From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Mon, 29 Oct 2018 16:56:56 +0100 From: Michal Hocko Subject: Re: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap Message-ID: <20181029155656.GK32673@dhcp22.suse.cz> References: <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: Sender: owner-linux-mm@kvack.org To: Dan Williams Cc: alexander.h.duyck@linux.intel.com, 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" , Zhang Yi List-ID: On Mon 29-10-18 08:49:46, Dan Williams wrote: > On Wed, Oct 17, 2018 at 8:02 AM 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. > > When David Hildebrand and I looked it was only the hibernation code > that we thought needed changing. More details please? -- Michal Hocko SUSE Labs