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

From: Vlastimil Babka
Date: Sat Apr 21 2018 - 19:15:42 EST

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?

>> + * PG_offline indicates that a page is offline and the backing storage
>> + * might already have been removed (virtualization). Don't touch!
> * PG_reserved is set for special pages, which can never be swapped out. Some
> * of them might not even exist...
> They seem pretty congruent to me.