mm, memory_hotplug: fix MMOP_ONLINE_KEEP behavior

mm, memory_hotplug: fix MMOP_ONLINE_KEEP behavior

Heiko Carstens has noticed that the MMOP_ONLINE_KEEP is broken currently

  $ grep . memory3?/valid_zones
  memory34/valid_zones:Normal Movable
  memory35/valid_zones:Normal Movable
  memory36/valid_zones:Normal Movable
  memory37/valid_zones:Normal Movable

  $ echo online_movable > memory34/state
  $ grep . memory3?/valid_zones
  memory34/valid_zones:Movable
  memory35/valid_zones:Movable
  memory36/valid_zones:Movable
  memory37/valid_zones:Movable

  $ echo online > memory36/state
  $ grep . memory3?/valid_zones
  memory34/valid_zones:Movable
  memory36/valid_zones:Normal
  memory37/valid_zones:Movable

so we have effectively punched a hole into the movable zone.

The problem is that move_pfn_range() check for MMOP_ONLINE_KEEP is
wrong.  It only checks whether the given range is already part of the
movable zone which is not the case here as only memory34 is in the zone.
Fix this by using allow_online_pfn_range(..., MMOP_ONLINE_KERNEL) if
that is false then we can be sure that movable onlining is the right
thing to do.

Fixes: "mm, memory_hotplug: do not associate hotadded memory to zones until online"
Link: http://lkml.kernel.org/r/20170601083746.4924-2-mhocko@kernel.org
Signed-off-by: Michal Hocko <mhocko@suse.com>
Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Tested-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Reza Arbab <arbab@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>