Kernel: Break retain cycle between Inode and VMObject.

There's no need for an Inode to keep its corresponding VMObject alive.
Obviously there are huge benefits to keeping a filesystem cache,
but leaking everything is hardly the right strategy. :^)

Kernel/FileSystem.cpp

@@ -148,7 +148,7 @@ void FS::sync()
     }
 }
 
-void Inode::set_vmo(RetainPtr<VMObject>&& vmo)
+void Inode::set_vmo(VMObject& vmo)
 {
-    m_vmo = move(vmo);
+    m_vmo = vmo.make_weak_ptr();
 }

Kernel/FileSystem.h

@@ -14,6 +14,7 @@
 #include <AK/Function.h>
 #include <AK/kstdio.h>
 #include <AK/Lock.h>
+#include <AK/WeakPtr.h>
 
 static const dword mepoch = 476763780;
 
@@ -103,7 +104,7 @@ public:
 
     void will_be_destroyed();
 
-    void set_vmo(RetainPtr<VMObject>&&);
+    void set_vmo(VMObject&);
     VMObject* vmo() { return m_vmo.ptr(); }
     const VMObject* vmo() const { return m_vmo.ptr(); }
 
@@ -118,7 +119,7 @@ protected:
 private:
     FS& m_fs;
     unsigned m_index { 0 };
-    RetainPtr<VMObject> m_vmo;
+    WeakPtr<VMObject> m_vmo;
     bool m_metadata_dirty { false };
 };
 

Kernel/MemoryManager.cpp

@@ -667,7 +667,7 @@ RetainPtr<VMObject> VMObject::create_file_backed(RetainPtr<Inode>&& inode)
     if (inode->vmo())
         return static_cast<VMObject*>(inode->vmo());
     auto vmo = adopt(*new VMObject(move(inode)));
-    vmo->inode()->set_vmo(vmo.ptr());
+    vmo->inode()->set_vmo(*vmo);
     return vmo;
 }
 
@@ -732,10 +732,8 @@ VMObject::VMObject(RetainPtr<Inode>&& inode)
 
 VMObject::~VMObject()
 {
-    if (m_inode) {
+    if (m_inode)
         ASSERT(m_inode->vmo() == this);
-        m_inode->set_vmo(nullptr);
-    }
     MM.unregister_vmo(*this);
 }
 

Kernel/MemoryManager.h

@@ -10,6 +10,7 @@
 #include <AK/HashTable.h>
 #include <AK/AKString.h>
 #include <AK/Badge.h>
+#include <AK/Weakable.h>
 #include <Kernel/VirtualFileSystem.h>
 
 #define PAGE_ROUND_UP(x) ((((dword)(x)) + PAGE_SIZE-1) & (~(PAGE_SIZE-1)))
@@ -77,7 +78,7 @@ private:
     HashMap<unsigned, RetainPtr<PhysicalPage>> m_physical_pages;
 };
 
-class VMObject : public Retainable<VMObject> {
+class VMObject : public Retainable<VMObject>, public Weakable<VMObject> {
     friend class MemoryManager;
 public:
     static RetainPtr<VMObject> create_file_backed(RetainPtr<Inode>&&);