Message ID | 20210311180057.1582638-1-seanjc@google.com (mailing list archive) |
---|---|
State | New, archived |
Headers | show |
Series | [v2] mm/mmu_notifiers: Esnure range_end() is paired with range_start() | expand |
On Thu, Mar 11, 2021 at 10:00:57AM -0800, Sean Christopherson wrote: > 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@ziepe.ca> > Cc: stable@vger.kernel.org > 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> > Signed-off-by: Sean Christopherson <seanjc@google.com> > > 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@google.com Looks fine, thanks. I think you need some commit message remark to discourage backporting, the added WARN_ON will trigger on older kernels that have many more things implementing invalidate_range_end(). It should not be backported to anything that has more invalidate_range_ends()'s than today's kernel. Reviewed-by: Jason Gunthorpe <jgg@nvidia.com> Jason
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;
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@ziepe.ca> Cc: stable@vger.kernel.org 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> Signed-off-by: Sean Christopherson <seanjc@google.com> --- 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@google.com include/linux/mmu_notifier.h | 10 +++++----- mm/mmu_notifier.c | 23 +++++++++++++++++++++++ 2 files changed, 28 insertions(+), 5 deletions(-)