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

[David Hildenbrand <david@redhat.com>]

On 13.04.2018 15:40, Michal Hocko wrote:
> On Fri 13-04-18 15:16:26, 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.
> 
> Well, theoretically possible but this would require a lot of auditing
> because the hotplug and per section assumption is quite a spread one.
> 
>> 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.
> 
> But you cannot use a page flag for that, I am afraid. Page flags are
> extremely scarce resource. I haven't looked at the rest of the series
> but _if_ we have a bit spare which I am not really sure about then you
> should prove there are no other ways around this.

BTW, looking at the possible layouts of page->flags, I don't think it
will be a problem adding this flag. Especially if we compile this flag
only if really needed.

We could glue this flag to CONFIG_MEMORY_HOTPLUG_SUBSECTION to something
like that, that will be set when our new driver is compiled. So this
would not affect anybody just wanting to use ordinary DIMM based hotplug
(CONFIG_MEMORY_HOTPLUG).

But I am open for other suggestions. I don't think PG_reserved is the
right thing to use. And storing for each section which parts are
online/offline is also something I would like to avoid.

-- 

Thanks,

David / dhildenb