Build lightweight virtual memory manager (vm_map/vm_unmap/vm_clone)

[pbalduino]

Goal

Create a lightweight virtual memory manager that provides high-level VM operations and wraps low-level page table manipulation.

Context

Currently, virtual memory operations are open-coded throughout the kernel. We need a clean abstraction layer that manages virtual memory regions and handles cleanup properly.

Definition of Done

• vm_map(): Map virtual memory regions with specified permissions
• vm_unmap(): Unmap and free virtual memory regions
• vm_clone(): Clone virtual memory spaces for process creation
• Region management: Update vm_region table during all operations
• Cleanup integration: Proper cleanup on process exit without page leaks
• 16 MiB support: Handle large footprints efficiently (for Doom requirements)

API Design

Implementation Details

• Wrap pmm_map_page_in_root() calls with higher-level interface
• Maintain vm_region metadata during all operations
• Implement proper reference counting for shared pages
• Add memory leak detection and prevention
• Support efficient mapping/unmapping of large regions
• Handle page table allocation and cleanup

Benefits

• Eliminates open-coded page table loops
• Prevents memory leaks through proper tracking
• Makes 16 MiB footprints manageable
• Provides clean interface for other kernel subsystems
• Enables easier debugging and validation

Files to Create

• vm_manager.h - Virtual memory manager interface
• vm_manager.c - Implementation of VM operations
• Integration with existing memory management
• Helper functions for common VM operations

Dependencies

• Formal VM layout system
• vm_region table implementation
• Existing page table management

Related Issues

This is part of the per-process virtual memory management overhaul (issue #28).