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

From: Michal Hocko
Date: Tue Oct 30 2018 - 02:29:26 EST

On Mon 29-10-18 12:59:11, Alexander Duyck wrote:
> On Mon, 2018-10-29 at 19:18 +0100, Michal Hocko wrote:

I will try to get to your other points later.

> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 89d2a2ab3fe6..048e4cc72fdf 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -5474,8 +5474,8 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
> > * Honor reservation requested by the driver for this ZONE_DEVICE
> > * memory
> > */
> > - if (altmap && start_pfn == altmap->base_pfn)
> > - start_pfn += altmap->reserve;
> > + if (pgmap && pgmap->get_memmap)
> > + start_pfn = pgmap->get_memmap(pgmap, start_pfn);
> >
> > for (pfn = start_pfn; pfn < end_pfn; pfn++) {
> > /*
> >
> > [...]
> The only reason why I hadn't bothered with these bits is that I was
> actually trying to leave this generic since I thought I had seen other
> discussions about hotplug scenerios where memory may want to change
> where the vmmemmap is initialized other than just the case of
> ZONE_DEVICE pages. So I was thinking at some point we may see altmap
> without the pgmap.

I wanted to abuse altmap to allocate struct pages from the physical
range to be added. In that case I would abstract the
allocation/initialization part of pgmap into a more abstract type.
Something trivially to be done without affecting end users of the
hotplug API.

> > Anyway we have gone into details while the primary problem here was that
> > the hotplug lock doesn't scale AFAIR. And my question was why cannot we
> > pull move_pfn_range_to_zone and what has to be done to achieve that.
> > That is a fundamental thing to address first. Then you can microptimize
> > on top.
> Yes, the hotplug lock was part of the original issue. However that
> starts to drift into the area I believe Oscar was working on as a part
> of his patch set in encapsulating the move_pfn_range_to_zone and other
> calls that were contained in the hotplug lock into their own functions.

Well, I would really love to keep the external API as simple as
possible. That means that we need arch_add_memory/add_pages and
move_pfn_range_to_zone to associate pages with a zone. The hotplug lock
should be preferably hidden from callers of those two and ideally it
shouldn't be a global lock. We should be good with a range lock.

> The patches Andrew pushed addressed the immediate issue so that now
> systems with nvdimm/DAX memory can at least initialize quick enough
> that systemd doesn't refuse to mount the root file system due to a
> timeout.

This is about the first time you actually mention that. I have re-read
the cover letter and all changelogs of patches in this serious. Unless I
have missed something there is nothing about real users hitting issues
out there. nvdimm is still considered a toy because there is no real HW
users can play with.

And hence my complains about half baked solutions rushed in just to fix
a performance regression. I can certainly understand that a pressing
problem might justify to rush things a bit but this should be always
carefuly justified.

> The next patch set I have refactors things to reduce code and
> allow us to reuse some of the hotplug code for the deferred page init,
> . After that I was planning to work on dealing with the PageReserved
> flag and trying to get that sorted out.
> I was hoping to wait until after Dan's HMM patches and Oscar's changes
> had been sorted before I get into any further refactor of this specific
> code.

Yes there is quite a lot going on here. I would really appreciate if we
all sit and actually try to come up with something robust rather than
hack here and there. I haven't yet seen your follow up series completely
so maybe you are indeed heading the correct direction.

Michal Hocko