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

[Alexander Duyck]

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.

> 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.

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.