From f7f1b08ec035ddef61218b51d8e9a9d4c1ee7b9a Mon Sep 17 00:00:00 2001
From: John Hodge <tpg@portege.thepowersgang.com>
Date: Tue, 22 Sep 2009 20:17:46 +0800
Subject: [PATCH] Changed COW to reuse current page if it is the last
 reference.

---
 Kernel/arch/x86/include/mm_phys.h |  3 ++-
 Kernel/arch/x86/mm_phys.c         | 19 +++++++++++++++++--
 Kernel/arch/x86/mm_virt.c         | 16 ++++++++++++----
 3 files changed, 31 insertions(+), 7 deletions(-)

diff --git a/Kernel/arch/x86/include/mm_phys.h b/Kernel/arch/x86/include/mm_phys.h
index 65ab091b..8d461098 100644
--- a/Kernel/arch/x86/include/mm_phys.h
+++ b/Kernel/arch/x86/include/mm_phys.h
@@ -6,8 +6,9 @@
 #define _MM_PHYS_H
 
 // === FUNCTIONS ===
-extern Uint32	MM_AllocPhys();
+extern tPAddr	MM_AllocPhys();
 extern void	MM_RefPhys(Uint32 Addr);
 extern void	MM_DerefPhys(Uint32 Addr);
+extern int	MM_GetRefCount(tPAddr Addr);
 
 #endif
diff --git a/Kernel/arch/x86/mm_phys.c b/Kernel/arch/x86/mm_phys.c
index 80e92fe6..e81c99e6 100644
--- a/Kernel/arch/x86/mm_phys.c
+++ b/Kernel/arch/x86/mm_phys.c
@@ -85,7 +85,7 @@ void MM_Install(tMBoot_Info *MBoot)
 }
 
 /**
- * \fn Uint32 MM_AllocPhys()
+ * \fn tPAddr MM_AllocPhys()
  * \brief Allocates a physical page
  */
 tPAddr MM_AllocPhys()
@@ -154,7 +154,7 @@ void MM_RefPhys(tPAddr Addr)
 /**
  * \fn void MM_DerefPhys(Uint32 Addr)
  */
-void MM_DerefPhys(Uint32 Addr)
+void MM_DerefPhys(tPAddr Addr)
 {
 	// Get page number
 	Addr >>= 12;
@@ -185,3 +185,18 @@ void MM_DerefPhys(Uint32 Addr)
 	// Release spinlock
 	RELEASE( &giPhysAlloc );
 }
+
+/**
+ * \fn int MM_GetRefCount(tPAddr Addr)
+ */
+int MM_GetRefCount(tPAddr Addr)
+{
+	// Get page number
+	Addr >>= 12;
+	
+	// We don't care about non-ram pages
+	if(Addr >= giPageCount)	return -1;
+	
+	// Check if it is freed
+	return gaPageReferences[ Addr ];
+}
diff --git a/Kernel/arch/x86/mm_virt.c b/Kernel/arch/x86/mm_virt.c
index 25da7433..8fe1b4aa 100644
--- a/Kernel/arch/x86/mm_virt.c
+++ b/Kernel/arch/x86/mm_virt.c
@@ -105,10 +105,18 @@ void MM_PageFault(Uint Addr, Uint ErrorCode, tRegs *Regs)
 	 && gaPageTable[Addr>>12] & PF_COW )
 	{
 		tPAddr	paddr;
-		paddr = MM_DuplicatePage( Addr );
-		MM_DerefPhys( gaPageTable[Addr>>12] & ~0xFFF );
-		gaPageTable[Addr>>12] &= PF_USER;
-		gaPageTable[Addr>>12] |= paddr|PF_PRESENT|PF_WRITE;
+		if(MM_GetRefCount( gaPageTable[Addr>>12] & ~0xFFF ) == 0)
+		{
+			gaPageTable[Addr>>12] &= ~PF_COW;
+			gaPageTable[Addr>>12] |= PF_PRESENT|PF_WRITE;
+		}
+		else
+		{
+			paddr = MM_DuplicatePage( Addr );
+			MM_DerefPhys( gaPageTable[Addr>>12] & ~0xFFF );
+			gaPageTable[Addr>>12] &= PF_USER;
+			gaPageTable[Addr>>12] |= paddr|PF_PRESENT|PF_WRITE;
+		}
 		INVLPG( Addr & ~0xFFF );
 		//LEAVE('-')
 		return;
-- 
2.20.1