[PATCH v2 0/1] mm/memory_hotplug: introduce and use PG_offline_skippable

[David Hildenbrand <david@redhat.com>]

This is a requirement for making PageOffline pages not have a refcount
in the long future ("frozen"), and for reworking non-folio page migration
in the near future.

I have patches mostly ready to go to handle the latter. For turning all
PageOffline() pages frozen, the non-folio page migration and memory
ballooning drivers will have to be reworked first, to no longer rely on
the refcount of PageOffline pages.

Introduce PG_offline_skippable that only applies to PageOffline() pages --
of course, reusing one of the existing PG_ flags for now -- and convert
virtio-mem to make use of the new way: to allow for skipping PageOffline
pages during memory offlining, treating them as if they would not be
allocated.

Note that the existing mechanism relied on the driver (virtio-mem)
dropping its reference during MEM_GOING_OFFLINE, which is complicated and
not compatible with the concept of frozen pages (no refcount).

Tested with virtio-mem on x86, including  partially hotplugging a memory
block (hotplugging 64MiB with a 128 MiB memory block size), and repeatedly
onlining+offlining the memory block. Also tested that forced driver
unloading with partially plugged memory blocks keeps working as is.

Cc: David Hildenbrand <david@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Cc: "Eugenio Pérez" <eperezma@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Zi Yan <ziy@nvidia.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>

v1 -> v2:
* Handle forced driver unloading of virtio-mem: clear the flag
* Added a comment for a PageOfflineSkippable check
* Added more details to the patch description
* Drop patch #2 ("mm/memory_hotplug: remove -EBUSY handling from
  scan_movable_pages()")

David Hildenbrand (1):
  mm/memory_hotplug: PG_offline_skippable for offlining memory blocks
    with PageOffline pages

 drivers/virtio/virtio_mem.c | 141 ++++++++----------------------------
 include/linux/page-flags.h  |  29 ++++++--
 mm/memory_hotplug.c         |  17 +++--
 mm/page_alloc.c             |   8 +-
 mm/page_isolation.c         |  21 ++----
 5 files changed, 74 insertions(+), 142 deletions(-)

-- 
2.49.0