similar has existed in the group scheduler in the form of shares.  The CPU
controllers interpretation of shares is very different though.  

Soft limits are the most useful feature to have for environments where the
administrator wants to overcommit the system, such that only on memory
contention do the limits become active.  The current soft limits
implementation provides a soft_limit_in_bytes interface for the memory
controller and not for memory+swap controller.  The implementation
maintains an RB-Tree of groups that exceed their soft limit and starts
reclaiming from the group that exceeds this limit by the maximum amount.

This patch:

Add documentation for soft limits
Signed-off-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>