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

[Yi Zhang]

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.

> 
> 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



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