[kernel] Replace buffer_cache with vm_area_buffers

In order to reduce the amount of tracked state, now use the
vm_area_buffers instead of a VMA with buffer_cache on top.

src/kernel/objects/vm_area.h

@@ -124,8 +124,9 @@ private:
 	kutil::vector<mapping> m_mappings;
 };
 
-/// Area split into standard-sized segments
-class vm_area_buffers :
+
+/// Area that allows open allocation (eg, kernel heap)
+class vm_area_open :
 	public vm_area
 {
 public:
@@ -133,7 +134,40 @@ public:
 	/// \arg size  Initial virtual size of the memory area
 	/// \arg space The address space this area belongs to
 	/// \arg flags Flags for this memory area
-	vm_area_buffers(size_t size, vm_space &space, vm_flags flags);
+	vm_area_open(size_t size, vm_space &space, vm_flags flags);
+
+	virtual vm_mapper & mapper() override { return m_mapper; }
+	virtual const vm_mapper & mapper() const override { return m_mapper; }
+
+	virtual void commit(uintptr_t phys, uintptr_t offset, size_t count) override;
+	virtual void uncommit(uintptr_t offset, size_t count) override;
+
+private:
+	vm_mapper_single m_mapper;
+};
+
+
+/// Area split into standard-sized segments
+class vm_area_buffers :
+	public vm_area
+{
+public:
+	/// Constructor.
+	/// \arg size      Initial virtual size of the memory area
+	/// \arg space     The address space this area belongs to
+	/// \arg flags     Flags for this memory area
+	/// \arg buf_pages Pages in an individual buffer
+	vm_area_buffers(
+		size_t size,
+		vm_space &space,
+		vm_flags flags,
+		size_t buf_pages);
+
+	/// Get an available stack address
+	uintptr_t get_buffer();
+
+	/// Return a buffer address to the available pool
+	void return_buffer(uintptr_t addr);
 
 	virtual vm_mapper & mapper() override { return m_mapper; }
 	virtual const vm_mapper & mapper() const override { return m_mapper; }
@@ -144,6 +178,9 @@ public:
 
 private:
 	vm_mapper_single m_mapper;
+	kutil::vector<uintptr_t> m_cache;
+	size_t m_pages;
+	uintptr_t m_next;
 };
 
 
@@ -167,26 +204,4 @@ private:
 	vm_mapper_multi m_mapper;
 };
 
-
-/// Area that allows open allocation (eg, kernel heap)
-class vm_area_open :
-	public vm_area
-{
-public:
-	/// Constructor.
-	/// \arg size  Initial virtual size of the memory area
-	/// \arg space The address space this area belongs to
-	/// \arg flags Flags for this memory area
-	vm_area_open(size_t size, vm_space &space, vm_flags flags);
-
-	virtual vm_mapper & mapper() override { return m_mapper; }
-	virtual const vm_mapper & mapper() const override { return m_mapper; }
-
-	virtual void commit(uintptr_t phys, uintptr_t offset, size_t count) override;
-	virtual void uncommit(uintptr_t offset, size_t count) override;
-
-private:
-	vm_mapper_single m_mapper;
-};
-
 IS_BITFIELD(vm_flags);