aarch64: fix memory unmapping

Author: zyuiop

src/arch/aarch64/mm/paging.rs

@@ -149,6 +149,11 @@ impl PageTableEntry {
 		(self.physical_address_and_flags & PageTableEntryFlags::TABLE_OR_4KIB_PAGE.bits()) != 0
 	}
 
+	/// Mark this as an invalid (not present) entry
+	fn unset(&mut self) {
+		self.physical_address_and_flags = 0;
+	}
+
 	/// Mark this as a valid (present) entry and set address translation and flags.
 	///
 	/// # Arguments
@@ -161,6 +166,10 @@ impl PageTableEntry {
 			physical_address.is_aligned_to(BasePageSize::SIZE),
 			"Physical address is not on a 4 KiB page boundary (physical_address = {physical_address:p})"
 		);
+		assert!(
+			!physical_address.is_null(),
+			"Cannot set a page table entry with null address"
+		);
 
 		let mut flags_to_set = flags;
 		flags_to_set.insert(PageTableEntryFlags::PRESENT);
@@ -414,7 +423,7 @@ impl<L: PageTableLevel> PageTableMethods for PageTable<L> {
 
 		if flags == PageTableEntryFlags::BLANK {
 			// in this case we unmap the pages
-			self.entries[index].set(physical_address, flags);
+			self.entries[index].unset()
 		} else {
 			self.entries[index].set(physical_address, S::MAP_EXTRA_FLAG | flags);
 		}