On Mon, 2020-03-02 at 14:49 +0100, David Hildenbrand wrote: > virtio-mem wants to allow to offline memory blocks of which some parts > were unplugged (allocated via alloc_contig_range()), especially, to later > offline and remove completely unplugged memory blocks. The important part > is that PageOffline() has to remain set until the section is offline, so > these pages will never get accessed (e.g., when dumping). The pages should > not be handed back to the buddy (which would require clearing PageOffline() > and result in issues if offlining fails and the pages are suddenly in the > buddy). > > Let's allow to do that by allowing to isolate any PageOffline() page > when offlining. This way, we can reach the memory hotplug notifier > MEM_GOING_OFFLINE, where the driver can signal that he is fine with > offlining this page by dropping its reference count. PageOffline() pages > with a reference count of 0 can then be skipped when offlining the > pages (like if they were free, however they are not in the buddy). > > Anybody who uses PageOffline() pages and does not agree to offline them > (e.g., Hyper-V balloon, XEN balloon, VMWare balloon for 2MB pages) will not > decrement the reference count and make offlining fail when trying to > migrate such an unmovable page. So there should be no observable change. > Same applies to balloon compaction users (movable PageOffline() pages), the > pages will simply be migrated. > > Note 1: If offlining fails, a driver has to increment the reference > count again in MEM_CANCEL_OFFLINE. > > Note 2: A driver that makes use of this has to be aware that re-onlining > the memory block has to be handled by hooking into onlining code > (online_page_callback_t), resetting the page PageOffline() and > not giving them to the buddy. > > Cc: Andrew Morton <[email protected]> > Cc: Juergen Gross <[email protected]> > Cc: Konrad Rzeszutek Wilk <[email protected]> > Cc: Pavel Tatashin <[email protected]> > Cc: Alexander Duyck <[email protected]> > Cc: Vlastimil Babka <[email protected]> > Cc: Johannes Weiner <[email protected]> > Cc: Anthony Yznaga <[email protected]> > Cc: Michal Hocko <[email protected]> > Cc: Oscar Salvador <[email protected]> > Cc: Mel Gorman <[email protected]> > Cc: Mike Rapoport <[email protected]> > Cc: Dan Williams <[email protected]> > Cc: Anshuman Khandual <[email protected]> > Cc: Qian Cai <[email protected]> > Cc: Pingfan Liu <[email protected]> > Signed-off-by: David Hildenbrand <[email protected]>
This addresses the core concerns I had with the patch. Reviewed-by: Alexander Duyck <[email protected]> --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
