* Introduces VMCache::CanWritePage() returning whether the given cache can

theoretically write the given page.
* page writer: Fixed the incorrect check whether a temporary page can be
  written by using the new CanWritePage().


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@30281 a95241bf-73f2-0310-859d-f6bbb57e9c96

headers/private/kernel/vm_types.h

@@ -219,6 +219,7 @@ public:
 	virtual	status_t	WriteAsync(off_t offset, const iovec* vecs,
 							size_t count, size_t numBytes, uint32 flags,
 							AsyncIOCallback* callback);
+	virtual	bool		CanWritePage(off_t offset);
 
 	virtual	status_t	Fault(struct vm_address_space *aspace, off_t offset);
 

src/system/kernel/cache/vnode_store.cpp

@@ -101,6 +101,14 @@ VMVnodeCache::Fault(struct vm_address_space *aspace, off_t offset)
 }
 
 
+bool
+VMVnodeCache::CanWritePage(off_t offset)
+{
+	// all pages can be written
+	return true;
+}
+
+
 status_t
 VMVnodeCache::AcquireUnreferencedStoreRef()
 {

src/system/kernel/cache/vnode_store.h

@@ -26,6 +26,7 @@ public:
 	virtual	status_t	WriteAsync(off_t offset, const iovec* vecs,
 							size_t count, size_t numBytes, uint32 flags,
 							AsyncIOCallback* callback);
+	virtual	bool		CanWritePage(off_t offset);
 
 	virtual	status_t	Fault(struct vm_address_space *aspace, off_t offset);
 

src/system/kernel/vm/VMAnonymousCache.cpp

@@ -641,6 +641,16 @@ VMAnonymousCache::WriteAsync(off_t offset, const iovec* vecs, size_t count,
 }
 
 
+bool
+VMAnonymousCache::CanWritePage(off_t offset)
+{
+	// We can write the page, if we have not used all of our committed swap
+	// space or the page already has a swap slot assigned.
+	return fAllocatedSwapSize < fCommittedSwapSize
+		|| _SwapBlockGetAddress(offset >> PAGE_SHIFT) != SWAP_SLOT_NONE;
+}
+
+
 status_t
 VMAnonymousCache::Fault(struct vm_address_space *aspace, off_t offset)
 {

src/system/kernel/vm/VMAnonymousCache.h

@@ -45,6 +45,7 @@ public:
 	virtual	status_t	WriteAsync(off_t offset, const iovec* vecs,
 							size_t count, size_t numBytes, uint32 flags,
 							AsyncIOCallback* callback);
+	virtual	bool		CanWritePage(off_t offset);
 
 	virtual	status_t	Fault(struct vm_address_space *aspace, off_t offset);
 

src/system/kernel/vm/vm_cache.cpp

@@ -892,6 +892,20 @@ VMCache::WriteAsync(off_t offset, const iovec* vecs, size_t count,
 }
 
 
+/*!	\brief Returns whether the cache can write the page at the given offset.
+
+	The cache must be locked when this function is invoked.
+
+	@param offset The page offset.
+	@return \c true, if the page can be written, \c false otherwise.
+*/
+bool
+VMCache::CanWritePage(off_t offset)
+{
+	return false;
+}
+
+
 status_t
 VMCache::Fault(struct vm_address_space *aspace, off_t offset)
 {

src/system/kernel/vm/vm_page.cpp

@@ -1247,7 +1247,8 @@ page_writer(void* /*unused*/)
 				|| (cache->temporary
 #if ENABLE_SWAP_SUPPORT
 					&& (!lowOnPages /*|| page->usage_count > 0*/
-						|| swap_available_pages() == 0)
+						|| !cache->CanWritePage(
+								(off_t)page->cache_offset << PAGE_SHIFT))
 #endif
 				)) {
 				continue;