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    Subject[PATCH 26/38] sysfs-namespaces: add a high-level Documentation file
    Date
    From: Serge E. Hallyn <serue@us.ibm.com>

    The first three paragraphs are almost verbatim taken from Eric's
    commit message on the patch introducing network ns tags. The next
    two paragraphs I wrote to be a brief high level overview. The last
    section is taken from the commit message on "Implement sysfs tagged
    directory support", but updated. Hopefully correctly.

    Signed-off-by: Serge E. Hallyn <serue@us.ibm.com>
    Cc: Eric W. Biederman <ebiederm@xmission.com>
    Cc: Tejun Heo <tj@kernel.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
    ---
    Documentation/filesystems/sysfs-tagging.txt | 42 +++++++++++++++++++++++++++
    1 files changed, 42 insertions(+), 0 deletions(-)
    create mode 100644 Documentation/filesystems/sysfs-tagging.txt

    diff --git a/Documentation/filesystems/sysfs-tagging.txt b/Documentation/filesystems/sysfs-tagging.txt
    new file mode 100644
    index 0000000..caaaf12
    --- /dev/null
    +++ b/Documentation/filesystems/sysfs-tagging.txt
    @@ -0,0 +1,42 @@
    +Sysfs tagging
    +-------------
    +
    +(Taken almost verbatim from Eric Biederman's netns tagging patch
    +commit msg)
    +
    +The problem. Network devices show up in sysfs and with the network
    +namespace active multiple devices with the same name can show up in
    +the same directory, ouch!
    +
    +To avoid that problem and allow existing applications in network
    +namespaces to see the same interface that is currently presented in
    +sysfs, sysfs now has tagging directory support.
    +
    +By using the network namespace pointers as tags to separate out the
    +the sysfs directory entries we ensure that we don't have conflicts
    +in the directories and applications only see a limited set of
    +the network devices.
    +
    +Each sysfs directory entry may be tagged with zero or one
    +namespaces. A sysfs_dirent is augmented with a void *s_ns. If a
    +directory entry is tagged, then sysfs_dirent->s_flags will have a
    +flag between KOBJ_NS_TYPE_NONE and KOBJ_NS_TYPES, and s_ns will
    +point to the namespace to which it belongs.
    +
    +Each sysfs superblock's sysfs_super_info contains an array void
    +*ns[KOBJ_NS_TYPES]. When a a task in a tagging namespace
    +kobj_nstype first mounts sysfs, a new superblock is created. It
    +will be differentiated from other sysfs mounts by having its
    +s_fs_info->ns[kobj_nstype] set to the new namespace. Note that
    +through bind mounting and mounts propagation, a task can easily view
    +the contents of other namespaces' sysfs mounts. Therefore, when a
    +namespace exits, it will call kobj_ns_exit() to invalidate any
    +sysfs_dirent->s_ns pointers pointing to it.
    +
    +Users of this interface:
    +- define a type in the kobj_ns_type enumeration.
    +- call kobj_ns_type_register() with its kobj_ns_type_operations which has
    + - current_ns() which returns current's namespace
    + - netlink_ns() which returns a socket's namespace
    + - initial_ns() which returns the initial namesapce
    +- call kobj_ns_exit() when an individual tag is no longer valid
    --
    1.7.0.3


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