Re: [PATCH 2/4] mm/hmm: allow snapshot of the special zero page

[Christoph Hellwig <hch@xxxxxx>]

On Wed, Sep 11, 2019 at 03:28:27PM -0700, Ralph Campbell wrote:
> Allow hmm_range_fault() to return success (0) when the CPU pagetable
> entry points to the special shared zero page.
> The caller can then handle the zero page by possibly clearing device
> private memory instead of DMAing a zero page.
> 
> Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
> Cc: "Jérôme Glisse" <jglisse@xxxxxxxxxx>
> Cc: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> ---
>  mm/hmm.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/hmm.c b/mm/hmm.c
> index 06041d4399ff..7217912bef13 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -532,7 +532,7 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
>  			return -EBUSY;
>  	} else if (IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL) && pte_special(pte)) {
>  		*pfn = range->values[HMM_PFN_SPECIAL];
> -		return -EFAULT;
> +		return is_zero_pfn(pte_pfn(pte)) ? 0 : -EFAULT;

Any chance to just use a normal if here:

		if (!is_zero_pfn(pte_pfn(pte)))
			return -EFAULT;
		return 0;