4 files changed, 65 insertions, 2 deletions

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index 2cd920f92e5e..2a99116edc47 100644
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -2374,8 +2374,14 @@ possible optional fields are:
 
 shared:X  mount is shared in peer group X
 master:X  mount is slave to peer group X
+propagate_from:X  mount is slave and receives propagation from peer group X (*)
 unbindable  mount is unbindable
 
+(*) X is the closest dominant peer group under the process's root.  If
+X is the immediate master of the mount, or if there's no dominant peer
+group under the same root, then only the "master:X" field is present
+and not the "propagate_from:X" field.
+
 For more information on mount propagation see:
 
   Documentation/filesystems/sharedsubtree.txt
diff --git a/fs/namespace.c b/fs/namespace.c
index c807b8d5f891..0505fb61aa74 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -850,8 +850,13 @@ static int show_mountinfo(struct seq_file *m, void *v)
         /* Tagged fields ("foo:X" or "bar") */
         if (IS_MNT_SHARED(mnt))
                 seq_printf(m, " shared:%i", mnt->mnt_group_id);
-        if (IS_MNT_SLAVE(mnt))
-                seq_printf(m, " master:%i", mnt->mnt_master->mnt_group_id);
+        if (IS_MNT_SLAVE(mnt)) {
+                int master = mnt->mnt_master->mnt_group_id;
+                int dom = get_dominating_id(mnt, &p->root);
+                seq_printf(m, " master:%i", master);
+                if (dom && dom != master)
+                        seq_printf(m, " propagate_from:%i", dom);
+        }
         if (IS_MNT_UNBINDABLE(mnt))
                 seq_puts(m, " unbindable");
 
diff --git a/fs/pnode.c b/fs/pnode.c
index d18d66491a01..8d5f392ec3d3 100644
--- a/fs/pnode.c
+++ b/fs/pnode.c
@@ -28,6 +28,57 @@ static inline struct vfsmount *next_slave(struct vfsmount *p)
         return list_entry(p->mnt_slave.next, struct vfsmount, mnt_slave);
 }
 
+/*
+ * Return true if path is reachable from root
+ *
+ * namespace_sem is held, and mnt is attached
+ */
+static bool is_path_reachable(struct vfsmount *mnt, struct dentry *dentry,
+                         const struct path *root)
+{
+        while (mnt != root->mnt && mnt->mnt_parent != mnt) {
+                dentry = mnt->mnt_mountpoint;
+                mnt = mnt->mnt_parent;
+        }
+        return mnt == root->mnt && is_subdir(dentry, root->dentry);
+}
+
+static struct vfsmount *get_peer_under_root(struct vfsmount *mnt,
+                                            struct mnt_namespace *ns,
+                                            const struct path *root)
+{
+        struct vfsmount *m = mnt;
+
+        do {
+                /* Check the namespace first for optimization */
+                if (m->mnt_ns == ns && is_path_reachable(m, m->mnt_root, root))
+                        return m;
+
+                m = next_peer(m);
+        } while (m != mnt);
+
+        return NULL;
+}
+
+/*
+ * Get ID of closest dominating peer group having a representative
+ * under the given root.
+ *
+ * Caller must hold namespace_sem
+ */
+int get_dominating_id(struct vfsmount *mnt, const struct path *root)
+{
+        struct vfsmount *m;
+
+        for (m = mnt->mnt_master; m != NULL; m = m->mnt_master) {
+                struct vfsmount *d = get_peer_under_root(m, mnt->mnt_ns, root);
+                if (d)
+                        return d->mnt_group_id;
+        }
+
+        return 0;
+}
+
 static int do_make_slave(struct vfsmount *mnt)
 {
         struct vfsmount *peer_mnt = mnt, *master = mnt->mnt_master;
diff --git a/fs/pnode.h b/fs/pnode.h
index 973c3f825e7d..958665d662af 100644
--- a/fs/pnode.h
+++ b/fs/pnode.h
@@ -36,4 +36,5 @@ int propagate_mnt(struct vfsmount *, struct dentry *, struct vfsmount *,
 int propagate_umount(struct list_head *);
 int propagate_mount_busy(struct vfsmount *, int);
 void mnt_release_group_id(struct vfsmount *);
+int get_dominating_id(struct vfsmount *mnt, const struct path *root);
 #endif /* _LINUX_PNODE_H */