Re: [PATCH v1 06/11] mm: Allow to offline unmovable PageOffline() pages via MEM_GOING_OFFLINE

[David Hildenbrand <david@redhat.com>]

On 10.03.20 12:47, Michal Hocko wrote:
> On Mon 02-03-20 14:49:36, 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@linux-foundation.org>
>> Cc: Juergen Gross <jgross@suse.com>
>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>> Cc: Pavel Tatashin <pavel.tatashin@microsoft.com>
>> Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
>> Cc: Vlastimil Babka <vbabka@suse.cz>
>> Cc: Johannes Weiner <hannes@cmpxchg.org>
>> Cc: Anthony Yznaga <anthony.yznaga@oracle.com>
>> Cc: Michal Hocko <mhocko@suse.com>
>> Cc: Oscar Salvador <osalvador@suse.de>
>> Cc: Mel Gorman <mgorman@techsingularity.net>
>> Cc: Mike Rapoport <rppt@linux.ibm.com>
>> Cc: Dan Williams <dan.j.williams@intel.com>
>> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
>> Cc: Qian Cai <cai@lca.pw>
>> Cc: Pingfan Liu <kernelfans@gmail.com>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
> 
> Looks good to me. Thanks for dropping __put_page hooks!
> Acked-by: Michal Hocko <mhocko@suse.com>

Thanks a lot Michal - looks much cleaner this way.

-- 
Thanks,

David / dhildenb