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

[Alexander Duyck]

On 10/10/2018 5:52 AM, Yi Zhang wrote:
> On 2018-10-09 at 14:19:32 -0700, Dan Williams wrote:
> > On Tue, Oct 9, 2018 at 1:34 PM Alexander Duyck
> > <alexander.h.duyck@xxxxxxxxxxxxxxx> wrote:
> > > On 10/9/2018 11:04 AM, Dan Williams wrote:
> > > > On Tue, Oct 9, 2018 at 3:21 AM Yi Zhang <yi.z.zhang@xxxxxxxxxxxxxxx> wrote:
> > [..]
> > > > That comment is incorrect, device-pages are never onlined. So I think
> > > > we can just skip that call to __SetPageReserved() unless the memory
> > > > range is MEMORY_DEVICE_{PRIVATE,PUBLIC}.
> > >
> > > When pages are "onlined" via __free_pages_boot_core they clear the
> > > reserved bit, that is the reason for the comment. The reserved bit is
> > > meant to indicate that the page cannot be swapped out or moved based on
> > > the description of the bit.
> >
> > ...but ZONE_DEVICE pages are never onlined so I would expect
> > memmap_init_zone_device() to know that detail.
> >
> > > I would think with that being the case we still probably need the call
> > > to __SetPageReserved to set the bit with the expectation that it will
> > > not be cleared for device-pages since the pages are not onlined.
> > > Removing the call to __SetPageReserved would probably introduce a number
> > > of regressions as there are multiple spots that use the reserved bit to
> > > determine if a page can be swapped out to disk, mapped as system memory,
> > > or migrated.
>
> Another things, it seems page_init/set_reserved already been done in the
> move_pfn_range_to_zone
>      |-->memmap_init_zone
>      	|-->for_each_page_in_pfn
> 		|-->__init_single_page
> 		|-->SetPageReserved
>
> Why we haven't remove these redundant initial in memmap_init_zone?
>
> Correct me if I missed something.

In this case it isn't redundant as only the vmmemmap pages are 
initialized in memmap_init_zone now. So all of the pages that are going 
to be used as device pages are not initialized until the call to 
memmap_init_zone_device. What I did is split the initialization of the 
pages into two parts in order to allow us to initialize the pages 
outside of the hotplug lock.

> > Right, this is what Yi is working on... the PageReserved flag is
> > problematic for KVM. Auditing those locations it seems as long as we
> > teach hibernation to avoid ZONE_DEVICE ranges we can safely not set
> > the reserved flag for DAX pages. What I'm trying to avoid is a local
> > KVM hack to check for DAX pages when the Reserved flag is not
> > otherwise needed.
> Thanks Dan. Provide the patch link.
>
> https://lore.kernel.org/lkml/cover.1536342881.git.yi.z.zhang@xxxxxxxxxxxxxxx

So it looks like your current logic is just working around the bit then 
since it just allows for reserved DAX pages.