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

From: Michal Hocko
Date: Wed Oct 17 2018 - 03:53:02 EST

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.

> > 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.

> For those two functions it currently makes much more sense to pass the
> dev_pagemap pointer and then reference the altmap from there. Otherwise we
> are likely starting to look at something that would be more of a dirty hack
> where we are passing a unused altmap in order to get to the dev_pagemap so
> that we could populate the page.

If dev_pagemap is a general abstraction then I agree.
Michal Hocko