kernel/vm: Track fault and copy counts in VMCache.

This solves a TODO for reporting CoW counts in area_info, and paves
the way for adaptive handling of pre-faulting based on how many
faults a cache has handled.

Change-Id: I4ecd7cf46b794c51acac87184fef49ea5ce76743
Reviewed-on: https://review.haiku-os.org/c/haiku/+/8873
Haiku-Format: Haiku-format Bot <[email protected]>
Reviewed-by: waddlesplash <[email protected]>

headers/private/kernel/vm/VMCache.h

@@ -174,6 +174,16 @@ public:
 	virtual	status_t			Fault(struct VMAddressSpace *aspace,
 									off_t offset);
 
+	inline	uint64				FaultCount() const
+									{ return fFaultCount; }
+	inline	void				IncrementFaultCount()
+									{ fFaultCount++; }
+
+	inline	uint64				CopiedPagesCount() const
+									{ return fCopiedPagesCount; }
+	inline	void				IncrementCopiedPagesCount()
+									{ fCopiedPagesCount++; }
+
 	virtual	void				Merge(VMCache* source);
 
 	virtual	status_t			AcquireUnreferencedStoreRef();
@@ -228,7 +238,10 @@ private:
 			PageEventWaiter*	fPageEventWaiters;
 			void*				fUserData;
 			VMCacheRef*			fCacheRef;
+
 			page_num_t			fWiredPagesCount;
+			uint64				fFaultCount;
+			uint64				fCopiedPagesCount;
 };