Currently, pcplists are drained during set_migratetype_isolate() which
means once per pageblock processed start_isolate_page_range().  This is
somewhat wasteful.  Moreover, the callers might need different guarantees,
and the draining is currently prone to races and does not guarantee that
no page from isolated pageblock will end up on the pcplist after the
drain.

Better guarantees are added by later patches and require explicit actions
by page isolation users that need them.  Thus it makes sense to move the
current imperfect draining to the callers also as a preparation step.

Link: https://lkml.kernel.org/r/20201111092812.11329-7-vbabka@suse.czSuggested-by: NDavid Hildenbrand <david@redhat.com>
Suggested-by: NPavel Tatashin <pasha.tatashin@soleen.com>
Signed-off-by: NVlastimil Babka <vbabka@suse.cz>
Reviewed-by: NDavid Hildenbrand <david@redhat.com>
Reviewed-by: NOscar Salvador <osalvador@suse.de>
Acked-by: NMichal Hocko <mhocko@suse.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>