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.
Fixes: 93065ac753e4 ("mm, oom: distinguish blockable mode for mmu notifiers")
Suggested-by: Jason Gunthorpe <jgg@xxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Ben Gardon <bgardon@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: "Jérôme Glisse" <jglisse@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Dimitri Sivanich <dimitri.sivanich@xxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
v2: Reimplemented as suggested by Jason. Only functional change relative
to Jason's suggestion is to check invalidate_range_end before calling to
avoid a NULL pointer dereference. I also added more comments, hopefully
they're helpful...
v1: https://lkml.kernel.org/r/20210310213117.1444147-1-seanjc@xxxxxxxxxx
include/linux/mmu_notifier.h | 10 +++++-----
mm/mmu_notifier.c | 23 +++++++++++++++++++++++
2 files changed, 28 insertions(+), 5 deletions(-)
diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
index b8200782dede..1a6a9eb6d3fa 100644
--- a/include/linux/mmu_notifier.h
+++ b/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);
diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
index 61ee40ed804e..459d195d2ff6 100644
--- a/mm/mmu_notifier.c
+++ b/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;
--
2.31.0.rc2.261.g7f71774620-goog
