@@ -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);
@@ -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;
The patch titled Subject: mm/mmu_notifiers: ensure range_end() is paired with range_start() has been added to the -mm tree. Its filename is mm-mmu_notifiers-esnure-range_end-is-paired-with-range_start.patch This patch should soon appear at https://ozlabs.org/~akpm/mmots/broken-out/mm-mmu_notifiers-esnure-range_end-is-paired-with-range_start.patch and later at https://ozlabs.org/~akpm/mmotm/broken-out/mm-mmu_notifiers-esnure-range_end-is-paired-with-range_start.patch Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next and is updated there every 3-4 working days ------------------------------------------------------ From: Sean Christopherson <seanjc@google.com> 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 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: 93065ac753e4 ("mm, oom: distinguish blockable mode for mmu notifiers") Signed-off-by: Sean Christopherson <seanjc@google.com> Suggested-by: Jason Gunthorpe <jgg@ziepe.ca> 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> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> --- include/linux/mmu_notifier.h | 10 +++++----- mm/mmu_notifier.c | 23 +++++++++++++++++++++++ 2 files changed, 28 insertions(+), 5 deletions(-)