mm/memory: return vm_fault_t result from migrate_to_ram() callback

The migrate_to_ram() callback should always succeed, but in rare cases can
fail usually returning VM_FAULT_SIGBUS.  Commit 16ce101
("mm/memory.c: fix race when faulting a device private page") incorrectly
stopped passing the return code up the stack.  Fix this by setting the ret
variable, restoring the previous behaviour on migrate_to_ram() failure.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 16ce101 ("mm/memory.c: fix race when faulting a device private page")
Signed-off-by: Alistair Popple <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Reviewed-by: Felix Kuehling <[email protected]>
Cc: Ralph Campbell <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Alex Sierra <[email protected]>
Cc: Ben Skeggs <[email protected]>
Cc: Lyude Paul <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Michael Ellerman <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/memory.c

@@ -3763,7 +3763,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 			 */
 			get_page(vmf->page);
 			pte_unmap_unlock(vmf->pte, vmf->ptl);
-			vmf->page->pgmap->ops->migrate_to_ram(vmf);
+			ret = vmf->page->pgmap->ops->migrate_to_ram(vmf);
 			put_page(vmf->page);
 		} else if (is_hwpoison_entry(entry)) {
 			ret = VM_FAULT_HWPOISON;