[PATCH] truncate: clear page dirtiness before running try_to_free_buffers()

truncate presently invalidates the dirty page's buffer_heads then shoots down
the page.  But try_to_free_buffers() will now bale out because the page is
dirty.

Net effect: the LRU gets filled with dirty pages which have invalidated
buffer_heads attached.  They have no ->mapping and hence cannot be cleaned.
The machine leaks memory at an enormous rate.

Fix this by cleaning the page before running try_to_free_buffers(), so
try_to_free_buffers() can do its work.

Also, remember to do dirty-page-acoounting in cancel_dirty_page() so the
machine won't wedge up trying to write non-existent dirty pages.

Probably still wrong, but now less so.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/truncate.c

@@ -60,11 +60,12 @@ void cancel_dirty_page(struct page *page, unsigned int account_size)
 		WARN_ON(++warncount < 5);
 	}
 		
-	if (TestClearPageDirty(page) && account_size)
+	if (TestClearPageDirty(page) && account_size) {
+		dec_zone_page_state(page, NR_FILE_DIRTY);
 		task_io_account_cancelled_write(account_size);
+	}
 }
 
-
 /*
  * If truncate cannot remove the fs-private metadata from the page, the page
  * becomes anonymous.  It will be left on the LRU and may even be mapped into
@@ -81,11 +82,11 @@ truncate_complete_page(struct address_space *mapping, struct page *page)
 	if (page->mapping != mapping)
 		return;
 
+	cancel_dirty_page(page, PAGE_CACHE_SIZE);
+
 	if (PagePrivate(page))
 		do_invalidatepage(page, 0);
 
-	cancel_dirty_page(page, PAGE_CACHE_SIZE);
-
 	ClearPageUptodate(page);
 	ClearPageMappedToDisk(page);
 	remove_from_page_cache(page);