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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 7E24CC19759 for ; Thu, 1 Aug 2019 08:04:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 52E2521726 for ; Thu, 1 Aug 2019 08:04:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1564646653; bh=iJ/jDfwskDyOu7dIT3HxTBTvS8lKujAL4owl/fbXJzE=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=bJsIGnoLk9DvmbLfLoje97JygBd9BRHzoyFa/pSHCZYTcDvPL7W+2DJc1AJxkzXce dRxMBlxnbqM2asueTa/lLPdPZoRjqXq2iwY1hsLXsUQX2sS7XIWYYRXGqjvwvCh+xa +A0DkyjrjKq/LyewgIaMju1qvQ3T5AkFDQpV/UjQ= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730164AbfHAIEM (ORCPT ); Thu, 1 Aug 2019 04:04:12 -0400 Received: from mx2.suse.de ([195.135.220.15]:41198 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1728582AbfHAIEL (ORCPT ); Thu, 1 Aug 2019 04:04:11 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 468E0AC91; Thu, 1 Aug 2019 08:04:10 +0000 (UTC) Date: Thu, 1 Aug 2019 10:04:07 +0200 From: Michal Hocko To: David Hildenbrand Cc: Rashmica Gupta , Oscar Salvador , Andrew Morton , Dan Williams , pasha.tatashin@soleen.com, Jonathan.Cameron@huawei.com, anshuman.khandual@arm.com, Vlastimil Babka , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 0/5] Allocate memmap from hotadded memory Message-ID: <20190801080407.GJ11627@dhcp22.suse.cz> References: <20190702074806.GA26836@linux> <20190731120859.GJ9330@dhcp22.suse.cz> <4ddee0dd719abd50350f997b8089fa26f6004c0c.camel@gmail.com> <20190801071709.GE11627@dhcp22.suse.cz> <9bcbd574-7e23-5cfe-f633-646a085f935a@redhat.com> <20190801072430.GF11627@dhcp22.suse.cz> <20190801073407.GG11627@dhcp22.suse.cz> <7c12f0b1-61a5-ed6f-2c64-4058e47860a3@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <7c12f0b1-61a5-ed6f-2c64-4058e47860a3@redhat.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 01-08-19 09:50:29, David Hildenbrand wrote: > On 01.08.19 09:34, Michal Hocko wrote: > > On Thu 01-08-19 09:26:35, David Hildenbrand wrote: > >> On 01.08.19 09:24, Michal Hocko wrote: > >>> On Thu 01-08-19 09:18:47, David Hildenbrand wrote: > >>>> On 01.08.19 09:17, Michal Hocko wrote: > >>>>> On Thu 01-08-19 09:06:40, Rashmica Gupta wrote: > >>>>>> On Wed, 2019-07-31 at 14:08 +0200, Michal Hocko wrote: > >>>>>>> On Tue 02-07-19 18:52:01, Rashmica Gupta wrote: > >>>>>>> [...] > >>>>>>>>> 2) Why it was designed, what is the goal of the interface? > >>>>>>>>> 3) When it is supposed to be used? > >>>>>>>>> > >>>>>>>>> > >>>>>>>> There is a hardware debugging facility (htm) on some power chips. > >>>>>>>> To use > >>>>>>>> this you need a contiguous portion of memory for the output to be > >>>>>>>> dumped > >>>>>>>> to - and we obviously don't want this memory to be simultaneously > >>>>>>>> used by > >>>>>>>> the kernel. > >>>>>>> > >>>>>>> How much memory are we talking about here? Just curious. > >>>>>> > >>>>>> From what I've seen a couple of GB per node, so maybe 2-10GB total. > >>>>> > >>>>> OK, that is really a lot to keep around unused just in case the > >>>>> debugging is going to be used. > >>>>> > >>>>> I am still not sure the current approach of (ab)using memory hotplug is > >>>>> ideal. Sure there is some overlap but you shouldn't really need to > >>>>> offline the required memory range at all. All you need is to isolate the > >>>>> memory from any existing user and the page allocator. Have you checked > >>>>> alloc_contig_range? > >>>>> > >>>> > >>>> Rashmica mentioned somewhere in this thread that the virtual mapping > >>>> must not be in place, otherwise the HW might prefetch some of this > >>>> memory, leading to errors with memtrace (which checks that in HW). > >>> > >>> Does anything prevent from unmapping the pfn range from the direct > >>> mapping? > >> > >> I am not sure about the implications of having > >> pfn_valid()/pfn_present()/pfn_online() return true but accessing it > >> results in crashes. (suspend, kdump, whatever other technology touches > >> online memory) > > > > If those pages are marked as Reserved then nobody should be touching > > them anyway. > > Which is not true as I remember we already discussed - I even documented > what PG_reserved can mean after that discussion in page-flags.h (e.g., > memmap of boot memory) - that's why we introduced PG_offline after all. Sorry, my statement was imprecise. What I meant is what we have documented: * PG_reserved is set for special pages. The "struct page" of such a page * should in general not be touched (e.g. set dirty) except by its owner. the owner part is important. -- Michal Hocko SUSE Labs