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

[Chao Peng]

On Tue, Mar 29, 2022 at 06:45:16PM +0000, Sean Christopherson wrote:
> 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?

Yes it's mandatory, will skip the check here.

> 
> > +			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.

The only currently supported backing store shmem does not need this so
can remove it for now.

> 
> > +}
> > +
> > +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.

Yes, I can change.

> 
> > +			      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.

Agreed, good suggestion.

> 
> 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;
>   }