memcg: avoid THP split in task migration

memcg: avoid THP split in task migration

Currently we can't do task migration among memory cgroups without THP
split, which means processes heavily using THP experience large overhead
in task migration.  This patch introduces the code for moving charge of
THP and makes THP more valuable.

Signed-off-by: Naoya Horiguchi <[email protected]>
Acked-by: Hillf Danton <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Acked-by: KAMEZAWA Hiroyuki <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Daisuke Nishimura <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>