There are a lot of kinds of objects in the kernel that don't have
  individual limits or that have limits that are ineffective when a set
  of processes is allowed to switch user ids.  With user namespaces
  enabled in a kernel for people who don't trust their users or their
  users programs to play nice this problems becomes more acute.
  
  Therefore it is recommended that memory control groups be enabled in
  kernels that enable user namespaces, and it is further recommended
  that userspace configure memory control groups to limit how much
  memory user's they don't trust to play nice can use.
  
  Memory control groups can be configured by installing the libcgroup
  package present on most distros editing /etc/cgrules.conf,
  /etc/cgconfig.conf and setting up libpam-cgroup.