[PATCH v9 3/3] Documentation: add docs for /proc/pidns_hierarchy

[Chen Hanxiao]

Signed-off-by: Chen Hanxiao <chenhanxiao@xxxxxxxxxxxxxx>
---
 Documentation/namespaces/pidns-hierarchy.txt | 51 ++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)
 create mode 100644 Documentation/namespaces/pidns-hierarchy.txt

diff --git a/Documentation/namespaces/pidns-hierarchy.txt b/Documentation/namespaces/pidns-hierarchy.txt
new file mode 100644
index 0000000..feb92a9
--- /dev/null
+++ b/Documentation/namespaces/pidns-hierarchy.txt
@@ -0,0 +1,51 @@
+This document is about how to use pid namespace hierarchy procfs.
+
+We knew whether two pids living in the same pid namespace
+by /proc/PID/ns/pid, but their relationships
+between pids were unknown:
+we couldn't tell that one pid was another one's parent/siblings...
+But /proc/pidns_hierarchy could tell us the answer.
+
+/proc/pidns_hierarchy will show the hierarchy of pid namespace
+in the form of:
+
+<init_PID> <parent_of_init_PID> <relative PID level>
+
+init_PID:            child reaper in a pid namespace
+parent_of_init_PID:  init_PID's parent, child reaper too
+relative PID level:  pid level relative to caller's ns,
+                     started from '1'.
+
+Here is a chart to describe the relationship between
+some pids:
+
+         init_pid_ns                          level 0
+              |
+              1
+              |
+ââââââââââââââ
+ns1                      ns2                  level 1
+|                         |
+1550                    18060
+                          |
+                          |
+                         ns3                  level 2
+                          |
+                        18102
+                          |
+                 ââââââââââââ
+                 ns4                   ns5    level 3
+                 |                     |
+                1534                  1600
+
+It will be showed by /proc/pidns_hierarchy as below:
+
+#cat /proc/pidns_hierarchy
+18060 1 1
+18102 18060 2
+1534  18102 3
+1600  18102 3
+1550  1 1
+
+Note: numbers in column 1 are pid numbers in current ns,
+    they represent the pid '1' in different ns
-- 
1.9.3

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