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author | Sean Christopherson <seanjc@google.com> | 2021-03-25 05:37:23 +0100 |
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committer | Linus Torvalds <torvalds@linux-foundation.org> | 2021-03-25 17:22:55 +0100 |
commit | c2655835fd8cabdfe7dab737253de3ffb88da126 (patch) | |
tree | 978841d56b0642a485277181117a4c5f12d9b8c7 /include | |
parent | kasan: fix per-page tags for non-page_alloc pages (diff) | |
download | linux-c2655835fd8cabdfe7dab737253de3ffb88da126.tar.xz linux-c2655835fd8cabdfe7dab737253de3ffb88da126.zip |
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.h | 10 |
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); |