Kernel: Use a shared physical page for zero-filled pages until written

This patch adds a globally shared zero-filled PhysicalPage that will
be mapped into every slot of every zero-filled AnonymousVMObject until
that page is written to, achieving CoW-like zero-filled pages.

Initial testing show that this doesn't actually achieve any sharing yet
but it seems like a good design regardless, since it may reduce the
number of page faults taken by programs.

If you look at the refcount of MM.shared_zero_page() it will have quite
a high refcount, but that's just because everything maps it everywhere.
If you want to see the "real" refcount, you can build with the
MAP_SHARED_ZERO_PAGE_LAZILY flag, and we'll defer mapping of the shared
zero page until the first NP read fault.

I've left this behavior behind a flag for future testing of this code.

1 files changed, 3 insertions, 1 deletions

diff --git a/Kernel/VM/MemoryManager.cpp b/Kernel/VM/MemoryManager.cpp
index 8573f52561..a4433ed036 100644
--- a/Kernel/VM/MemoryManager.cpp
+++ b/Kernel/VM/MemoryManager.cpp
@@ -53,6 +53,8 @@ MemoryManager::MemoryManager()
     write_cr3(kernel_page_directory().cr3());
     setup_low_identity_mapping();
     protect_kernel_image();
+
+    m_shared_zero_page = allocate_user_physical_page();
 }
 
 MemoryManager::~MemoryManager()
@@ -297,7 +299,7 @@ OwnPtr<Region> MemoryManager::allocate_kernel_region(size_t size, const StringVi
         region = Region::create_user_accessible(range, name, access, cacheable);
     else
         region = Region::create_kernel_only(range, name, access, cacheable);
-    region->set_page_directory(kernel_page_directory());
+    region->map(kernel_page_directory());
     if (should_commit)
         region->commit();
     return region;