* vm_page: Swapped members usage_count and wired_count. We lost 4 bytes

due to alignment padding before.
* Reorganized merging of caches a bit. Renamed MergeStore() to Merge()
  and moved some more functionality into it. The method also moves the
  pages from source to consumer, now. This is necessary, since
  VMAnonymousCache needs to consider both physical pages and swap pages
  at the same time. Before we first moved the physical pages and the
  swap pages later, which was broken for two reasons: (1) A swap page in
  the consumer cache shadows a physical page of the source cache, which
  we ignored. (2) A source cache's physical page that also had a swap
  page would lose the latter in the process when moved to the consumer
  cache, i.e. if the page was not marked modified, it could be stolen
  and its data would be lost.

These changes improve the situation when building Haiku with 256 MB RAM
in that jam doesn't crash anymore, but in my test the system became
totally unusable after about an hour or 7000 targets (GUI froze). For
some reason it didn't manage to free pages anymore although swapping
heavily.


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@27179 a95241bf-73f2-0310-859d-f6bbb57e9c96

headers/private/kernel/vm_types.h

@@ -110,9 +110,11 @@ struct vm_page {
 	uint8				is_cleared : 1;
 		// is currently only used in vm_page_allocate_page_run()
 	uint8				busy_writing : 1;
+	uint8				merge_swap : 1;
+		// used in VMAnonymousCache::Merge()
 
-	uint16				wired_count;
 	int8				usage_count;
+	uint16				wired_count;
 };
 
 enum {
@@ -232,7 +234,7 @@ public:
 
 	virtual	status_t	Fault(struct vm_address_space *aspace, off_t offset);
 
-	virtual	void		MergeStore(VMCache* source);
+	virtual	void		Merge(VMCache* source);
 
 	virtual	status_t	AcquireUnreferencedStoreRef();
 	virtual	void		AcquireStoreRef();