mm/hmm: allow snapshot of the special zero page

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.

Link: https://lore.kernel.org/r/20191023195515.13168-3-rcampbell@nvidia.com
Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: "Jérôme Glisse" <jglisse@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

diff --git a/mm/hmm.c b/mm/hmm.c
index 902f5fa..6b01366 100644 (file)
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -532,8 +532,14 @@ 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;