On Thu 08-08-19 12:04:07, Jason Gunthorpe wrote: > On Thu, Aug 08, 2019 at 10:18:27AM +0200, Michal Hocko wrote: > > On Wed 07-08-19 19:16:32, Jason Gunthorpe wrote: > > > Many users of the mmu_notifier invalidate_range callbacks maintain > > > locking/counters/etc on a paired basis and have long expected that > > > invalidate_range start/end are always paired. > > > > > > The recent change to add non-blocking notifiers breaks this assumption > > > when multiple notifiers are present in the list as an EAGAIN return from a > > > later notifier causes all earlier notifiers to get their > > > invalidate_range_end() skipped. > > > > > > During the development of non-blocking each user was audited to be sure > > > they can skip their invalidate_range_end() if their start returns -EAGAIN, > > > so the only place that has a problem is when there are multiple > > > subscriptions. > > > > > > Due to the RCU locking we can't reliably generate a subset of the linked > > > list representing the notifiers already called, and generate an > > > invalidate_range_end() pairing. > > > > > > Rather than design an elaborate fix, for now, just block non-blocking > > > requests early on if there are multiple subscriptions. > > > > Which means that the oom path cannot really release any memory for > > ranges covered by these notifiers which is really unfortunate because > > that might cover a lot of memory. Especially when the particular range > > might not be tracked at all, right? > > Yes, it is a very big hammer to avoid a bug where the locking schemes > get corrupted and the impacted drivers deadlock. > > If you really don't like it then we have to push ahead on either an > rcu-safe undo algorithm or some locking thing. I've been looking at > the locking thing, so we can wait a bit more and see. Well, I do not like it but I understand that an over reaction for OOM is much less of a pain than a deadlock or similar misbehavior. So go ahead with this as a stop gap with Cc: stable but please let's do not stop there and let's come up with something of a less hamery kind. That being said, feel free to add Acked-by: Michal Hocko <mhocko@xxxxxxxx> with a printk_once to explain what is going on and a TODO note that this is just a stop gap. Thanks! -- Michal Hocko SUSE Labs
