[RFC PATCH] touch_mnt_namespace when the mount flags change

Daemons that need to be launched while the rootfs is read-only can now
poll /proc/mounts to be notified when their O_RDWR requests may no
longer end in EROFS.

Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: Neil Brown <neilb@suse.de>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>

fs/namespace.c

@@ -1550,8 +1550,13 @@ static int do_remount(struct path *path, int flags, int mnt_flags,
 	if (!err)
 		path->mnt->mnt_flags = mnt_flags;
 	up_write(&sb->s_umount);
-	if (!err)
+	if (!err) {
 		security_sb_post_remount(path->mnt, flags, data);
+
+		spin_lock(&vfsmount_lock);
+		touch_mnt_namespace(path->mnt->mnt_ns);
+		spin_unlock(&vfsmount_lock);
+	}
 	return err;
 }