fs: add fastpath for dissolve_on_fput()

Instead of acquiring the namespace semaphore and the mount lock
everytime we close a file with FMODE_NEED_UNMOUNT set add a fastpath
that checks whether we need to at all. Most of the time the caller will
have attached the mount to the filesystem hierarchy and there's nothing
to do.

Link: https://lore.kernel.org/r/20250221-brauner-open_tree-v1-4-dbcfcb98c676@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/namespace.c b/fs/namespace.c
index c9670a198b507f47b7bb1077d3d55e29441021fa..8cb15539aaaf0e8c4b728e41f63f2e0f3e97ca50 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2246,22 +2246,57 @@ struct vfsmount *collect_mounts(const struct path *path)
 static void free_mnt_ns(struct mnt_namespace *);
 static struct mnt_namespace *alloc_mnt_ns(struct user_namespace *, bool);
 
+static inline bool must_dissolve(struct mnt_namespace *mnt_ns)
+{
+       /*
+        * This mount belonged to an anonymous mount namespace
+        * but was moved to a non-anonymous mount namespace and
+        * then unmounted.
+        */
+       if (unlikely(!mnt_ns))
+               return false;
+
+       /*
+        * This mount belongs to a non-anonymous mount namespace
+        * and we know that such a mount can never transition to
+        * an anonymous mount namespace again.
+        */
+       if (!is_anon_ns(mnt_ns)) {
+               /*
+                * A detached mount either belongs to an anonymous mount
+                * namespace or a non-anonymous mount namespace. It
+                * should never belong to something purely internal.
+                */
+               VFS_WARN_ON_ONCE(mnt_ns == MNT_NS_INTERNAL);
+               return false;
+       }
+
+       return true;
+}
+
 void dissolve_on_fput(struct vfsmount *mnt)
 {
        struct mnt_namespace *ns;
-       namespace_lock();
-       lock_mount_hash();
-       ns = real_mount(mnt)->mnt_ns;
-       if (ns) {
-               if (is_anon_ns(ns))
-                       umount_tree(real_mount(mnt), UMOUNT_CONNECTED);
-               else
-                       ns = NULL;
+       struct mount *m = real_mount(mnt);
+
+       scoped_guard(rcu) {
+               if (!must_dissolve(READ_ONCE(m->mnt_ns)))
+                       return;
        }
-       unlock_mount_hash();
-       namespace_unlock();
-       if (ns)
-               free_mnt_ns(ns);
+
+       scoped_guard(rwsem_write, &namespace_sem) {
+               ns = m->mnt_ns;
+               if (!must_dissolve(ns))
+                       return;
+
+               lock_mount_hash();
+               umount_tree(m, UMOUNT_CONNECTED);
+               unlock_mount_hash();
+       }
+
+       /* Make sure we notice when we leak mounts. */
+       VFS_WARN_ON_ONCE(!mnt_ns_empty(ns));
+       free_mnt_ns(ns);
 }
 
 void drop_collected_mounts(struct vfsmount *mnt)