[Devel] [PATCH rh7 3/2] kvm: fix race between mmu_shrink_scan() and VM destroy

[Kirill Tkhai]

On 07.06.2019 14:49, Konstantin Khorenko wrote:
> Honor the race between VM memory shrink and VM destroy,
> just skip dying VMs, shrink alive ones.
> 
> Fixes: 05a623470d4b ("kvm: move actual VM memory shrink out of kvm_lock")
> https://jira.sw.ru/browse/PSBM-95077
> 
> Reported-by: Kirill Tkhai <ktkhai at virtuozzo.com>
> Signed-off-by: Konstantin Khorenko <khorenko at virtuozzo.com>

Reviewed-by: Kirill Tkhai <ktkhai at virtuozzo.com>

> ---
>  arch/x86/kvm/mmu.c       | 7 ++++++-
>  include/linux/kvm_host.h | 1 +
>  virt/kvm/kvm_main.c      | 6 ++++++
>  3 files changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 0c3ded90dd38..18c7f63fcccd 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -5383,7 +5383,12 @@ mmu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
>  		      !kvm_has_zapped_obsolete_pages(kvm))
>  			continue;
>  
> -		kvm_get_kvm(kvm);
> +		/*
> +		 * If try_get fails, we race with last kvm_put_kvm(),
> +		 * so skip the VM, it will die soon anyway.
> +		 */
> +		if (!kvm_try_get_kvm(kvm))
> +			continue;
>  		spin_unlock(&kvm_lock);
>  
>  		idx = srcu_read_lock(&kvm->srcu);
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index ff50020dc91f..f0d028e6f9fb 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -542,6 +542,7 @@ int kvm_init(void *opaque, unsigned vcpu_size, unsigned vcpu_align,
>  void kvm_exit(void);
>  
>  void kvm_get_kvm(struct kvm *kvm);
> +int kvm_try_get_kvm(struct kvm *kvm);
>  void kvm_put_kvm(struct kvm *kvm);
>  
>  static inline struct kvm_memslots *__kvm_memslots(struct kvm *kvm, int as_id)
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index ccab89847623..b8b0146f7c8b 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -733,6 +733,12 @@ void kvm_get_kvm(struct kvm *kvm)
>  }
>  EXPORT_SYMBOL_GPL(kvm_get_kvm);
>  
> +int kvm_try_get_kvm(struct kvm *kvm)
> +{
> +	return atomic_inc_not_zero(&kvm->users_count);
> +}
> +EXPORT_SYMBOL_GPL(kvm_try_get_kvm);
> +
>  void kvm_put_kvm(struct kvm *kvm)
>  {
>  	if (atomic_dec_and_test(&kvm->users_count))
>