Clean up and make try_to_free_buffers() not race with dirty pages

This is preparatory work in our continuing saga on some hard-to-trigger
file corruption with shared writable mmap() after the dirty page
tracking changes (commit d08b3851da41d0ee60851f2c75b118e1f7a5fc89 etc)
were merged.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/buffer.c

@@ -2834,7 +2834,7 @@ int try_to_free_buffers(struct page *page)
 	int ret = 0;
 
 	BUG_ON(!PageLocked(page));
-	if (PageWriteback(page))
+	if (PageDirty(page) || PageWriteback(page))
 		return 0;
 
 	if (mapping == NULL) {		/* can this still happen? */
@@ -2845,22 +2845,6 @@ int try_to_free_buffers(struct page *page)
 	spin_lock(&mapping->private_lock);
 	ret = drop_buffers(page, &buffers_to_free);
 	spin_unlock(&mapping->private_lock);
-	if (ret) {
-		/*
-		 * If the filesystem writes its buffers by hand (eg ext3)
-		 * then we can have clean buffers against a dirty page.  We
-		 * clean the page here; otherwise later reattachment of buffers
-		 * could encounter a non-uptodate page, which is unresolvable.
-		 * This only applies in the rare case where try_to_free_buffers
-		 * succeeds but the page is not freed.
-		 *
-		 * Also, during truncate, discard_buffer will have marked all
-		 * the page's buffers clean.  We discover that here and clean
-		 * the page also.
-		 */
-		if (test_clear_page_dirty(page))
-			task_io_account_cancelled_write(PAGE_CACHE_SIZE);
-	}
 out:
 	if (buffers_to_free) {
 		struct buffer_head *bh = buffers_to_free;