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

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Date:   Mon, 29 Oct 2018 16:56:56 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     alexander.h.duyck@...ux.intel.com, Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Pasha Tatashin <pavel.tatashin@...rosoft.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        rppt@...ux.vnet.ibm.com, Ingo Molnar <mingo@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Zhang Yi <yi.z.zhang@...ux.intel.com>
Subject: Re: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the
 point where we init pgmap

On Mon 29-10-18 08:49:46, Dan Williams wrote:
> On Wed, Oct 17, 2018 at 8:02 AM Alexander Duyck
> <alexander.h.duyck@...ux.intel.com> 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