Fix delay memory handler: set present=true before unmapping

crate/memory/src/memory_set/handler/delay.rs

@@ -27,8 +27,11 @@ impl<T: FrameAllocator> MemoryHandler for Delay<T> {
         let entry = pt.get_entry(addr).expect("failed to get entry");
         if entry.present() {
             self.allocator.dealloc(entry.target());
-            pt.unmap(addr);
         }
+
+        // PageTable::unmap requires page to be present
+        entry.set_present(true);
+        pt.unmap(addr);
     }
 
     fn page_fault_handler(&self, pt: &mut PageTable, addr: VirtAddr) -> bool {
@@ -40,6 +43,7 @@ impl<T: FrameAllocator> MemoryHandler for Delay<T> {
         let frame = self.allocator.alloc().expect("failed to alloc frame");
         entry.set_target(frame);
         entry.set_present(true);
+        entry.update();
         true
     }
 }