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

    Cc: Li Zefan <lizefan@huawei.com>
    Cc: Tejun Heo <tj@kernel.org>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Hillf Danton <dhillf@gmail.com>
    Cc: Lennart Poettering <lpoetter@redhat.com>
    Signed-off-by: Aristeu Rozanski <aris@redhat.com>

    ---
    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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