From 9e62eb48566f0c5e77460ed5923ff9d228fcd195 Mon Sep 17 00:00:00 2001
From: Andreas Kling <awesomekling@gmail.com>
Date: Mon, 5 Nov 2018 14:10:18 +0100
Subject: [PATCH] Only COW on fault if the physical page has retain_count > 1.

This makes COW pages lazily-but-transparently revert back to read/write.
---
 Kernel/MemoryManager.cpp  | 10 ++++++++++
 Kernel/MemoryManager.h    |  2 ++
 Kernel/ProcFileSystem.cpp |  5 +++--
 Kernel/i386.cpp           |  2 ++
 4 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/Kernel/MemoryManager.cpp b/Kernel/MemoryManager.cpp
index 93989f8d0d..1b0d801ec9 100644
--- a/Kernel/MemoryManager.cpp
+++ b/Kernel/MemoryManager.cpp
@@ -223,6 +223,16 @@ Region* MemoryManager::region_from_laddr(Process& process, LinearAddress laddr)
 
 bool MemoryManager::copy_on_write(Process& process, Region& region, unsigned page_index_in_region)
 {
+    ASSERT_INTERRUPTS_DISABLED();
+    if (region.physical_pages[page_index_in_region]->retain_count() == 1) {
+#ifdef PAGE_FAULT_DEBUG
+        dbgprintf("    >> It's a COW page but nobody is sharing it anymore. Remap r/w\n");
+#endif
+        region.cow_map.set(page_index_in_region, false);
+        remap_region_page(process.m_page_directory, region, page_index_in_region, true);
+        return true;
+    }
+
 #ifdef PAGE_FAULT_DEBUG
     dbgprintf("    >> It's a COW page and it's time to COW!\n");
 #endif
diff --git a/Kernel/MemoryManager.h b/Kernel/MemoryManager.h
index 415fa83a28..9477e420d2 100644
--- a/Kernel/MemoryManager.h
+++ b/Kernel/MemoryManager.h
@@ -37,6 +37,8 @@ public:
             return_to_freelist();
     }
 
+    unsigned retain_count() const { return m_retain_count; }
+
 private:
     PhysicalPage(PhysicalAddress paddr)
         : m_paddr(paddr)
diff --git a/Kernel/ProcFileSystem.cpp b/Kernel/ProcFileSystem.cpp
index 0516089dbd..23e05d3604 100644
--- a/Kernel/ProcFileSystem.cpp
+++ b/Kernel/ProcFileSystem.cpp
@@ -61,9 +61,10 @@ ByteBuffer procfs$pid_vm(Process& process)
             region->name.characters());
         for (size_t i = 0; i < region->physical_pages.size(); ++i) {
             auto& physical_page = region->physical_pages[i];
-            ptr += ksprintf(ptr, "P%x%s ",
+            ptr += ksprintf(ptr, "P%x%s(%u) ",
                 physical_page ? physical_page->paddr().get() : 0,
-                region->cow_map.get(i) ? "!" : ""
+                region->cow_map.get(i) ? "!" : "",
+                physical_page->retain_count()
             );
         }
         ptr += ksprintf(ptr, "\n");
diff --git a/Kernel/i386.cpp b/Kernel/i386.cpp
index dfc903a48c..244a5b4c71 100644
--- a/Kernel/i386.cpp
+++ b/Kernel/i386.cpp
@@ -8,6 +8,8 @@
 #include "IRQHandler.h"
 #include "PIC.h"
 
+//#define PAGE_FAULT_DEBUG
+
 struct DescriptorTablePointer {
     WORD size;
     void* address;