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
