mm/mmu_notifiers: ensure range_end() is paired with range_start()

If one or more notifiers fails .invalidate_range_start(), invoke
.invalidate_range_end() for "all" notifiers.  If there are multiple
notifiers, those that did not fail are expecting _start() and _end() to
be paired, e.g.  KVM's mmu_notifier_count would become imbalanced.
Disallow notifiers that can fail _start() from implementing _end() so
that it's unnecessary to either track which notifiers rejected _start(),
or had already succeeded prior to a failed _start().

Note, the existing behavior of calling _start() on all notifiers even
after a previous notifier failed _start() was an unintented "feature".
Make it canon now that the behavior is depended on for correctness.

As of today, the bug is likely benign:

  1. The only caller of the non-blocking notifier is OOM kill.
  2. The only notifiers that can fail _start() are the i915 and Nouveau
     drivers.
  3. The only notifiers that utilize _end() are the SGI UV GRU driver
     and KVM.
  4. The GRU driver will never coincide with the i195/Nouveau drivers.
  5. An imbalanced kvm->mmu_notifier_count only causes soft lockup in the
     _guest_, and the guest is already doomed due to being an OOM victim.

Fix the bug now to play nice with future usage, e.g.  KVM has a
potential use case for blocking memslot updates in KVM while an
invalidation is in-progress, and failure to unblock would result in said
updates being blocked indefinitely and hanging.

Found by inspection.  Verified by adding a second notifier in KVM that
periodically returns -EAGAIN on non-blockable ranges, triggering OOM,
and observing that KVM exits with an elevated notifier count.

Link: https://lkml.kernel.org/r/20210311180057.1582638-1-seanjc@google.com
Fixes: 93065ac753 ("mm, oom: distinguish blockable mode for mmu notifiers")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Suggested-by: Jason Gunthorpe <jgg@ziepe.ca>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Ben Gardon <bgardon@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: "Jérôme Glisse" <jglisse@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Dimitri Sivanich <dimitri.sivanich@hpe.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mmu_notifier.h

@@ -169,11 +169,11 @@ struct mmu_notifier_ops {
 	 * the last refcount is dropped.
 	 *
 	 * If blockable argument is set to false then the callback cannot
-	 * sleep and has to return with -EAGAIN. 0 should be returned
-	 * otherwise. Please note that if invalidate_range_start approves
-	 * a non-blocking behavior then the same applies to
-	 * invalidate_range_end.
-	 *
+	 * sleep and has to return with -EAGAIN if sleeping would be required.
+	 * 0 should be returned otherwise. Please note that notifiers that can
+	 * fail invalidate_range_start are not allowed to implement
+	 * invalidate_range_end, as there is no mechanism for informing the
+	 * notifier that its start failed.
 	 */
 	int (*invalidate_range_start)(struct mmu_notifier *subscription,
 				      const struct mmu_notifier_range *range);

mm/mmu_notifier.c

@@ -501,10 +501,33 @@ static int mn_hlist_invalidate_range_start(
 						"");
 				WARN_ON(mmu_notifier_range_blockable(range) ||
 					_ret != -EAGAIN);
+				/*
+				 * We call all the notifiers on any EAGAIN,
+				 * there is no way for a notifier to know if
+				 * its start method failed, thus a start that
+				 * does EAGAIN can't also do end.
+				 */
+				WARN_ON(ops->invalidate_range_end);
 				ret = _ret;
 			}
 		}
 	}
+
+	if (ret) {
+		/*
+		 * Must be non-blocking to get here.  If there are multiple
+		 * notifiers and one or more failed start, any that succeeded
+		 * start are expecting their end to be called.  Do so now.
+		 */
+		hlist_for_each_entry_rcu(subscription, &subscriptions->list,
+					 hlist, srcu_read_lock_held(&srcu)) {
+			if (!subscription->ops->invalidate_range_end)
+				continue;
+
+			subscription->ops->invalidate_range_end(subscription,
+								range);
+		}
+	}
 	srcu_read_unlock(&srcu, id);
 
 	return ret;