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(¬ifier->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(¬ifier->bs->list->lock);
> list_del_rcu(¬ifier->list);
> spin_unlock(¬ifier->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(¬ifier->list, &list->head);
> spin_unlock(&list->lock);
> return 0;
> }
> }
>
> return -EOPNOTSUPP;
> }
