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fs: start all kthreads in nullfs

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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.

Link: https://patch.msgid.link/20260601-work-kthread-nullfs-v4-23-77ee053060e0@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
This commit is contained in:
Christian Brauner
2026-06-29 10:54:43 +02:00
parent e435696d57
commit 32750c77e8
1 changed files with 10 additions and 6 deletions
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fs/namespace.c
+10 -6
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@@ -6184,12 +6184,14 @@ 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))
@@ -6229,12 +6231,14 @@ 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);