setup_mnt(): primitive for connecting a mount to filesystem

Take the identical logics in vfs_create_mount() and clone_mnt() into
a new helper that takes an empty struct mount and attaches it to
given dentry (sub)tree.

Should be called once in the lifetime of every mount, prior to making
it visible in any data structures.

After that point ->mnt_root and ->mnt_sb never change; ->mnt_root
is a counting reference to dentry and ->mnt_sb - an active reference
to superblock.

Mount remains associated with that dentry tree all the way until
the call of cleanup_mnt(), when the refcount eventually drops
to zero.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Author: Al Viro

fs/namespace.c

@@ -1196,6 +1196,21 @@ static void commit_tree(struct mount *mnt)
 	touch_mnt_namespace(n);
 }
 
+static void setup_mnt(struct mount *m, struct dentry *root)
+{
+	struct super_block *s = root->d_sb;
+
+	atomic_inc(&s->s_active);
+	m->mnt.mnt_sb = s;
+	m->mnt.mnt_root = dget(root);
+	m->mnt_mountpoint = m->mnt.mnt_root;
+	m->mnt_parent = m;
+
+	lock_mount_hash();
+	list_add_tail(&m->mnt_instance, &s->s_mounts);
+	unlock_mount_hash();
+}
+
 /**
  * vfs_create_mount - Create a mount for a configured superblock
  * @fc: The configuration context with the superblock attached
@@ -1219,15 +1234,8 @@ struct vfsmount *vfs_create_mount(struct fs_context *fc)
 	if (fc->sb_flags & SB_KERNMOUNT)
 		mnt->mnt.mnt_flags = MNT_INTERNAL;
 
-	atomic_inc(&fc->root->d_sb->s_active);
-	mnt->mnt.mnt_sb		= fc->root->d_sb;
-	mnt->mnt.mnt_root	= dget(fc->root);
-	mnt->mnt_mountpoint	= mnt->mnt.mnt_root;
-	mnt->mnt_parent		= mnt;
+	setup_mnt(mnt, fc->root);
 
-	lock_mount_hash();
-	list_add_tail(&mnt->mnt_instance, &mnt->mnt.mnt_sb->s_mounts);
-	unlock_mount_hash();
 	return &mnt->mnt;
 }
 EXPORT_SYMBOL(vfs_create_mount);
@@ -1285,7 +1293,6 @@ EXPORT_SYMBOL_GPL(vfs_kern_mount);
 static struct mount *clone_mnt(struct mount *old, struct dentry *root,
 					int flag)
 {
-	struct super_block *sb = old->mnt.mnt_sb;
 	struct mount *mnt;
 	int err;
 
@@ -1310,16 +1317,9 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root,
 	if (mnt->mnt_group_id)
 		set_mnt_shared(mnt);
 
-	atomic_inc(&sb->s_active);
 	mnt->mnt.mnt_idmap = mnt_idmap_get(mnt_idmap(&old->mnt));
 
-	mnt->mnt.mnt_sb = sb;
-	mnt->mnt.mnt_root = dget(root);
-	mnt->mnt_mountpoint = mnt->mnt.mnt_root;
-	mnt->mnt_parent = mnt;
-	lock_mount_hash();
-	list_add_tail(&mnt->mnt_instance, &sb->s_mounts);
-	unlock_mount_hash();
+	setup_mnt(mnt, root);
 
 	if (flag & CL_PRIVATE)	// we are done with it
 		return mnt;