Re: [PATCH v3 2/3] mm/hmm: allow snapshot of the special zero page

[Jerome Glisse]

On Wed, Oct 23, 2019 at 12:55:14PM -0700, Ralph Campbell wrote:
> If a device driver like nouveau tries to use hmm_range_fault() to access
> the special shared zero page in system memory, hmm_range_fault() will
> return -EFAULT and kill the process.
> Allow hmm_range_fault() to return success (0) when the CPU pagetable
> entry points to the special shared zero page.
> page_to_pfn() and pfn_to_page() are defined on the zero page so just
> handle it like any other page.
> 
> Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>

Reviewed-by: "Jérôme Glisse" <jglisse@xxxxxxxxxx>

> Cc: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
> ---
>  mm/hmm.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/hmm.c b/mm/hmm.c
> index acf7a664b38c..8c96c9ddcae5 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -529,8 +529,14 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
>  		if (unlikely(!hmm_vma_walk->pgmap))
>  			return -EBUSY;
>  	} else if (IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL) && pte_special(pte)) {
> -		*pfn = range->values[HMM_PFN_SPECIAL];
> -		return -EFAULT;
> +		if (!is_zero_pfn(pte_pfn(pte))) {
> +			*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 = hmm_device_entry_from_pfn(range, pte_pfn(pte)) | cpu_flags;
> -- 
> 2.20.1
>