Re: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap

From: Dan Williams
Date: Mon Oct 29 2018 - 11:50:01 EST

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?