First steps towards being able of collapse vm_cache objects after forking:

* a vm_cache now maintains a list of its "consumer" caches.
* introduced to new functions that add/remove consumer to a cache (instead
  of only maintaining the vm_cache::source field).
* fixed the incorrect reference counting when doing copy-on-write; we kept
  one ref too many of the lower cache.
* minor cleanup.


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@19035 a95241bf-73f2-0310-859d-f6bbb57e9c96

headers/private/kernel/vm_cache.h

@@ -27,6 +27,8 @@ void vm_cache_release_ref(vm_cache_ref *cache_ref);
 vm_page *vm_cache_lookup_page(vm_cache_ref *cacheRef, off_t page);
 void vm_cache_insert_page(vm_cache_ref *cacheRef, vm_page *page, off_t offset);
 void vm_cache_remove_page(vm_cache_ref *cacheRef, vm_page *page);
+void vm_cache_remove_consumer(vm_cache_ref *cacheRef, vm_cache *consumer);
+void vm_cache_add_consumer(vm_cache_ref *cacheRef, vm_cache *consumer);
 status_t vm_cache_write_modified(vm_cache_ref *ref, bool fsReenter);
 status_t vm_cache_set_minimal_commitment(vm_cache_ref *ref, off_t commitment);
 status_t vm_cache_resize(vm_cache_ref *cacheRef, off_t newSize);

headers/private/kernel/vm_types.h

@@ -68,6 +68,9 @@ typedef struct vm_cache_ref {
 
 // vm_cache
 typedef struct vm_cache {
+	struct list_link	consumer_link;
+	struct list			consumers;
+		// list of caches that use this cache as a source
 	vm_page				*page_list;
 	vm_cache_ref		*ref;
 	struct vm_cache		*source;