On Wed, Mar 10, 2021, Jason Gunthorpe wrote: > On Wed, Mar 10, 2021 at 01:31:17PM -0800, Sean Christopherson wrote: > > Invoke the MMU notifier's .invalidate_range_end() callbacks even if one > > of the .invalidate_range_start() callbacks failed. If there are multiple > > notifiers, the notifier that did not fail may have performed actions in > > its ...start() that it expects to unwind via ...end(). Per the > > mmu_notifier_ops documentation, ...start() and ...end() must be paired. > > No this is not OK, if invalidate_start returns EBUSY invalidate_end > should *not* be called. > > As you observed: > > > The only in-kernel usage that is fatally broken is the SGI UV GRU driver, > > which effectively blocks and sleeps fault handlers during ...start(), and > > unblocks/wakes the handlers during ...end(). But, the only users that > > can fail ...start() are the i915 and Nouveau drivers, which are unlikely > > to collide with the SGI driver. > > It used to be worse but I've since moved most of the other problematic > users to the itree notifier which doesn't have the problem. > > > KVM is the only other user of ...end(), and while KVM also blocks fault > > handlers in ...start(), the fault handlers do not sleep and originate in > > KVM will have its mmu_notifier_count become imbalanced: > > static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn, > const struct mmu_notifier_range *range) > { > kvm->mmu_notifier_count++; > > static void kvm_mmu_notifier_invalidate_range_end(struct mmu_notifier *mn, > const struct mmu_notifier_range *range) > { > kvm->mmu_notifier_count--; > > Which I believe is fatal to kvm? These notifiers certainly do not only > happen at process exit. My point about the process dying is that the existing bug that causes mmu_notifier_count to become imbalanced is benign only because the process is being killed, and thus KVM will stop running its vCPUs. > So, both of the remaining _end users become corrupted with this patch! I don't follow. mn_hlist_invalidate_range_start() iterates over all notifiers, even if a notifier earlier in the chain failed. How will KVM become imbalanced? The existing _end users never fail their _start. If KVM started failing its start, then yes, it could get corrupted. But my assumption/expection is that, if KVM were to ever reject _start, it would be responsible for knowing that it must also skip _end. I'm happy to kick that one down the road though, as I can't think of a scenario where KVM would _need_ to sleep. > I've tried to fix this before, the only thing that seems like it will > work is to sort the hlist and only call ends that have succeeded their > starts by comparing pointers with <. > > This is because the hlist can have items removed concurrently under > SRCU so there is no easy way to compute the subset that succeeded in > calling start. > > I had a prior effort to just ban more than 1 hlist notifier with end, > but it turns out kvm on ARM uses two all the time (IIRC) > > > Found by inspection. Verified by adding a second notifier in KVM > > that > > AFAIK it is a non-problem in real life because kvm is not mixed with > notifier_start's that fail (and GRU is dead?). Everything else was > fixed by moving to itree. > > Jason