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From: Andrew Morton <>
Subject: [patch 03/14] mm/mmu_notifiers: ensure range_end() is paired with range_start()
Date: Wed, 24 Mar 2021 21:36:43 -0700	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

From: Sean Christopherson <>
Subject: 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
  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.

Fixes: 93065ac753e4 ("mm, oom: distinguish blockable mode for mmu notifiers")
Signed-off-by: Sean Christopherson <>
Suggested-by: Jason Gunthorpe <>
Reviewed-by: Jason Gunthorpe <>
Cc: David Rientjes <>
Cc: Ben Gardon <>
Cc: Michal Hocko <>
Cc: "Jérôme Glisse" <>
Cc: Andrea Arcangeli <>
Cc: Johannes Weiner <>
Cc: Dimitri Sivanich <>
Signed-off-by: Andrew Morton <>

 include/linux/mmu_notifier.h |   10 +++++-----
 mm/mmu_notifier.c            |   23 +++++++++++++++++++++++
 2 files changed, 28 insertions(+), 5 deletions(-)

--- a/include/linux/mmu_notifier.h~mm-mmu_notifiers-esnure-range_end-is-paired-with-range_start
+++ a/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);
--- a/mm/mmu_notifier.c~mm-mmu_notifiers-esnure-range_end-is-paired-with-range_start
+++ a/mm/mmu_notifier.c
@@ -501,10 +501,33 @@ static int mn_hlist_invalidate_range_sta
 				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;

  parent reply	other threads:[~2021-03-25  4:36 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <>
2021-03-25  4:36 ` [patch 01/14] hugetlb_cgroup: fix imbalanced css_get and css_put pair for shared mappings Andrew Morton
2021-03-25  4:36 ` [patch 02/14] kasan: fix per-page tags for non-page_alloc pages Andrew Morton
2021-03-25  4:36 ` Andrew Morton [this message]
2021-03-25  4:36 ` [patch 04/14] selftests/vm: fix out-of-tree build Andrew Morton
2021-03-25  4:36 ` [patch 05/14] z3fold: prevent reclaim/free race for headless pages Andrew Morton
2021-03-25  4:36 incoming Andrew Morton
2021-03-25  4:37 ` [patch 03/14] mm/mmu_notifiers: ensure range_end() is paired with range_start() Andrew Morton

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