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

[Michal Hocko]

On Wed 17-10-18 08:02:20, 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.

Please be more specific. Unless I misremember, I have added this
PageReserved just to be sure (f1dd2cd13c4bb) because pages where just
half initialized back then. I am not aware anybody is depending on this.
If there is somebody then be explicit about that. The last thing I want
to see is to preserve a cargo cult and build a design around it.

> > > > 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.
> 
> I'm just basing my statement on the use of the move_pfn_range_to_zone call.
> The only caller that is actually passing the altmap is devm_memremap_pages
> and if I understand things correctly that is only used when we want to stare
> the vmmemmap on the same memory that we just hotplugged.

Yes, and that is what I've called as allocator callback earlier.

> That is why it made more sense to me to just create a ZONE_DEVICE specific
> function for handling the page initialization because the one value I do
> have to pass is the dev_pagemap in both HMM and memremap case, and that has
> the altmap already embedded inside of it.

And I have argued that this is a wrong approach to the problem. If you
need a very specific struct page initialization then create a init
(constructor) callback.
-- 
Michal Hocko
SUSE Labs