It's possible for free pages to become stranded on per-cpu pagesets
(pcps) that, if drained, could be merged with buddy pages on the zone's
free area to form large order pages, including up to MAX_ORDER.

Consider a verbose example using the tools/vm/page-types tool at the
beginning of a ZONE_NORMAL ('B' indicates a buddy page and 'S' indicates
a slab page).  Pages on pcps do not have any page flags set.

  109954  1       _______S________________________________________________________
  109955  2       __________B_____________________________________________________
  109957  1       ________________________________________________________________
  109958  1       __________B_____________________________________________________
  109959  7       ________________________________________________________________
  109960  1       __________B_____________________________________________________
  109961  9       ________________________________________________________________
  10996a  1       __________B_____________________________________________________
  10996b  3       ________________________________________________________________
  10996e  1       __________B_____________________________________________________
  10996f  1       ________________________________________________________________
  ...
  109f8c  1       __________B_____________________________________________________
  109f8d  2       ________________________________________________________________
  109f8f  2       __________B_____________________________________________________
  109f91  f       ________________________________________________________________
  109fa0  1       __________B_____________________________________________________
  109fa1  7       ________________________________________________________________
  109fa8  1       __________B_____________________________________________________
  109fa9  1       ________________________________________________________________
  109faa  1       __________B_____________________________________________________
  109fab  1       _______S________________________________________________________

The compaction migration scanner is attempting to defragment this memory
since it is at the beginning of the zone.  It has done so quite well,
all movable pages have been migrated.  From pfn [0x109955, 0x109fab),
there are only buddy pages and pages without flags set.

These pages may be stranded on pcps that could otherwise allow this
memory to be coalesced if freed back to the zone free area.  It is
possible that some of these pages may not be on pcps and that something
has called alloc_pages() and used the memory directly, but we rely on
the absence of __GFP_MOVABLE in these cases to allocate from
MIGATE_UNMOVABLE pageblocks to try to keep these MIGRATE_MOVABLE
pageblocks as free as possible.

These buddy and pcp pages, spanning 1,621 pages, could be coalesced and
allow for three transparent hugepages to be dynamically allocated.
Running the numbers for all such spans on the system, it was found that
there were over 400 such spans of only buddy pages and pages without
flags set at the time this /proc/kpageflags sample was collected.
Without this support, there were _no_ order-9 or order-10 pages free.

When kcompactd fails to defragment memory such that a cc.order page can
be allocated, drain all pcps for the zone back to the buddy allocator so
this stranding cannot occur.  Compaction for that order will
subsequently be deferred, which acts as a ratelimit on this drain.

Link: http://lkml.kernel.org/r/alpine.DEB.2.20.1803010340100.88270@chino.kir.corp.google.comSigned-off-by: David Rientjes <rientjes@google.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>