Fix contiguous chunk iterator in monotonic PR (#1299)

Fixed a bug where the iterator defined in
`MonotonicPageResource::iterate_allocated_regions` erronious thinks the
cursor is within the current contiguous region, while the cursor is
actually at the end of the previous chunk. This bug will cause side
metadata (such as VO-bits) to be left over after releasing a CopySpace.

Author: wks

src/util/heap/monotonepageresource.rs

@@ -350,12 +350,18 @@ impl<VM: VMBinding> MonotonePageResource<VM> {
                 } else {
                     let start = self.discontiguous_start;
                     self.discontiguous_start = self.pr.vm_map().get_next_contiguous_region(start);
-                    // If the current cursor is within the current discontiguous region (i.e. chunk),
-                    // then return the size till the cursor
-                    let size = if self.pr.cursor().chunk_index() == start.chunk_index() {
-                        self.pr.cursor() - start
+
+                    let contiguous_region_size = self.pr.vm_map().get_contiguous_region_size(start);
+                    let cursor = self.pr.cursor();
+                    let size = if start < cursor && cursor < start + contiguous_region_size {
+                        // If the current cursor is within the current discontiguous region,
+                        // then return the size till the cursor.
+                        // This is sufficient for sweeping the memory and clearing side metadata.
+                        // Note that if cursor == start,
+                        // it means the cursor is at the end of the previous chunk.
+                        cursor - start
                     } else {
-                        self.pr.vm_map().get_contiguous_region_size(start)
+                        contiguous_region_size
                     };
                     Some((start, size))
                 }