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authorSerge E. Hallyn <serue@us.ibm.com>2010-05-04 21:45:38 -0500
committerGreg Kroah-Hartman <gregkh@suse.de>2010-05-21 09:37:31 -0700
commitb9d8b45ee3c5f62cdbd34ee006f3dd2ac51f7018 (patch)
tree9e941a38200988eb7721517fa41322518034114a /Documentation/filesystems
parentbe867b194a3ae3c680c29521287ae49b4d44d420 (diff)
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sysfs-namespaces: add a high-level Documentation file
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>
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diff --git a/Documentation/filesystems/sysfs-tagging.txt b/Documentation/filesystems/sysfs-tagging.txt
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+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
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