Subject: Re: [v3 1/5] KVM: PPC: Book3S HV: Disable page merging in H_SVM_INIT_START

From: Bharata B Rao <[email-masked]>
Date: Mon, 13 Jul 2020 10:59:41 +0530

On Sat, Jul 11, 2020 at 02:13:43AM -0700, Ram Pai wrote:
> Merging of pages associated with each memslot of a SVM is
> disabled the page is migrated in H_SVM_PAGE_IN handler.
> 
> This operation should have been done much earlier; the moment the VM
> is initiated for secure-transition. Delaying this operation, increases
> the probability for those pages to acquire new references , making it
> impossible to migrate those pages in H_SVM_PAGE_IN handler.
> 
> Disable page-migration in H_SVM_INIT_START handling.

While it is a good idea to disable KSM merging for all VMAs during
H_SVM_INIT_START, I am curious if you did observe an actual case of
ksm_madvise() failing which resulted in subsequent H_SVM_PAGE_IN
failing to migrate?

> 
> Signed-off-by: Ram Pai <[email-masked]>
> ---
>  arch/powerpc/kvm/book3s_hv_uvmem.c | 96 +++++++++++++++++++++++++++++---------
>  1 file changed, 74 insertions(+), 22 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_hv_uvmem.c b/arch/powerpc/kvm/book3s_hv_uvmem.c
> index 3d987b1..bfc3841 100644
> --- a/arch/powerpc/kvm/book3s_hv_uvmem.c
> +++ b/arch/powerpc/kvm/book3s_hv_uvmem.c
> @@ -211,6 +211,65 @@ static bool kvmppc_gfn_is_uvmem_pfn(unsigned long gfn, struct kvm *kvm,
>  	return false;
>  }
>  
> +static int kvmppc_memslot_page_merge(struct kvm *kvm,
> +		struct kvm_memory_slot *memslot, bool merge)
> +{
> +	unsigned long gfn = memslot->base_gfn;
> +	unsigned long end, start = gfn_to_hva(kvm, gfn);
> +	int ret = 0;
> +	struct vm_area_struct *vma;
> +	int merge_flag = (merge) ? MADV_MERGEABLE : MADV_UNMERGEABLE;
> +
> +	if (kvm_is_error_hva(start))
> +		return H_STATE;

This and other cases below seem to be a new return value from
H_SVM_INIT_START. May be update the documentation too along with
this patch?

> +
> +	end = start + (memslot->npages << PAGE_SHIFT);
> +
> +	down_write(&kvm->mm->mmap_sem);

When you rebase the patches against latest upstream you may want to
replace the above and other instances by mmap_write/read_lock().

> +	do {
> +		vma = find_vma_intersection(kvm->mm, start, end);
> +		if (!vma) {
> +			ret = H_STATE;
> +			break;
> +		}
> +		ret = ksm_madvise(vma, vma->vm_start, vma->vm_end,
> +			  merge_flag, &vma->vm_flags);
> +		if (ret) {
> +			ret = H_STATE;
> +			break;
> +		}
> +		start = vma->vm_end + 1;
> +	} while (end > vma->vm_end);
> +
> +	up_write(&kvm->mm->mmap_sem);
> +	return ret;
> +}
> +
> +static int __kvmppc_page_merge(struct kvm *kvm, bool merge)
> +{
> +	struct kvm_memslots *slots;
> +	struct kvm_memory_slot *memslot;
> +	int ret = 0;
> +
> +	slots = kvm_memslots(kvm);
> +	kvm_for_each_memslot(memslot, slots) {
> +		ret = kvmppc_memslot_page_merge(kvm, memslot, merge);
> +		if (ret)
> +			break;
> +	}
> +	return ret;
> +}
> +
> +static inline int kvmppc_disable_page_merge(struct kvm *kvm)
> +{
> +	return __kvmppc_page_merge(kvm, false);
> +}
> +
> +static inline int kvmppc_enable_page_merge(struct kvm *kvm)
> +{
> +	return __kvmppc_page_merge(kvm, true);
> +}
> +
>  unsigned long kvmppc_h_svm_init_start(struct kvm *kvm)
>  {
>  	struct kvm_memslots *slots;
> @@ -232,11 +291,18 @@ unsigned long kvmppc_h_svm_init_start(struct kvm *kvm)
>  		return H_AUTHORITY;
>  
>  	srcu_idx = srcu_read_lock(&kvm->srcu);
> +
> +	/* disable page-merging for all memslot */
> +	ret = kvmppc_disable_page_merge(kvm);
> +	if (ret)
> +		goto out;
> +
> +	/* register the memslot */
>  	slots = kvm_memslots(kvm);
>  	kvm_for_each_memslot(memslot, slots) {
>  		if (kvmppc_uvmem_slot_init(kvm, memslot)) {
>  			ret = H_PARAMETER;
> -			goto out;
> +			break;
>  		}
>  		ret = uv_register_mem_slot(kvm->arch.lpid,
>  					   memslot->base_gfn << PAGE_SHIFT,
> @@ -245,9 +311,12 @@ unsigned long kvmppc_h_svm_init_start(struct kvm *kvm)
>  		if (ret < 0) {
>  			kvmppc_uvmem_slot_free(kvm, memslot);
>  			ret = H_PARAMETER;
> -			goto out;
> +			break;
>  		}
>  	}
> +
> +	if (ret)
> +		kvmppc_enable_page_merge(kvm);

Is there any use of enabling KSM merging in the failure path here?
Won't UV terminate the VM if H_SVM_INIT_START fails? If there is no need,
you can do away with some extra routines above.

>  out:
>  	srcu_read_unlock(&kvm->srcu, srcu_idx);
>  	return ret;
> @@ -384,7 +453,7 @@ static struct page *kvmppc_uvmem_get_page(unsigned long gpa, struct kvm *kvm)
>   */
>  static int kvmppc_svm_page_in(struct vm_area_struct *vma, unsigned long start,
>  		   unsigned long end, unsigned long gpa, struct kvm *kvm,
> -		   unsigned long page_shift, bool *downgrade)
> +		   unsigned long page_shift)
>  {
>  	unsigned long src_pfn, dst_pfn = 0;
>  	struct migrate_vma mig;
> @@ -400,18 +469,6 @@ static int kvmppc_svm_page_in(struct vm_area_struct *vma, unsigned long start,
>  	mig.src = &src_pfn;
>  	mig.dst = &dst_pfn;
>  
> -	/*
> -	 * We come here with mmap_sem write lock held just for
> -	 * ksm_madvise(), otherwise we only need read mmap_sem.
> -	 * Hence downgrade to read lock once ksm_madvise() is done.
> -	 */
> -	ret = ksm_madvise(vma, vma->vm_start, vma->vm_end,
> -			  MADV_UNMERGEABLE, &vma->vm_flags);

I haven't seen the subsequent patches yet, but guess you are
taking care of disabling KSM mering for hot-plugged memory too.

Regards,
Bharata.