Kernel: Uncommit a shared COW page when discovering it was unshared

When we hit a COW fault and discover than no other process is sharing
the physical page, we simply remap it r/w and save ourselves the
trouble. When this happens, we can also give back (uncommit) one of our
shared committed COW pages, since we won't be needing it.

We had this optimization before, but I mistakenly removed it in
50472fd69f since I had misunderstood
it to be the reason for a panic.

Kernel/VM/AnonymousVMObject.cpp

@@ -315,6 +315,12 @@ PageFaultResponse AnonymousVMObject::handle_cow_fault(size_t page_index, Virtual
     if (page_slot->ref_count() == 1) {
         dbgln_if(PAGE_FAULT_DEBUG, "    >> It's a COW page but nobody is sharing it anymore. Remap r/w");
         set_should_cow(page_index, false);
+
+        if (m_shared_committed_cow_pages) {
+            m_shared_committed_cow_pages->uncommit_one();
+            if (!m_shared_committed_cow_pages->is_empty())
+                m_shared_committed_cow_pages = nullptr;
+        }
         return PageFaultResponse::Continue;
     }
 
@@ -368,4 +374,10 @@ NonnullRefPtr<PhysicalPage> AnonymousVMObject::SharedCommittedCowPages::take_one
     return m_committed_pages.take_one();
 }
 
+void AnonymousVMObject::SharedCommittedCowPages::uncommit_one()
+{
+    ScopedSpinLock locker(m_lock);
+    m_committed_pages.uncommit_one();
+}
+
 }

Kernel/VM/AnonymousVMObject.h

@@ -70,9 +70,11 @@ private:
         explicit SharedCommittedCowPages(CommittedPhysicalPageSet&&);
         ~SharedCommittedCowPages();
 
-        [[nodiscard]] NonnullRefPtr<PhysicalPage> take_one();
         [[nodiscard]] bool is_empty() const { return m_committed_pages.is_empty(); }
 
+        [[nodiscard]] NonnullRefPtr<PhysicalPage> take_one();
+        void uncommit_one();
+
     public:
         SpinLock<u8> m_lock;
         CommittedPhysicalPageSet m_committed_pages;

Kernel/VM/MemoryManager.cpp

@@ -1137,4 +1137,11 @@ NonnullRefPtr<PhysicalPage> CommittedPhysicalPageSet::take_one()
     return MM.allocate_committed_user_physical_page(MemoryManager::ShouldZeroFill::Yes);
 }
 
+void CommittedPhysicalPageSet::uncommit_one()
+{
+    VERIFY(m_page_count > 0);
+    --m_page_count;
+    MM.uncommit_user_physical_pages({}, 1);
+}
+
 }

Kernel/VM/MemoryManager.h

@@ -126,6 +126,7 @@ public:
     size_t page_count() const { return m_page_count; }
 
     [[nodiscard]] NonnullRefPtr<PhysicalPage> take_one();
+    void uncommit_one();
 
     void operator=(CommittedPhysicalPageSet&&) = delete;