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-... and later at https://ozlabs.org/~akpm/mmotm/broken-out/mm-mmu_notifiers-esnure-range_end-...

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------------------------------------------------------ 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(-)

--- 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; _

Patches currently in -mm which might be from seanjc@google.com are

mm-mmu_notifiers-esnure-range_end-is-paired-with-range_start.patch