VFS: Clean up shared mount flag propagation

The handling of mount flags in set_mnt_shared() got a little tangled
up during previous cleanups, with the following problems:

* MNT_PNODE_MASK is defined as a literal constant when it should be a
bitwise xor of other MNT_* flags
* set_mnt_shared() clears and then sets MNT_SHARED (part of MNT_PNODE_MASK)
* MNT_PNODE_MASK could use a comment in mount.h
* MNT_PNODE_MASK is a terrible name, change to MNT_SHARED_MASK

This patch fixes these problems.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namespace.c

@@ -1538,7 +1538,7 @@ static int do_remount(struct path *path, int flags, int mnt_flags,
 		err = do_remount_sb(sb, flags, data, 0);
 	if (!err) {
 		spin_lock(&vfsmount_lock);
-		mnt_flags |= path->mnt->mnt_flags & MNT_PNODE_MASK;
+		mnt_flags |= path->mnt->mnt_flags & MNT_PROPAGATION_MASK;
 		path->mnt->mnt_flags = mnt_flags;
 		spin_unlock(&vfsmount_lock);
 	}

fs/pnode.h

@@ -25,7 +25,7 @@
 
 static inline void set_mnt_shared(struct vfsmount *mnt)
 {
-	mnt->mnt_flags &= ~MNT_PNODE_MASK;
+	mnt->mnt_flags &= ~MNT_SHARED_MASK;
 	mnt->mnt_flags |= MNT_SHARED;
 }
 

include/linux/mount.h

@@ -34,7 +34,16 @@ struct mnt_namespace;
 
 #define MNT_SHARED	0x1000	/* if the vfsmount is a shared mount */
 #define MNT_UNBINDABLE	0x2000	/* if the vfsmount is a unbindable mount */
-#define MNT_PNODE_MASK	0x3000	/* propagation flag mask */
+/*
+ * MNT_SHARED_MASK is the set of flags that should be cleared when a
+ * mount becomes shared.  Currently, this is only the flag that says a
+ * mount cannot be bind mounted, since this is how we create a mount
+ * that shares events with another mount.  If you add a new MNT_*
+ * flag, consider how it interacts with shared mounts.
+ */
+#define MNT_SHARED_MASK	(MNT_UNBINDABLE)
+#define MNT_PROPAGATION_MASK	(MNT_SHARED | MNT_UNBINDABLE)
+
 
 struct vfsmount {
 	struct list_head mnt_hash;