In the current implementation, there are two places to isolate a range of
page: __offline_pages() and alloc_contig_range().  During this procedure,
it will drain pages on pcp list.

Below is a brief call flow:

  __offline_pages()/alloc_contig_range()
      start_isolate_page_range()
          set_migratetype_isolate()
              drain_all_pages()
      drain_all_pages()                 <--- A

This snippet shows the current logic is isolate and drain pcp list for
each pageblock and drain pcp list again for the whole range.

start_isolate_page_range is responsible for isolating the given pfn range.
One part of that job is to make sure that also pages that are on the
allocator pcp lists are properly isolated.  Otherwise they could be reused
and the range wouldn't be completely isolated until the memory is freed
back.  While there is no strict guarantee here because pages might get
allocated at any time before drain_all_pages is called there doesn't seem
to be any strong demand for such a guarantee.

In any case, draining is already done at the isolation level and there is
no need to do it again later by start_isolate_page_range callers (memory
hotplug and CMA allocator currently).  Therefore remove pointless draining
in existing callers to make the code more clear and functionally correct.

[mhocko@suse.com: provide a clearer changelog for the last two paragraphs]
Link: http://lkml.kernel.org/r/20190105233141.2329-1-richard.weiyang@gmail.com


Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>