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 <akpm@xxxxxxxxxxxxxxxxxxxx> > Cc: Juergen Gross <jgross@xxxxxxxx> > Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> > Cc: Pavel Tatashin <pavel.tatashin@xxxxxxxxxxxxx> > Cc: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx> > Cc: Vlastimil Babka <vbabka@xxxxxxx> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx> > Cc: Anthony Yznaga <anthony.yznaga@xxxxxxxxxx> > Cc: Michal Hocko <mhocko@xxxxxxxx> > Cc: Oscar Salvador <osalvador@xxxxxxx> > Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> > Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx> > Cc: Dan Williams <dan.j.williams@xxxxxxxxx> > Cc: Anshuman Khandual <anshuman.khandual@xxxxxxx> > Cc: Qian Cai <cai@xxxxxx> > Cc: Pingfan Liu <kernelfans@xxxxxxxxx> > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx> This addresses the core concerns I had with the patch. Reviewed-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
