Subject: Re: [v3 4/5] KVM: PPC: Book3S HV: retry page migration before erroring-out H_SVM_PAGE_IN

From: Bharata B Rao <[email-masked]>
Date: Mon, 13 Jul 2020 15:20:43 +0530

On Sat, Jul 11, 2020 at 02:13:46AM -0700, Ram Pai wrote:
> The page requested for page-in; sometimes, can have transient
> references, and hence cannot migrate immediately. Retry a few times
> before returning error.

As I noted in the previous patch, we need to understand what are these
transient errors and they occur on what type of pages?

The previous patch also introduced a bit of retry logic in the
page-in path. Can you consolidate the retry logic into a separate
patch?

> 
> H_SVM_PAGE_IN interface is enhanced to return H_BUSY if the page is
> not in a migratable state.
> 
> Cc: Paul Mackerras <[email-masked]>
> Cc: Benjamin Herrenschmidt <[email-masked]>
> Cc: Michael Ellerman <[email-masked]>
> Cc: Bharata B Rao <[email-masked]>
> Cc: Aneesh Kumar K.V <[email-masked]>
> Cc: Sukadev Bhattiprolu <[email-masked]>
> Cc: Laurent Dufour <[email-masked]>
> Cc: Thiago Jung Bauermann <[email-masked]>
> Cc: David Gibson <[email-masked]>
> Cc: Claudio Carvalho <[email-masked]>
> Cc: [email-masked]
> Cc: [email-masked]
> 
> Signed-off-by: Ram Pai <[email-masked]>
> ---
>  Documentation/powerpc/ultravisor.rst |  1 +
>  arch/powerpc/kvm/book3s_hv_uvmem.c   | 54 +++++++++++++++++++++---------------
>  2 files changed, 33 insertions(+), 22 deletions(-)
> 
> diff --git a/Documentation/powerpc/ultravisor.rst b/Documentation/powerpc/ultravisor.rst
> index d98fc85..638d1a7 100644
> --- a/Documentation/powerpc/ultravisor.rst
> +++ b/Documentation/powerpc/ultravisor.rst
> @@ -1034,6 +1034,7 @@ Return values
>  	* H_PARAMETER	if ``guest_pa`` is invalid.
>  	* H_P2		if ``flags`` is invalid.
>  	* H_P3		if ``order`` of page is invalid.
> +	* H_BUSY	if ``page`` is not in a state to pagein
>  
>  Description
>  ~~~~~~~~~~~
> diff --git a/arch/powerpc/kvm/book3s_hv_uvmem.c b/arch/powerpc/kvm/book3s_hv_uvmem.c
> index 12ed52a..c9bdef6 100644
> --- a/arch/powerpc/kvm/book3s_hv_uvmem.c
> +++ b/arch/powerpc/kvm/book3s_hv_uvmem.c
> @@ -843,7 +843,7 @@ unsigned long kvmppc_h_svm_page_in(struct kvm *kvm, unsigned long gpa,
>  	struct vm_area_struct *vma;
>  	int srcu_idx;
>  	unsigned long gfn = gpa >> page_shift;
> -	int ret;
> +	int ret, repeat_count = REPEAT_COUNT;
>  
>  	if (!(kvm->arch.secure_guest & KVMPPC_SECURE_INIT_START))
>  		return H_UNSUPPORTED;
> @@ -857,34 +857,44 @@ unsigned long kvmppc_h_svm_page_in(struct kvm *kvm, unsigned long gpa,
>  	if (flags & H_PAGE_IN_SHARED)
>  		return kvmppc_share_page(kvm, gpa, page_shift);
>  
> -	ret = H_PARAMETER;
>  	srcu_idx = srcu_read_lock(&kvm->srcu);
> -	down_write(&kvm->mm->mmap_sem);
>  
> -	start = gfn_to_hva(kvm, gfn);
> -	if (kvm_is_error_hva(start))
> -		goto out;
> -
> -	mutex_lock(&kvm->arch.uvmem_lock);
>  	/* Fail the page-in request of an already paged-in page */
> -	if (kvmppc_gfn_is_uvmem_pfn(gfn, kvm, NULL))
> -		goto out_unlock;
> +	mutex_lock(&kvm->arch.uvmem_lock);
> +	ret = kvmppc_gfn_is_uvmem_pfn(gfn, kvm, NULL);
> +	mutex_unlock(&kvm->arch.uvmem_lock);
> +	if (ret) {
> +		srcu_read_unlock(&kvm->srcu, srcu_idx);
> +		return H_PARAMETER;
> +	}
>  
> -	end = start + (1UL << page_shift);
> -	vma = find_vma_intersection(kvm->mm, start, end);
> -	if (!vma || vma->vm_start > start || vma->vm_end < end)
> -		goto out_unlock;
> +	do {
> +		ret = H_PARAMETER;
> +		down_write(&kvm->mm->mmap_sem);

Again with ksm_madvise() moved to init time, check if you still need
write mmap_sem here.

Regards,
Bharata.