From de2e6b4e32d6be7ed2218c1b20a9f81f8859ec2a Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Wed, 24 Mar 2021 21:37:23 -0700 Subject: [PATCH] mm/mmu_notifiers: ensure range_end() is paired with range_start() MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit [ Upstream commit c2655835fd8cabdfe7dab737253de3ffb88da126 ] 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 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 Signed-off-by: Linus Torvalds Signed-off-by: Sasha Levin --- 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 b820078..1a6a9eb 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 5654dd1..07f42a7 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; -- 2.7.4