[PATCH RFC v3 24/26] fs: start all kthreads in nullfs

Christian Brauner
2026-03-11T21:57:10Z

Point init_task's fs_struct (root and pwd) at a private nullfs instance
instead of the mutable rootfs. All kthreads now start isolated in nullfs
and must use scoped_with_init_fs() for any path resolution.

PID 1 is moved from nullfs into the initramfs by init_userspace_fs().
Usermodehelper threads use userspace_init_fs via the umh flag in
copy_fs(). All subsystems that need init's filesystem state for path
resolution already use scoped_with_init_fs() from earlier commits in
this series.

This isolates kthreads from userspace filesystem state and makes it
hard to perform filesystem operations from kthread context.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
fs/namespace.c | 16 ++++++++++------
1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index e23d2fa7e255..5d318e2e1e4a 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -6143,12 +6143,14 @@ static void __init init_mount_tree(void)
struct path root;

/*
- * We create two mounts:
+ * We create three mounts:
*
* (1) nullfs with mount id 1
* (2) mutable rootfs with mount id 2
+ * (3) private nullfs for kthreads (SB_KERNMOUNT)
*
- * with (2) mounted on top of (1).
+ * with (2) mounted on top of (1). The init_task's root and pwd
+ * are pointed at (3) so all kthreads start isolated in nullfs.
*/
nullfs_mnt = vfs_kern_mount(&nullfs_fs_type, 0, "nullfs", NULL);
if (IS_ERR(nullfs_mnt))
@@ -6188,12 +6190,14 @@ static void __init init_mount_tree(void)
init_mnt_ns.nr_mounts++;
}

+ nullfs_mnt = kern_mount(&nullfs_fs_type);
+ if (IS_ERR(nullfs_mnt))
+ panic("VFS: Failed to create private nullfs instance");
+ root.mnt = nullfs_mnt;
+ root.dentry = nullfs_mnt->mnt_root;
+
init_task.nsproxy->mnt_ns = &init_mnt_ns;
get_mnt_ns(&init_mnt_ns);
-
- /* The root and pwd always point to the mutable rootfs. */
- root.mnt = mnt;
- root.dentry = mnt->mnt_root;
set_fs_pwd(current->fs, &root);
set_fs_root(current->fs, &root);

--
2.47.3