memcg: do not create memsw files if swap accounting is disabled

memcg: do not create memsw files if swap accounting is disabled

Zhouping Liu has reported that memsw files are exported even though swap
accounting is runtime disabled if CONFIG_MEMCG_SWAP is enabled.  This
behavior has been introduced by af36f906 (memcg: always create memsw files
if CONFIG_CGROUP_MEM_RES_CTLR_SWAP) and it causes any attempt to open the
file to return EOPNOTSUPP.  Although EOPNOTSUPP should say be clear that
memsw operations are not supported in the given configuration it is fair
to say that this behavior could be quite confusing.

Let's tear memsw files out of default cgroup files and add them only if
the swap accounting is really enabled (either by CONFIG_MEMCG_SWAP_ENABLED
or swapaccount=1 boot parameter).  We can hook into mem_cgroup_init which
is called when the memcg subsystem is initialized and which happens after
boot command line is processed.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Reported-by: Zhouping Liu <zliu@redhat.com>
Tested-by: Zhouping Liu <zliu@redhat.com>
Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Li Zefan <lizefan@huawei.com>
Cc: CAI Qian <caiqian@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>