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Subject: Re: [PATCH v3 2/3] mm/hmm: allow snapshot of the special zero page
To:     Ralph Campbell <rcampbell@nvidia.com>,
        Jerome Glisse <jglisse@redhat.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Christoph Hellwig <hch@lst.de>,
        Jason Gunthorpe <jgg@mellanox.com>
Cc:     linux-rdma@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20191023195515.13168-1-rcampbell@nvidia.com>
 <20191023195515.13168-3-rcampbell@nvidia.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <8e051e7f-fb41-4026-cc55-45eee85f8829@redhat.com>
Date:   Thu, 24 Oct 2019 11:27:59 +0200
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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.
>=20
> Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: "J=C3=A9r=C3=B4me Glisse" <jglisse@redhat.com>
> Cc: Jason Gunthorpe <jgg@mellanox.com>
> ---
>   mm/hmm.c | 10 ++++++++--
>   1 file changed, 8 insertions(+), 2 deletions(-)
>=20
> 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,
>   =09=09if (unlikely(!hmm_vma_walk->pgmap))
>   =09=09=09return -EBUSY;
>   =09} else if (IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL) && pte_special(pt=
e)) {
> -=09=09*pfn =3D range->values[HMM_PFN_SPECIAL];
> -=09=09return -EFAULT;
> +=09=09if (!is_zero_pfn(pte_pfn(pte))) {
> +=09=09=09*pfn =3D range->values[HMM_PFN_SPECIAL];
> +=09=09=09return -EFAULT;
> +=09=09}
> +=09=09/*
> +=09=09 * Since each architecture defines a struct page for the zero
> +=09=09 * page, just fall through and treat it like a normal page.
> +=09=09 */
>   =09}
>  =20
>   =09*pfn =3D hmm_device_entry_from_pfn(range, pte_pfn(pte)) | cpu_flags;
>=20

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

--=20

Thanks,

David / dhildenb