Kernel: Rename Kernel/VM/ to Kernel/Memory/

This directory isn't just about virtual memory, it's about all kinds
of memory management.

Kernel/Memory/PhysicalPage.h

@@ -0,0 +1,71 @@
+/*
+ * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/NonnullRefPtr.h>
+#include <Kernel/PhysicalAddress.h>
+
+namespace Kernel {
+
+enum class MayReturnToFreeList : bool {
+    No,
+    Yes
+};
+
+class PhysicalPage {
+    AK_MAKE_NONCOPYABLE(PhysicalPage);
+    AK_MAKE_NONMOVABLE(PhysicalPage);
+
+    friend class MemoryManager;
+
+public:
+    PhysicalAddress paddr() const;
+
+    void ref()
+    {
+        m_ref_count.fetch_add(1, AK::memory_order_acq_rel);
+    }
+
+    void unref()
+    {
+        if (m_ref_count.fetch_sub(1, AK::memory_order_acq_rel) == 1)
+            free_this();
+    }
+
+    static NonnullRefPtr<PhysicalPage> create(PhysicalAddress, MayReturnToFreeList may_return_to_freelist = MayReturnToFreeList::Yes);
+
+    u32 ref_count() const { return m_ref_count.load(AK::memory_order_consume); }
+
+    bool is_shared_zero_page() const;
+    bool is_lazy_committed_page() const;
+
+private:
+    explicit PhysicalPage(MayReturnToFreeList may_return_to_freelist);
+    ~PhysicalPage() = default;
+
+    void free_this();
+
+    Atomic<u32> m_ref_count { 1 };
+    MayReturnToFreeList m_may_return_to_freelist { MayReturnToFreeList::Yes };
+};
+
+struct PhysicalPageEntry {
+    union {
+        // If it's a live PhysicalPage object:
+        struct {
+            PhysicalPage physical_page;
+        } allocated;
+
+        // If it's an entry in a PhysicalZone::Bucket's freelist.
+        struct {
+            i16 next_index;
+            i16 prev_index;
+        } freelist;
+    };
+};
+
+}