vfs: Add a function to lazily unmount all mounts from any dentry.

The new function detach_mounts comes in two pieces.  The first piece
is a static inline test of d_mounpoint that returns immediately
without taking any locks if d_mounpoint is not set.  In the common
case when mountpoints are absent this allows the vfs to continue
running with it's same cacheline foot print.

The second piece of detach_mounts __detach_mounts actually does the
work and it assumes that a mountpoint is present so it is slow and
takes namespace_sem for write, and then locks the mount hash (aka
mount_lock) after a struct mountpoint has been found.

With those two locks held each entry on the list of mounts on a
mountpoint is selected and lazily unmounted until all of the mount
have been lazily unmounted.

v7: Wrote a proper change description and removed the changelog
    documenting deleted wrong turns.

Signed-off-by: Eric W. Biederman <ebiederman@twitter.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

2 files changed, 40 insertions, 0 deletions

diff --git a/fs/mount.h b/fs/mount.h
index 68bb03ed7f19..f82c62840905 100644
--- a/fs/mount.h
+++ b/fs/mount.h
@@ -87,6 +87,15 @@ extern struct mount *__lookup_mnt_last(struct vfsmount *, struct dentry *);
 
 extern bool legitimize_mnt(struct vfsmount *, unsigned);
 
+extern void __detach_mounts(struct dentry *dentry);
+
+static inline void detach_mounts(struct dentry *dentry)
+{
+        if (!d_mountpoint(dentry))
+                return;
+        __detach_mounts(dentry);
+}
+
 static inline void get_mnt_ns(struct mnt_namespace *ns)
 {
         atomic_inc(&ns->count);
diff --git a/fs/namespace.c b/fs/namespace.c
index 88fc3f4d77ed..00e5b1efa591 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1468,6 +1468,37 @@ static int do_umount(struct mount *mnt, int flags)
         return retval;
 }
 
+/*
+ * __detach_mounts - lazily unmount all mounts on the specified dentry
+ *
+ * During unlink, rmdir, and d_drop it is possible to loose the path
+ * to an existing mountpoint, and wind up leaking the mount.
+ * detach_mounts allows lazily unmounting those mounts instead of
+ * leaking them.
+ *
+ * The caller may hold dentry->d_inode->i_mutex.
+ */
+void __detach_mounts(struct dentry *dentry)
+{
+        struct mountpoint *mp;
+        struct mount *mnt;
+
+        namespace_lock();
+        mp = lookup_mountpoint(dentry);
+        if (!mp)
+                goto out_unlock;
+
+        lock_mount_hash();
+        while (!hlist_empty(&mp->m_list)) {
+                mnt = hlist_entry(mp->m_list.first, struct mount, mnt_mp_list);
+                umount_tree(mnt, 2);
+        }
+        unlock_mount_hash();
+        put_mountpoint(mp);
+out_unlock:
+        namespace_unlock();
+}
+
 /* 
  * Is the caller allowed to modify his namespace?
  */