Kernel: Add basic fixed width allocator for userspace

We have to move process stacks to the general heap and maybe map
kernel to higher half.

kernel/include/kernel/Memory/FixedWidthAllocator.h

@@ -0,0 +1,48 @@
+#pragma once
+
+#include <kernel/Memory/Heap.h>
+
+namespace Kernel
+{
+
+	class Process;
+
+	class FixedWidthAllocator
+	{
+		BAN_NON_COPYABLE(FixedWidthAllocator);
+
+	public:
+		FixedWidthAllocator(Process*, uint32_t);
+		FixedWidthAllocator(FixedWidthAllocator&&);
+		~FixedWidthAllocator();
+
+		vaddr_t allocate();
+		void deallocate(vaddr_t);
+
+		uint32_t allocation_size() const { return m_allocation_size; }
+
+	private:
+		struct node
+		{
+			node* prev { nullptr };
+			node* next { nullptr };
+		};
+		vaddr_t address_of(const node*) const;
+		void allocate_page_for_node_if_needed(const node*);
+
+	private:
+		static constexpr uint32_t m_min_allocation_size = 16;
+
+		Process* m_process;
+		const uint32_t m_allocation_size;
+		
+		vaddr_t m_nodes_page { 0 };
+		vaddr_t m_allocated_pages { 0 };
+
+		node* m_free_list { nullptr };
+		node* m_used_list { nullptr };
+
+		uint32_t m_allocated { 0 };
+	};
+
+}

kernel/include/kernel/Process.h

@@ -4,6 +4,7 @@
 #include <BAN/StringView.h>
 #include <BAN/Vector.h>
 #include <kernel/FS/Inode.h>
+#include <kernel/Memory/FixedWidthAllocator.h>
 #include <kernel/Memory/Heap.h>
 #include <kernel/Memory/MMU.h>
 #include <kernel/SpinLock.h>
@@ -55,6 +56,8 @@ namespace Kernel
 		BAN::ErrorOr<BAN::String> working_directory() const;
 		BAN::ErrorOr<void> set_working_directory(BAN::StringView);
 
+		BAN::ErrorOr<void*> allocate(size_t);
+
 		void termid(char*) const;
 
 		static Process& current() { return Thread::current().process(); }
@@ -90,6 +93,8 @@ namespace Kernel
 		BAN::String m_working_directory;
 		BAN::Vector<Thread*> m_threads;
 
+		BAN::Vector<FixedWidthAllocator> m_fixed_width_allocators;
+
 		MMU* m_mmu { nullptr };
 		TTY* m_tty { nullptr };
 	};

kernel/include/kernel/Syscall.h

@@ -7,6 +7,7 @@
 #define SYS_CLOSE 5
 #define SYS_SEEK 6
 #define SYS_OPEN 7
+#define SYS_ALLOC 8
 
 #include <stdint.h>