On Wed, Oct 17, 2018 at 8:02 AM Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx> 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. We either need to audit the removal or go back to adding a special case hack for kvm because this is a blocking issue for them. What do you see beyond the hibernation change?
