memcg: remove the overhead associated with the root cgroup

Change the memory cgroup to remove the overhead associated with accounting
all pages in the root cgroup.  As a side-effect, we can no longer set a
memory hard limit in the root cgroup.

A new flag to track whether the page has been accounted or not has been
added as well.  Flags are now set atomically for page_cgroup,
pcg_default_flags is now obsolete and removed.

[akpm@linux-foundation.org: fix a few documentation glitches]
Signed-off-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 0 deletions

diff --git a/Documentation/cgroups/memory.txt b/Documentation/cgroups/memory.txt
index 23d1262c0775..ab0a02172cf4 100644
--- a/Documentation/cgroups/memory.txt
+++ b/Documentation/cgroups/memory.txt
@@ -179,6 +179,9 @@ The reclaim algorithm has not been modified for cgroups, except that
 pages that are selected for reclaiming come from the per cgroup LRU
 list.
 
+NOTE: Reclaim does not work for the root cgroup, since we cannot set any
+limits on the root cgroup.
+
 2. Locking
 
 The memory controller uses the following hierarchy
@@ -210,6 +213,7 @@ We can alter the memory limit:
 NOTE: We can use a suffix (k, K, m, M, g or G) to indicate values in kilo,
 mega or gigabytes.
 NOTE: We can write "-1" to reset the *.limit_in_bytes(unlimited).
+NOTE: We cannot set limits on the root cgroup any more.
 
 # cat /cgroups/0/memory.limit_in_bytes
 4194304