 4 Oct 2017 11:10:38 -0400 Received: from aserp1040.oracle.com ([141.146.126.69]:37727 "EHLO aserp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752330AbdJDPKg (ORCPT ); Wed, 4 Oct 2017 11:10:36 -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> <9198a33d-cd40-dd70-4823-7f70c57ef9a2@oracle.com> <20171004125743.fm6mf2artbga76et@dhcp22.suse.cz> <20171004140410.2w2zf2gbutdxunir@dhcp22.suse.cz> From: Pasha Tatashin Message-ID: Date: Wed, 4 Oct 2017 11:08:59 -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: <20171004140410.2w2zf2gbutdxunir@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Source-IP: aserv0022.oracle.com [141.146.126.234] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/04/2017 10:04 AM, Michal Hocko wrote: > On Wed 04-10-17 09:28:55, Pasha Tatashin wrote: >> >>> I am not really familiar with the trim_low_memory_range code path. I am >>> not even sure we have to care about it because nobody should be walking >>> pfns outside of any zone. >> >> According to commit comments first 4K belongs to BIOS, so I think the memory >> exists but BIOS may or may not report it to Linux. So, reserve it to make >> sure we never touch it. > > Yes and that memory should be outside of any zones, no? I am not totally sure, I think some x86 expert could help us here. But, in either case this issue can be fixed separately from the rest of the series. > >>> I am worried that this patch adds a code which >>> is not really used and it will just stay that way for ever because >>> nobody will dare to change it as it is too obscure and not explained >>> very well. >> >> I could explain mine code better. Perhaps add more comments, and explain >> when it can be removed? > > More explanation would be definitely helpful > >>> trim_low_memory_range is a good example of this. Why do we >>> even reserve this range from the memory block allocator? The memory >>> shouldn't be backed by any real memory and thus not in the allocator in >>> the first place, no? >>> >> >> Since it is not enforced in memblock that everything in reserved list must >> be part of memory list, we can have it, and we need to make sure kernel does >> not panic. Otherwise, it is very hard to detect such bugs. > > So, should we report such a memblock reservation API (ab)use to the log? > Are you actually sure that trim_low_memory_range is doing a sane and > really needed thing? In other words do we have a zone which contains > this no-memory backed pfns? > And, this patch reports it already: + pr_info("Reserved but unavailable: %lld pages", pgcnt); I could add a comment above this print call, explain that such memory is probably bogus and must be studied/fixed. Also, add that this code can be removed once memblock is changed to allow reserve only memory that is backed by physical memory i.e. in "memory" list. Pasha