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;
>   }