[PATCH] orphaned pagecache memleak fix

Chris found that with data journaling a reiserfs pagecache may be truncate
while still pinned.  The truncation removes the page->mapping, but the page
is still listed in the VM queues because it still has buffers.  Then during
the journaling process, a buffer is marked dirty and that sets the PG_dirty
bitflag as well (in mark_buffer_dirty).  After that the page is leaked
because it's both dirty and without a mapping.

So we must allow pages without mapping and dirty to reach the PagePrivate
check.  The page->mapping will be checked again right after the PagePrivate
check.
Signed-off-by: Andrea Arcangeli <andrea@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/vmscan.c

@@ -313,8 +313,20 @@ static pageout_t pageout(struct page *page, struct address_space *mapping)
 	 */
 	if (!is_page_cache_freeable(page))
 		return PAGE_KEEP;
-	if (!mapping)
+	if (!mapping) {
+		/*
+		 * Some data journaling orphaned pages can have
+		 * page->mapping == NULL while being dirty with clean buffers.
+		 */
+		if (PageDirty(page) && PagePrivate(page)) {
+			if (try_to_free_buffers(page)) {
+				ClearPageDirty(page);
+				printk("%s: orphaned page\n", __FUNCTION__);
+				return PAGE_CLEAN;
+			}
+		}
 		return PAGE_KEEP;
+	}
 	if (mapping->a_ops->writepage == NULL)
 		return PAGE_ACTIVATE;
 	if (!may_write_to_queue(mapping->backing_dev_info))