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

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Date:   Thu, 24 Oct 2019 11:27:59 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Ralph Campbell <rcampbell@...dia.com>,
        Jerome Glisse <jglisse@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Christoph Hellwig <hch@....de>,
        Jason Gunthorpe <jgg@...lanox.com>
Cc:     linux-rdma@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/3] mm/hmm: allow snapshot of the special zero page

On 23.10.19 21:55, 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@...dia.com>
> Cc: Christoph Hellwig <hch@....de>
> Cc: "Jérôme Glisse" <jglisse@...hat.com>
> Cc: Jason Gunthorpe <jgg@...lanox.com>
> ---
>   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;
> 

Acked-by: David Hildenbrand <david@...hat.com>

-- 

Thanks,

David / dhildenb