Re: [PATCH v5 02/13] mm: Introduce memfile_notifier

[Sean Christopherson <seanjc@google.com>]

On Thu, Mar 10, 2022, Chao Peng wrote:
> diff --git a/mm/Makefile b/mm/Makefile
> index 70d4309c9ce3..f628256dce0d 100644
> +void memfile_notifier_invalidate(struct memfile_notifier_list *list,
> +				 pgoff_t start, pgoff_t end)
> +{
> +	struct memfile_notifier *notifier;
> +	int id;
> +
> +	id = srcu_read_lock(&srcu);
> +	list_for_each_entry_srcu(notifier, &list->head, list,
> +				 srcu_read_lock_held(&srcu)) {
> +		if (notifier->ops && notifier->ops->invalidate)

Any reason notifier->ops isn't mandatory?

> +			notifier->ops->invalidate(notifier, start, end);
> +	}
> +	srcu_read_unlock(&srcu, id);
> +}
> +
> +void memfile_notifier_fallocate(struct memfile_notifier_list *list,
> +				pgoff_t start, pgoff_t end)
> +{
> +	struct memfile_notifier *notifier;
> +	int id;
> +
> +	id = srcu_read_lock(&srcu);
> +	list_for_each_entry_srcu(notifier, &list->head, list,
> +				 srcu_read_lock_held(&srcu)) {
> +		if (notifier->ops && notifier->ops->fallocate)
> +			notifier->ops->fallocate(notifier, start, end);
> +	}
> +	srcu_read_unlock(&srcu, id);
> +}
> +
> +void memfile_register_backing_store(struct memfile_backing_store *bs)
> +{
> +	BUG_ON(!bs || !bs->get_notifier_list);
> +
> +	list_add_tail(&bs->list, &backing_store_list);
> +}
> +
> +void memfile_unregister_backing_store(struct memfile_backing_store *bs)
> +{
> +	list_del(&bs->list);

Allowing unregistration of a backing store is broken.  Using the _safe() variant
is not sufficient to guard against concurrent modification.  I don't see any reason
to support this out of the gate, the only reason to support unregistering a backing
store is if the backing store is implemented as a module, and AFAIK none of the
backing stores we plan on supporting initially support being built as a module.
These aren't exported, so it's not like that's even possible.  Registration would
also be broken if modules are allowed, I'm pretty sure module init doesn't run
under a global lock.

We can always add this complexity if it's needed in the future, but for now the
easiest thing would be to tag memfile_register_backing_store() with __init and
make backing_store_list __ro_after_init.

> +}
> +
> +static int memfile_get_notifier_info(struct inode *inode,
> +				     struct memfile_notifier_list **list,
> +				     struct memfile_pfn_ops **ops)
> +{
> +	struct memfile_backing_store *bs, *iter;
> +	struct memfile_notifier_list *tmp;
> +
> +	list_for_each_entry_safe(bs, iter, &backing_store_list, list) {
> +		tmp = bs->get_notifier_list(inode);
> +		if (tmp) {
> +			*list = tmp;
> +			if (ops)
> +				*ops = &bs->pfn_ops;
> +			return 0;
> +		}
> +	}
> +	return -EOPNOTSUPP;
> +}
> +
> +int memfile_register_notifier(struct inode *inode,

Taking an inode is a bit odd from a user perspective.  Any reason not to take a
"struct file *" and get the inode here?  That would give callers a hint that they
need to hold a reference to the file for the lifetime of the registration.

> +			      struct memfile_notifier *notifier,
> +			      struct memfile_pfn_ops **pfn_ops)
> +{
> +	struct memfile_notifier_list *list;
> +	int ret;
> +
> +	if (!inode || !notifier | !pfn_ops)

Bitwise | instead of logical ||.  But IMO taking in a pfn_ops pointer is silly.
More below.

> +		return -EINVAL;
> +
> +	ret = memfile_get_notifier_info(inode, &list, pfn_ops);
> +	if (ret)
> +		return ret;
> +
> +	spin_lock(&list->lock);
> +	list_add_rcu(&notifier->list, &list->head);
> +	spin_unlock(&list->lock);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(memfile_register_notifier);
> +
> +void memfile_unregister_notifier(struct inode *inode,
> +				 struct memfile_notifier *notifier)
> +{
> +	struct memfile_notifier_list *list;
> +
> +	if (!inode || !notifier)
> +		return;
> +
> +	BUG_ON(memfile_get_notifier_info(inode, &list, NULL));

Eww.  Rather than force the caller to provide the inode/file and the notifier,
what about grabbing the backing store itself in the notifier?

	struct memfile_notifier {
		struct list_head list;
		struct memfile_notifier_ops *ops;

		struct memfile_backing_store *bs;
	};

That also helps avoid confusing between "ops" and "pfn_ops".  IMO, exposing
memfile_backing_store to the caller isn't a big deal, and is preferable to having
to rewalk multiple lists just to delete a notifier.

Then this can become:

  void memfile_unregister_notifier(struct memfile_notifier *notifier)
  {
	spin_lock(&notifier->bs->list->lock);
	list_del_rcu(&notifier->list);
	spin_unlock(&notifier->bs->list->lock);

	synchronize_srcu(&srcu);
  }

and registration can be:

  int memfile_register_notifier(const struct file *file,
			      struct memfile_notifier *notifier)
  {
	struct memfile_notifier_list *list;
	struct memfile_backing_store *bs;
	int ret;

	if (!file || !notifier)
		return -EINVAL;

	list_for_each_entry(bs, &backing_store_list, list) {
		list = bs->get_notifier_list(file_inode(file));
		if (list) {
			notifier->bs = bs;

			spin_lock(&list->lock);
			list_add_rcu(&notifier->list, &list->head);
			spin_unlock(&list->lock);
			return 0;
		}
	}

	return -EOPNOTSUPP;
  }