[PATCH 1/2] cgroups: add documentation on extended attributes usage

[Aristeu Rozanski]

Cc: Li Zefan <lizefan@xxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Hillf Danton <dhillf@xxxxxxxxx>
Cc: Lennart Poettering <lpoetter@xxxxxxxxxx>
Signed-off-by: Aristeu Rozanski <aris@xxxxxxxxxx>

---
 Documentation/cgroups/00-INDEX  |    2 ++
 Documentation/cgroups/xattr.txt |   21 +++++++++++++++++++++
 2 files changed, 23 insertions(+)

--- a/Documentation/cgroups/00-INDEX	2010-07-29 22:53:28.000000000 -0400
+++ b/Documentation/cgroups/00-INDEX	2012-08-30 12:32:18.419879863 -0400
@@ -16,3 +16,5 @@ memory.txt
 	- Memory Resource Controller; design, accounting, interface, testing.
 resource_counter.txt
 	- Resource Counter API.
+xattr.txt
+	- Extended attributes support and usage in cgroup filesystem
--- /dev/null	1970-01-01 00:00:00.000000000 +0000
+++ b/Documentation/cgroups/xattr.txt	2012-08-30 13:02:56.585865106 -0400
@@ -0,0 +1,21 @@
+Extended Attributes Usage in cgroup Filesystem
+
+1. Usage
+
+cgroup filesystem supports certain types of extended attributes in its
+directories and files.  The current supported types are:
+	- Trusted (XATTR_TRUSTED)
+	- Security (XATTR_SECURITY)
+
+Both require CAP_SYS_ADMIN capability to set.
+
+Like in tmpfs, the extended attributes in cgroup filesystem are stored
+using kernel memory and it's advised to keep the usage at minimum.  This
+is the reason why user defined extended attributes are not supported, since
+any user can do it and there's no limit in the value size.
+
+2. Users
+
+The current known users for this feature are SELinux to limit cgroup usage
+in containers and systemd for assorted meta data like main PID in a cgroup
+(systemd creates a cgroup per service).

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