Kernel: Use a FixedArray for VMObject::m_physical_pages

This makes VMObject 8 bytes smaller since we can use the array size as
the page count.

The size() is now also computed from the page count instead of being
a separate value. This makes sizes always be a multiple of PAGE_SIZE,
which is sane.

Kernel/FileSystem/ProcFS.cpp

@@ -315,7 +315,7 @@ Optional<KBuffer> procfs$pid_vmo(InodeIdentifier identifier)
             region.vmo().is_anonymous() ? "anonymous" : "file-backed",
             &region.vmo(),
             region.vmo().ref_count());
-        for (int i = 0; i < region.vmo().page_count(); ++i) {
+        for (size_t i = 0; i < region.vmo().page_count(); ++i) {
             auto& physical_page = region.vmo().physical_pages()[i];
             builder.appendf("P%x%s(%u) ",
                 physical_page ? physical_page->paddr().get() : 0,

Kernel/VM/AnonymousVMObject.cpp

@@ -3,27 +3,25 @@
 
 NonnullRefPtr<AnonymousVMObject> AnonymousVMObject::create_with_size(size_t size)
 {
-    size = ceil_div(size, PAGE_SIZE) * PAGE_SIZE;
     return adopt(*new AnonymousVMObject(size));
 }
 
 NonnullRefPtr<AnonymousVMObject> AnonymousVMObject::create_for_physical_range(PhysicalAddress paddr, size_t size)
 {
-    size = ceil_div(size, PAGE_SIZE) * PAGE_SIZE;
     return adopt(*new AnonymousVMObject(paddr, size));
 }
 
 AnonymousVMObject::AnonymousVMObject(size_t size)
-    : VMObject(size, ShouldFillPhysicalPages::Yes)
+    : VMObject(size)
 {
 }
 
 AnonymousVMObject::AnonymousVMObject(PhysicalAddress paddr, size_t size)
-    : VMObject(size, ShouldFillPhysicalPages::No)
+    : VMObject(size)
 {
-    for (size_t i = 0; i < size; i += PAGE_SIZE)
-        m_physical_pages.append(PhysicalPage::create(paddr.offset(i), false, false));
-    ASSERT(m_physical_pages.size() == page_count());
+    ASSERT(paddr.page_base() == paddr.get());
+    for (size_t i = 0; i < page_count(); ++i)
+        physical_pages()[i] = PhysicalPage::create(paddr.offset(i * PAGE_SIZE), false, false);
 }
 
 AnonymousVMObject::AnonymousVMObject(const AnonymousVMObject& other)

Kernel/VM/InodeVMObject.cpp

@@ -19,7 +19,7 @@ NonnullRefPtr<VMObject> InodeVMObject::clone()
 }
 
 InodeVMObject::InodeVMObject(Inode& inode)
-    : VMObject(ceil_div(inode.size(), PAGE_SIZE) * PAGE_SIZE, ShouldFillPhysicalPages::Yes)
+    : VMObject(inode.size())
     , m_inode(inode)
 {
 }
@@ -43,18 +43,8 @@ void InodeVMObject::inode_size_changed(Badge<Inode>, size_t old_size, size_t new
 
     InterruptDisabler disabler;
 
-    auto old_page_count = page_count();
-    m_size = new_size;
-
-    if (page_count() > old_page_count) {
-        // Add null pages and let the fault handler page these in when that day comes.
-        for (auto i = old_page_count; i < page_count(); ++i)
-            m_physical_pages.append(nullptr);
-    } else {
-        // Prune the no-longer valid pages. I'm not sure this is actually correct behavior.
-        for (auto i = page_count(); i < old_page_count; ++i)
-            m_physical_pages.take_last();
-    }
+    auto new_page_count = PAGE_ROUND_UP(new_size);
+    m_physical_pages.resize(new_page_count);
 
     // FIXME: Consolidate with inode_contents_changed() so we only do a single walk.
     for_each_region([](Region& region) {

Kernel/VM/VMObject.cpp

@@ -4,18 +4,15 @@
 #include <Kernel/VM/VMObject.h>
 
 VMObject::VMObject(const VMObject& other)
-    : m_size(other.m_size)
-    , m_physical_pages(other.m_physical_pages)
+    : m_physical_pages(other.m_physical_pages)
 {
     MM.register_vmo(*this);
 }
 
-VMObject::VMObject(size_t size, ShouldFillPhysicalPages should_fill_physical_pages)
-    : m_size(size)
+VMObject::VMObject(size_t size)
+    : m_physical_pages(ceil_div(size, PAGE_SIZE))
 {
     MM.register_vmo(*this);
-    if (should_fill_physical_pages == ShouldFillPhysicalPages::Yes)
-        m_physical_pages.resize(page_count());
 }
 
 VMObject::~VMObject()

Kernel/VM/VMObject.h

@@ -2,7 +2,7 @@
 
 #include <AK/RefCounted.h>
 #include <AK/RefPtr.h>
-#include <AK/Vector.h>
+#include <AK/FixedArray.h>
 #include <AK/Weakable.h>
 #include <Kernel/Lock.h>
 
@@ -21,25 +21,20 @@ public:
     virtual bool is_anonymous() const { return false; }
     virtual bool is_inode() const { return false; }
 
-    int page_count() const { return m_size / PAGE_SIZE; }
-    const Vector<RefPtr<PhysicalPage>>& physical_pages() const { return m_physical_pages; }
-    Vector<RefPtr<PhysicalPage>>& physical_pages() { return m_physical_pages; }
+    size_t page_count() const { return m_physical_pages.size(); }
+    const FixedArray<RefPtr<PhysicalPage>>& physical_pages() const { return m_physical_pages; }
+    FixedArray<RefPtr<PhysicalPage>>& physical_pages() { return m_physical_pages; }
 
-    size_t size() const { return m_size; }
+    size_t size() const { return m_physical_pages.size() * PAGE_SIZE; }
 
 protected:
-    enum ShouldFillPhysicalPages {
-        No = 0,
-        Yes
-    };
-    VMObject(size_t, ShouldFillPhysicalPages);
+    explicit VMObject(size_t);
     explicit VMObject(const VMObject&);
 
     template<typename Callback>
     void for_each_region(Callback);
 
-    size_t m_size { 0 };
-    Vector<RefPtr<PhysicalPage>> m_physical_pages;
+    FixedArray<RefPtr<PhysicalPage>> m_physical_pages;
 
 private:
     VMObject& operator=(const VMObject&) = delete;