+static struct file *restrictedmem_file_create(struct file *memfd)
+{
+	struct restrictedmem_data *data;
+	struct address_space *mapping;
+	struct inode *inode;
+	struct file *file;
+
+	data = kzalloc(sizeof(*data), GFP_KERNEL);
+	if (!data)
+		return ERR_PTR(-ENOMEM);
+
+	data->memfd = memfd;
+	mutex_init(&data->lock);
+	INIT_LIST_HEAD(&data->notifiers);
+
+	inode = alloc_anon_inode(restrictedmem_mnt->mnt_sb);
+	if (IS_ERR(inode)) {
+		kfree(data);
+		return ERR_CAST(inode);
+	}

alloc_anon_inode() uses new_pseudo_inode() to get the inode. As per the comment, new inode 
is not added to the superblock s_inodes list.

/**
 *	new_inode_pseudo 	- obtain an inode
 *	@sb: superblock
 *
 *	Allocates a new inode for given superblock.
 *	Inode wont be chained in superblock s_inodes list
 *	This means :
 *	- fs can't be unmount
 *	- quotas, fsnotify, writeback can't work
 */

So the restrictedmem_error_page will not find the inode as it was never added to the s_inodes list.

We might need to add the inode after allocating.

	inode_sb_list_add(inode);

+void restrictedmem_error_page(struct page *page, struct address_space *mapping)
+{
+	struct super_block *sb = restrictedmem_mnt->mnt_sb;
+	struct inode *inode, *next;
+
+	if (!shmem_mapping(mapping))
+		return;
+
+	spin_lock(&sb->s_inode_list_lock);
+	list_for_each_entry_safe(inode, next, &sb->s_inodes, i_sb_list) {
+		struct restrictedmem_data *data = inode->i_mapping->private_data;
+		struct file *memfd = data->memfd;
+
+		if (memfd->f_mapping == mapping) {
+			pgoff_t start, end;
+
+			spin_unlock(&sb->s_inode_list_lock);
+
+			start = page->index;
+			end = start + thp_nr_pages(page);
+			restrictedmem_notifier_error(data, start, end);
+			return;
+		}
+	}
+	spin_unlock(&sb->s_inode_list_lock);
+}

Regards
Nikunj