Unverified Commit b73ec10a authored by Christian Brauner's avatar Christian Brauner
Browse files

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: default avatarChristian Brauner <brauner@kernel.org>
parent 043bc81e

fs/namespace.c

+47 −12
Original line number Diff line number Diff line
@@ -2246,21 +2246,56 @@ 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;

	}



	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();

	namespace_unlock();

	if (ns)

	}



	/* Make sure we notice when we leak mounts. */

	VFS_WARN_ON_ONCE(!mnt_ns_empty(ns));

	free_mnt_ns(ns);

}