Re: [PATCH hmm 8/8] mm/hmm: add missing call to hmm_pte_need_fault in HMM_PFN_SPECIAL handling

[Ralph Campbell <rcampbell@nvidia.com>]

On 3/11/20 11:35 AM, Jason Gunthorpe wrote:
> From: Jason Gunthorpe <jgg@mellanox.com>
> 
> Currently if a special PTE is encountered hmm_range_fault() immediately
> returns EFAULT and sets the HMM_PFN_SPECIAL error output (which nothing
> uses).
> 
> EFAULT should only be returned after testing with hmm_pte_need_fault().
> 
> Also pte_devmap() and pte_special() are exclusive, and there is no need to
> check IS_ENABLED, pte_special() is stubbed out to return false on
> unsupported architectures.
> 
> Fixes: 992de9a8b751 ("mm/hmm: allow to mirror vma of a file on a DAX backed filesystem")
> Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>

> ---
>   mm/hmm.c | 19 ++++++++++++-------
>   1 file changed, 12 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/hmm.c b/mm/hmm.c
> index f61fddf2ef6505..ca33d086bdc190 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -335,16 +335,21 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
>   			pte_unmap(ptep);
>   			return -EBUSY;
>   		}
> -	} else if (IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL) && pte_special(pte)) {
> -		if (!is_zero_pfn(pte_pfn(pte))) {
> +	}
> +
> +	/*
> +	 * Since each architecture defines a struct page for the zero page, just
> +	 * fall through and treat it like a normal page.
> +	 */
> +	if (pte_special(pte) && !is_zero_pfn(pte_pfn(pte))) {
> +		hmm_pte_need_fault(hmm_vma_walk, orig_pfn, 0, &fault,
> +				   &write_fault);
> +		if (fault || write_fault) {
>   			pte_unmap(ptep);
> -			*pfn = range->values[HMM_PFN_SPECIAL];
>   			return -EFAULT;
>   		}
> -		/*
> -		 * Since each architecture defines a struct page for the zero
> -		 * page, just fall through and treat it like a normal page.
> -		 */
> +		*pfn = range->values[HMM_PFN_SPECIAL];
> +		return 0;
>   	}
>   
>   	*pfn = hmm_device_entry_from_pfn(range, pte_pfn(pte)) | cpu_flags;
>