Kernel: Defer creation of Region CoW bitmaps until they're needed

Instead of allocating and populating a Copy-on-Write bitmap for each
Region up front, wait until we actually clone the Region for sharing
with another process.

In most cases, we never need any CoW bits and we save ourselves a lot
of kmalloc() memory and time.

Kernel/VM/MemoryManager.cpp

@@ -307,7 +307,6 @@ bool MemoryManager::zero_page(Region& region, unsigned page_index_in_region)
 #ifdef PAGE_FAULT_DEBUG
     dbgprintf("      >> ZERO P%p\n", physical_page->paddr().get());
 #endif
-    region.set_should_cow(page_index_in_region, false);
     vmo.physical_pages()[page_index_in_region] = move(physical_page);
     remap_region_page(region, page_index_in_region);
     return true;
@@ -468,9 +467,9 @@ OwnPtr<Region> MemoryManager::allocate_kernel_region(size_t size, const StringVi
     ASSERT(range.is_valid());
     OwnPtr<Region> region;
     if (user_accessible)
-        region = Region::create_user_accessible(range, name, PROT_READ | PROT_WRITE | PROT_EXEC, false);
+        region = Region::create_user_accessible(range, name, PROT_READ | PROT_WRITE | PROT_EXEC);
     else
-        region = Region::create_kernel_only(range, name, PROT_READ | PROT_WRITE | PROT_EXEC, false);
+        region = Region::create_kernel_only(range, name, PROT_READ | PROT_WRITE | PROT_EXEC);
     MM.map_region_at_address(*m_kernel_page_directory, *region, range.base());
     // FIXME: It would be cool if these could zero-fill on demand instead.
     if (should_commit)

Kernel/VM/Region.cpp

@@ -6,33 +6,30 @@
 #include <Kernel/VM/MemoryManager.h>
 #include <Kernel/VM/Region.h>
 
-Region::Region(const Range& range, const String& name, u8 access, bool cow)
+Region::Region(const Range& range, const String& name, u8 access)
     : m_range(range)
     , m_vmobject(AnonymousVMObject::create_with_size(size()))
     , m_name(name)
     , m_access(access)
-    , m_cow_map(Bitmap::create(m_vmobject->page_count(), cow))
 {
     MM.register_region(*this);
 }
 
-Region::Region(const Range& range, RefPtr<Inode>&& inode, const String& name, u8 access, bool cow)
+Region::Region(const Range& range, RefPtr<Inode>&& inode, const String& name, u8 access)
     : m_range(range)
     , m_vmobject(InodeVMObject::create_with_inode(*inode))
     , m_name(name)
     , m_access(access)
-    , m_cow_map(Bitmap::create(m_vmobject->page_count(), cow))
 {
     MM.register_region(*this);
 }
 
-Region::Region(const Range& range, NonnullRefPtr<VMObject> vmo, size_t offset_in_vmo, const String& name, u8 access, bool cow)
+Region::Region(const Range& range, NonnullRefPtr<VMObject> vmo, size_t offset_in_vmo, const String& name, u8 access)
     : m_range(range)
     , m_offset_in_vmo(offset_in_vmo)
     , m_vmobject(move(vmo))
     , m_name(name)
     , m_access(access)
-    , m_cow_map(Bitmap::create(m_vmobject->page_count(), cow))
 {
     MM.register_region(*this);
 }
@@ -77,9 +74,11 @@ NonnullOwnPtr<Region> Region::clone()
         vaddr().get());
 #endif
     // Set up a COW region. The parent (this) region becomes COW as well!
-    m_cow_map.fill(true);
+    ensure_cow_map().fill(true);
     MM.remap_region(current->process().page_directory(), *this);
-    return Region::create_user_accessible(m_range, m_vmobject->clone(), m_offset_in_vmo, m_name, m_access, true);
+    auto clone_region = Region::create_user_accessible(m_range, m_vmobject->clone(), m_offset_in_vmo, m_name, m_access);
+    clone_region->ensure_cow_map();
+    return clone_region;
 }
 
 int Region::commit()
@@ -123,30 +122,50 @@ size_t Region::amount_shared() const
     return bytes;
 }
 
-NonnullOwnPtr<Region> Region::create_user_accessible(const Range& range, const StringView& name, u8 access, bool cow)
+NonnullOwnPtr<Region> Region::create_user_accessible(const Range& range, const StringView& name, u8 access)
 {
-    auto region = make<Region>(range, name, access, cow);
+    auto region = make<Region>(range, name, access);
     region->m_user_accessible = true;
     return region;
 }
 
-NonnullOwnPtr<Region> Region::create_user_accessible(const Range& range, NonnullRefPtr<VMObject> vmobject, size_t offset_in_vmobject, const StringView& name, u8 access, bool cow)
+NonnullOwnPtr<Region> Region::create_user_accessible(const Range& range, NonnullRefPtr<VMObject> vmobject, size_t offset_in_vmobject, const StringView& name, u8 access)
 {
-    auto region = make<Region>(range, move(vmobject), offset_in_vmobject, name, access, cow);
+    auto region = make<Region>(range, move(vmobject), offset_in_vmobject, name, access);
     region->m_user_accessible = true;
     return region;
 }
 
-NonnullOwnPtr<Region> Region::create_user_accessible(const Range& range, NonnullRefPtr<Inode> inode, const StringView& name, u8 access, bool cow)
+NonnullOwnPtr<Region> Region::create_user_accessible(const Range& range, NonnullRefPtr<Inode> inode, const StringView& name, u8 access)
 {
-    auto region = make<Region>(range, move(inode), name, access, cow);
+    auto region = make<Region>(range, move(inode), name, access);
     region->m_user_accessible = true;
     return region;
 }
 
-NonnullOwnPtr<Region> Region::create_kernel_only(const Range& range, const StringView& name, u8 access, bool cow)
+NonnullOwnPtr<Region> Region::create_kernel_only(const Range& range, const StringView& name, u8 access)
 {
-    auto region = make<Region>(range, name, access, cow);
+    auto region = make<Region>(range, name, access);
     region->m_user_accessible = false;
     return region;
 }
+
+bool Region::should_cow(size_t page_index) const
+{
+    if (m_shared)
+        return false;
+    return m_cow_map && m_cow_map->get(page_index);
+}
+
+void Region::set_should_cow(size_t page_index, bool cow)
+{
+    ASSERT(!m_shared);
+    ensure_cow_map().set(page_index, cow);
+}
+
+Bitmap& Region::ensure_cow_map() const
+{
+    if (!m_cow_map)
+        m_cow_map = make<Bitmap>(page_count(), true);
+    return *m_cow_map;
+}

Kernel/VM/Region.h

@@ -21,10 +21,10 @@ public:
         Execute = 4,
     };
 
-    static NonnullOwnPtr<Region> create_user_accessible(const Range&, const StringView& name, u8 access, bool cow = false);
-    static NonnullOwnPtr<Region> create_user_accessible(const Range&, NonnullRefPtr<VMObject>, size_t offset_in_vmobject, const StringView& name, u8 access, bool cow = false);
-    static NonnullOwnPtr<Region> create_user_accessible(const Range&, NonnullRefPtr<Inode>, const StringView& name, u8 access, bool cow = false);
-    static NonnullOwnPtr<Region> create_kernel_only(const Range&, const StringView& name, u8 access, bool cow = false);
+    static NonnullOwnPtr<Region> create_user_accessible(const Range&, const StringView& name, u8 access);
+    static NonnullOwnPtr<Region> create_user_accessible(const Range&, NonnullRefPtr<VMObject>, size_t offset_in_vmobject, const StringView& name, u8 access);
+    static NonnullOwnPtr<Region> create_user_accessible(const Range&, NonnullRefPtr<Inode>, const StringView& name, u8 access);
+    static NonnullOwnPtr<Region> create_kernel_only(const Range&, const StringView& name, u8 access);
 
     ~Region();
 
@@ -103,8 +103,8 @@ public:
         m_page_directory.clear();
     }
 
-    bool should_cow(size_t page_index) const { return m_cow_map.get(page_index); }
-    void set_should_cow(size_t page_index, bool cow) { m_cow_map.set(page_index, cow); }
+    bool should_cow(size_t page_index) const;
+    void set_should_cow(size_t page_index, bool);
 
     void set_writable(bool b)
     {
@@ -119,11 +119,13 @@ public:
     Region* m_prev { nullptr };
 
     // NOTE: These are public so we can make<> them.
-    Region(const Range&, const String&, u8 access, bool cow = false);
-    Region(const Range&, NonnullRefPtr<VMObject>, size_t offset_in_vmo, const String&, u8 access, bool cow = false);
-    Region(const Range&, RefPtr<Inode>&&, const String&, u8 access, bool cow = false);
+    Region(const Range&, const String&, u8 access);
+    Region(const Range&, NonnullRefPtr<VMObject>, size_t offset_in_vmo, const String&, u8 access);
+    Region(const Range&, RefPtr<Inode>&&, const String&, u8 access);
 
 private:
+    Bitmap& ensure_cow_map() const;
+
     RefPtr<PageDirectory> m_page_directory;
     Range m_range;
     size_t m_offset_in_vmo { 0 };
@@ -132,5 +134,5 @@ private:
     u8 m_access { 0 };
     bool m_shared { false };
     bool m_user_accessible { false };
-    Bitmap m_cow_map;
+    mutable OwnPtr<Bitmap> m_cow_map;
 };