Subject: Re: [PATCH] powerpc/book3s_hv_uvmem: Check for failed page migration

From: Ram Pai <[email-masked]>
Date: Tue, 8 Dec 2020 06:20:22 -0800

On Thu, Dec 03, 2020 at 04:08:12PM +1100, Alistair Popple wrote:
> migrate_vma_pages() may still clear MIGRATE_PFN_MIGRATE on pages which
> are not able to be migrated. Drivers may safely copy data prior to
> calling migrate_vma_pages() however a remote mapping must not be
> established until after migrate_vma_pages() has returned as the
> migration could still fail.
> 
> UV_PAGE_IN_in both copies and maps the data page, therefore it should
> only be called after checking the results of migrate_vma_pages().
> 
> Signed-off-by: Alistair Popple <[email-masked]>
> ---
>  arch/powerpc/kvm/book3s_hv_uvmem.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_hv_uvmem.c b/arch/powerpc/kvm/book3s_hv_uvmem.c
> index 84e5a2dc8be5..08aa6a90c525 100644
> --- a/arch/powerpc/kvm/book3s_hv_uvmem.c
> +++ b/arch/powerpc/kvm/book3s_hv_uvmem.c
> @@ -762,7 +762,10 @@ static int kvmppc_svm_page_in(struct vm_area_struct *vma,
>  		goto out_finalize;
>  	}
> 
> -	if (pagein) {
> +	*mig.dst = migrate_pfn(page_to_pfn(dpage)) | MIGRATE_PFN_LOCKED;
> +	migrate_vma_pages(&mig);
> +
> +	if ((*mig.src & MIGRATE_PFN_MIGRATE) && pagein) {
>  		pfn = *mig.src >> MIGRATE_PFN_SHIFT;
>  		spage = migrate_pfn_to_page(*mig.src);
>  		if (spage) {
> @@ -773,8 +776,6 @@ static int kvmppc_svm_page_in(struct vm_area_struct *vma,
>  		}
>  	}
> 
> -	*mig.dst = migrate_pfn(page_to_pfn(dpage)) | MIGRATE_PFN_LOCKED;
> -	migrate_vma_pages(&mig);
>  out_finalize:
>  	migrate_vma_finalize(&mig);
>  	return ret;

Though this patch did not solve the specific problem, I am running into,
my tests did not expose any regression.

Tested-by: Ram Pai <[email-masked]>

Reviewed-by: Ram Pai <[email-masked]>