fix max memory region calculation

Author: Wasabi375

common/src/legacy_memory_region.rs

@@ -152,6 +152,14 @@ where
             .unwrap()
     }
 
+    /// Calculate the maximum number of regions produced by [Self::construct_memory_map]
+    pub fn memory_map_max_region_count(&self) -> usize {
+        // every used slice can split an original region into 3 new regions,
+        // this means we need to reserve 2 extra spaces for each slice.
+        // There are 3 additional slices (kernel, ramdisk and the bootloader heap)
+        self.len() + (3 + self.used_slices.clone().count()) * 2
+    }
+
     /// Converts this type to a boot info memory map.
     ///
     /// The memory map is placed in the given `regions` slice. The length of the given slice

common/src/lib.rs

@@ -487,9 +487,7 @@ where
     // allocate and map space for the boot info
     let (boot_info, memory_regions) = {
         let boot_info_layout = Layout::new::<BootInfo>();
-        // TODO the assumption here about the regions is wrong
-        // figure out what the correct region count should be
-        let regions = frame_allocator.len() + 4 + 30; // up to 4 regions might be split into used/unused
+        let regions = frame_allocator.memory_map_max_region_count();
         let memory_regions_layout = Layout::array::<MemoryRegion>(regions).unwrap();
         let (combined, memory_regions_offset) =
             boot_info_layout.extend(memory_regions_layout).unwrap();