mm/hmm: fault non-owner device private entries

If hmm_range_fault() is called with the HMM_PFN_REQ_FAULT flag and a
device private PTE is found, the hmm_range::dev_private_owner page is used
to determine if the device private page should not be faulted in. 
However, if the device private page is not owned by the caller,
hmm_range_fault() returns an error instead of calling migrate_to_ram() to
fault in the page.

For example, if a page is migrated to GPU private memory and a RDMA fault
capable NIC tries to read the migrated page, without this patch it will
get an error.  With this patch, the page will be migrated back to system
memory and the NIC will be able to read the data.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 08ddddd ("mm/hmm: check the device private page owner in hmm_range_fault()")
Signed-off-by: Ralph Campbell <[email protected]>
Reported-by: Felix Kuehling <[email protected]>
Reviewed-by: Alistair Popple <[email protected]>
Cc: Philip Yang <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/hmm.c

@@ -212,14 +212,6 @@ int hmm_vma_handle_pmd(struct mm_walk *walk, unsigned long addr,
 		unsigned long end, unsigned long hmm_pfns[], pmd_t pmd);
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 
-static inline bool hmm_is_device_private_entry(struct hmm_range *range,
-		swp_entry_t entry)
-{
-	return is_device_private_entry(entry) &&
-		pfn_swap_entry_to_page(entry)->pgmap->owner ==
-		range->dev_private_owner;
-}
-
 static inline unsigned long pte_to_hmm_pfn_flags(struct hmm_range *range,
 						 pte_t pte)
 {
@@ -252,10 +244,12 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
 		swp_entry_t entry = pte_to_swp_entry(pte);
 
 		/*
-		 * Never fault in device private pages, but just report
-		 * the PFN even if not present.
+		 * Don't fault in device private pages owned by the caller,
+		 * just report the PFN.
 		 */
-		if (hmm_is_device_private_entry(range, entry)) {
+		if (is_device_private_entry(entry) &&
+		    pfn_swap_entry_to_page(entry)->pgmap->owner ==
+		    range->dev_private_owner) {
 			cpu_flags = HMM_PFN_VALID;
 			if (is_writable_device_private_entry(entry))
 				cpu_flags |= HMM_PFN_WRITE;
@@ -273,6 +267,9 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
 		if (!non_swap_entry(entry))
 			goto fault;
 
+		if (is_device_private_entry(entry))
+			goto fault;
+
 		if (is_device_exclusive_entry(entry))
 			goto fault;