mm/vmscan: never demote for memcg reclaim

Global reclaim aims to reduce the amount of memory used on a given node or
set of nodes.  Migrating pages to another node serves this purpose.

memcg reclaim is different.  Its goal is to reduce the total memory
consumption of the entire memcg, across all nodes.  Migration does not
assist memcg reclaim because it just moves page contents between nodes
rather than actually reducing memory consumption.

Link: https://lkml.kernel.org/r/20210715055145.195411-9-ying.huang@intel.com


Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Suggested-by: Yang Shi <yang.shi@linux.alibaba.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Wei Xu <weixugc@google.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: David Rientjes <rientjes@google.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: Keith Busch <kbusch@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>