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

[Ralph Campbell <rcampbell@nvidia.com>]

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@nvidia.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: "Jérôme Glisse" <jglisse@redhat.com>
Cc: Jason Gunthorpe <jgg@mellanox.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;
-- 
2.20.1