* Added a TODO about a problematic use of vm_page_allocate_page() in combination

with vm_cache_acquire_locked_page_cache().
* Added new function vm_page_num_unused_pages() which returns the pages that are
  actually completely free and unused.


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@30514 a95241bf-73f2-0310-859d-f6bbb57e9c96

headers/private/kernel/vm_page.h

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2008, Axel Dörfler, [email protected].
+ * Copyright 2002-2009, Axel Dörfler, [email protected].
  * Distributed under the terms of the MIT License.
  *
  * Copyright 2001-2002, Travis Geiselbrecht. All rights reserved.
@@ -35,6 +35,7 @@ void vm_page_requeue(struct vm_page *page, bool tail);
 size_t vm_page_num_pages(void);
 size_t vm_page_num_free_pages(void);
 size_t vm_page_num_available_pages(void);
+size_t vm_page_num_unused_pages(void);
 void vm_page_get_stats(system_info *info);
 
 status_t vm_page_write_modified_page_range(struct VMCache *cache,

src/system/kernel/vm/vm_cache.cpp

@@ -345,6 +345,9 @@ vm_cache_acquire_locked_page_cache(vm_page* page, bool dontWait)
 			return NULL;
 		}
 
+		// TODO: this is problematic, as it requires the caller not to have
+		// a lock on this cache (it might be called via
+		// vm_page_allocate_page(..., false)).
 		if (!cache->SwitchLock(&sCacheListLock)) {
 			// cache has been deleted
 			mutex_lock(&sCacheListLock);

src/system/kernel/vm/vm_page.cpp

@@ -1893,6 +1893,8 @@ vm_page_try_reserve_pages(uint32 count)
 }
 
 
+// TODO: you must not have locked a cache when calling this function with
+// reserved == false. See vm_cache_acquire_locked_page_cache().
 vm_page *
 vm_page_allocate_page(int pageState, bool reserved)
 {
@@ -2167,13 +2169,20 @@ vm_page_num_free_pages(void)
 {
 	size_t reservedPages = sReservedPages;
 	size_t count = free_page_queue_count() + sInactivePageQueue.count;
-	if (reservedPages > count)
+	if (reservedPages >= count)
 		return 0;
 
 	return count - reservedPages;
 }
 
 
+size_t
+vm_page_num_unused_pages(void)
+{
+	return free_page_queue_count() - sReservedPages;
+}
+
+
 void
 vm_page_get_stats(system_info *info)
 {