Re: [PATCH RFC 2/8] mm: introduce PG_offline

[Matthew Wilcox]

On Sat, Apr 21, 2018 at 06:52:18PM +0200, Vlastimil Babka wrote:
> On 04/13/2018 07:11 PM, Matthew Wilcox wrote:
> > On Fri, Apr 13, 2018 at 03:16:26PM +0200, David Hildenbrand wrote:
> >> online_pages()/offline_pages() theoretically allows us to work on
> >> sub-section sizes. This is especially relevant in the context of
> >> virtualization. It e.g. allows us to add/remove memory to Linux in a VM in
> >> 4MB chunks.
> >>
> >> While the whole section is marked as online/offline, we have to know
> >> the state of each page. E.g. to not read memory that is not online
> >> during kexec() or to properly mark a section as offline as soon as all
> >> contained pages are offline.
> > 
> > Can you not use PG_reserved for this purpose?
> 
> Sounds like your newly introduced "page types" could be useful here? I
> don't suppose those offline pages would be using mapcount which is
> aliased there?

Oh, that's a good point!  Yes, this is a perfect use for page_type.
We have something like twenty bits available there.

Now you've got me thinking that we can move PG_hwpoison and PG_reserved
to be page_type flags too.  That'll take us from 23 to 21 bits (on 32-bit,
with PG_UNCACHED)