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authorSean Christopherson <seanjc@google.com>2021-03-25 07:37:23 +0300
committerLinus Torvalds <torvalds@linux-foundation.org>2021-03-25 19:22:55 +0300
commitc2655835fd8cabdfe7dab737253de3ffb88da126 (patch)
tree978841d56b0642a485277181117a4c5f12d9b8c7 /include
parentcf10bd4c4aff8dd64d1aa7f2a529d0c672bc16af (diff)
downloadlinux-c2655835fd8cabdfe7dab737253de3ffb88da126.tar.xz
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> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com> 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: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'include')
-rw-r--r--include/linux/mmu_notifier.h10
1 files changed, 5 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);