add support for unassigning a mapping

This will permit repurposing an already assigned mapping, allowing
multiboot to preemptively assigning "free memory" regions, resizing
ranges by deleting and recreating them, or making temporary regions.
Could also be useful for NUMA.

Author: mazunki

api/kernel/memmap.hpp

@@ -339,6 +339,12 @@ class Memory_map {
    */
   Fixed_memory_range& assign_range(const Fixed_memory_range::size_type size);
 
+  /**
+   * Removes a memory range previously defined
+   * Useful to redefine the purpose of a range
+   */
+  void unassign_range(const Fixed_memory_range& range);
+
   /**
    * Check if an address is within a range in the map
    *

src/kernel/memmap.cpp

@@ -173,6 +173,21 @@ Fixed_memory_range& Memory_map::assign_range(Fixed_memory_range&& rng) {
   return new_entry.first->second;
 }
 
+///////////////////////////////////////////////////////////////////////////////
+void Memory_map::unassign_range(const Fixed_memory_range& range) {
+  auto it = map_.find(range.addr_start());
+
+  if (it != map_.end()) {
+    if (it->second.addr_end() == range.addr_end()) {
+        map_.erase(it);
+    } else {
+        throw Memory_range_exception{"Range mismatch at address " + std::to_string(range.addr_start())};
+    }
+  } else {
+    throw Memory_range_exception{"No range found to erase at " + std::to_string(range.addr_start())};
+  }
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 Fixed_memory_range& Memory_map::at(const Key key) {
   return const_cast<Fixed_memory_range&>(static_cast<const Memory_map*>(this)->at(key));