Re: [PATCH] mm/oom_kill: Ensure MMU notifier range_end() is paired with range_start()

[Ben Gardon <bgardon@google.com>]

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On Wed, Mar 10, 2021 at 1:31 PM Sean Christopherson <seanjc@google.com>
wrote:

> Invoke the MMU notifier's .invalidate_range_end() callbacks even if one
> of the .invalidate_range_start() callbacks failed.  If there are multiple
> notifiers, the notifier that did not fail may have performed actions in
> its ...start() that it expects to unwind via ...end().  Per the
> mmu_notifier_ops documentation, ...start() and ...end() must be paired.
>
> The only in-kernel usage that is fatally broken is the SGI UV GRU driver,
> which effectively blocks and sleeps fault handlers during ...start(), and
> unblocks/wakes the handlers during ...end().  But, the only users that
> can fail ...start() are the i915 and Nouveau drivers, which are unlikely
> to collide with the SGI driver.
>
> KVM is the only other user of ...end(), and while KVM also blocks fault
> handlers in ...start(), the fault handlers do not sleep and originate in
> killable ioctl() calls.  So while it's possible for the i915 and Nouveau
> drivers to collide with KVM, the bug is benign for KVM since the process
> is dying and KVM's guest is about to be terminated.
>
> So, as of today, the bug is likely benign.  But, that may not always be
> true, e.g. there is 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")
> Cc: stable@vger.kernel.org
> Cc: David Rientjes <rientjes@google.com>
> Cc: Ben Gardon <bgardon@google.com>
> Cc: Jason Gunthorpe <jgg@ziepe.ca>
> 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>
>

Reviewed-by: Ben Gardon <bgardon@google.com>

Thanks for catching and fixing this.


> ---
>  mm/oom_kill.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
>
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index bc65ba4f5192..acc3ba8b2ed7 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -546,12 +546,10 @@ bool __oom_reap_task_mm(struct mm_struct *mm)
>                                                 vma, mm, vma->vm_start,
>                                                 vma->vm_end);
>                         tlb_gather_mmu(&tlb, mm);
> -                       if
> (mmu_notifier_invalidate_range_start_nonblock(&range)) {
> -                               tlb_finish_mmu(&tlb);
> +                       if
> (!mmu_notifier_invalidate_range_start_nonblock(&range))
> +                               unmap_page_range(&tlb, vma, range.start,
> range.end, NULL);
> +                       else
>                                 ret = false;
> -                               continue;
> -                       }
> -                       unmap_page_range(&tlb, vma, range.start,
> range.end, NULL);
>                         mmu_notifier_invalidate_range_end(&range);
>                         tlb_finish_mmu(&tlb);
>                 }
> --
> 2.30.1.766.gb4fecdf3b7-goog
>
>

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