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=-2.5 required=3.0 tests=MAILING_LIST_MULTI,SPF_PASS, USER_AGENT_MUTT autolearn=ham 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 212F8C0044C for ; Mon, 29 Oct 2018 14:12:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C1E222082D for ; Mon, 29 Oct 2018 14:12:15 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C1E222082D Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726214AbeJ2XBD (ORCPT ); Mon, 29 Oct 2018 19:01:03 -0400 Received: from mx2.suse.de ([195.135.220.15]:45272 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1725977AbeJ2XBC (ORCPT ); Mon, 29 Oct 2018 19:01:02 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay1.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 81AAFAFFA; Mon, 29 Oct 2018 14:12:11 +0000 (UTC) Date: Mon, 29 Oct 2018 15:12:10 +0100 From: Michal Hocko To: Alexander Duyck Cc: Dan Williams , Linux MM , Andrew Morton , Linux Kernel Mailing List , linux-nvdimm , Pasha Tatashin , Dave Hansen , =?iso-8859-1?B?Suly9G1l?= Glisse , rppt@linux.vnet.ibm.com, Ingo Molnar , "Kirill A. Shutemov" , yi.z.zhang@linux.intel.com Subject: Re: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap Message-ID: <20181029141210.GJ32673@dhcp22.suse.cz> References: <25092df0-b7b4-d456-8409-9c004cb6e422@linux.intel.com> <20181010095838.GG5873@dhcp22.suse.cz> <20181010172451.GK5873@dhcp22.suse.cz> <98c35e19-13b9-0913-87d9-b3f1ab738b61@linux.intel.com> <20181010185242.GP5873@dhcp22.suse.cz> <20181011085509.GS5873@dhcp22.suse.cz> <6f32f23c-c21c-9d42-7dda-a1d18613cd3c@linux.intel.com> <20181017075257.GF18839@dhcp22.suse.cz> <971729e6-bcfe-a386-361b-d662951e69a7@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <971729e6-bcfe-a386-361b-d662951e69a7@linux.intel.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 Wed 17-10-18 08:02:20, Alexander Duyck wrote: > On 10/17/2018 12:52 AM, Michal Hocko wrote: > > On Thu 11-10-18 10:38:39, Alexander Duyck wrote: > > > On 10/11/2018 1:55 AM, Michal Hocko wrote: > > > > On Wed 10-10-18 20:52:42, Michal Hocko wrote: > > > > [...] > > > > > My recollection was that we do clear the reserved bit in > > > > > move_pfn_range_to_zone and we indeed do in __init_single_page. But then > > > > > we set the bit back right afterwards. This seems to be the case since > > > > > d0dc12e86b319 which reorganized the code. I have to study this some more > > > > > obviously. > > > > > > > > so my recollection was wrong and d0dc12e86b319 hasn't really changed > > > > much because __init_single_page wouldn't zero out the struct page for > > > > the hotplug contex. A comment in move_pfn_range_to_zone explains that we > > > > want the reserved bit because pfn walkers already do see the pfn range > > > > and the page is not fully associated with the zone until it is onlined. > > > > > > > > I am thinking that we might be overzealous here. With the full state > > > > initialized we shouldn't actually care. pfn_to_online_page should return > > > > NULL regardless of the reserved bit and normal pfn walkers shouldn't > > > > touch pages they do not recognize and a plain page with ref. count 1 > > > > doesn't tell much to anybody. So I _suspect_ that we can simply drop the > > > > reserved bit setting here. > > > > > > So this has me a bit hesitant to want to just drop the bit entirely. If > > > nothing else I think I may wan to make that a patch onto itself so that if > > > we aren't going to set it we just drop it there. That way if it does cause > > > issues we can bisect it to that patch and pinpoint the cause. > > > > Yes a patch on its own make sense for bisectability. > > For now I think I am going to back off of this. There is a bunch of other > changes that need to happen in order for us to make this work. As far as I > can tell there are several places that are relying on this reserved bit. Please be more specific. Unless I misremember, I have added this PageReserved just to be sure (f1dd2cd13c4bb) because pages where just half initialized back then. I am not aware anybody is depending on this. If there is somebody then be explicit about that. The last thing I want to see is to preserve a cargo cult and build a design around it. > > > > Regarding the post initialization required by devm_memremap_pages and > > > > potentially others. Can we update the altmap which is already a way how > > > > to get alternative struct pages by a constructor which we could call > > > > from memmap_init_zone and do the post initialization? This would reduce > > > > the additional loop in the caller while it would still fit the overall > > > > design of the altmap and the core hotplug doesn't have to know anything > > > > about DAX or whatever needs a special treatment. > > > > > > > > Does that make any sense? > > > > > > I think the only thing that is currently using the altmap is the ZONE_DEVICE > > > memory init. Specifically I think it is only really used by the > > > devm_memremap_pages version of things, and then only under certain > > > circumstances. Also the HMM driver doesn't pass an altmap. What we would > > > really need is a non-ZONE_DEVICE users of the altmap to really justify > > > sticking with that as the preferred argument to pass. > > > > I am not aware of any upstream HMM user so I am not sure what are the > > expectations there. But I thought that ZONE_DEVICE users use altmap. If > > that is not generally true then we certainly have to think about a > > better interface. > > I'm just basing my statement on the use of the move_pfn_range_to_zone call. > The only caller that is actually passing the altmap is devm_memremap_pages > and if I understand things correctly that is only used when we want to stare > the vmmemmap on the same memory that we just hotplugged. Yes, and that is what I've called as allocator callback earlier. > That is why it made more sense to me to just create a ZONE_DEVICE specific > function for handling the page initialization because the one value I do > have to pass is the dev_pagemap in both HMM and memremap case, and that has > the altmap already embedded inside of it. And I have argued that this is a wrong approach to the problem. If you need a very specific struct page initialization then create a init (constructor) callback. -- Michal Hocko SUSE Labs