Commit c2655835 authored Mar 24, 2021 by Sean Christopherson Committed by Linus Torvalds Mar 25, 2021

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: 93065ac7 ("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>

parent cf10bd4c

include/linux/mmu_notifier.h

+5 −5

Original line number	Diff line number	Diff line
		@@ -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

+23 −0

Original line number	Diff line number	Diff line
		@@ -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;