ext4: Fix dirtying of journalled buffers in data=journal mode

In data=journal mode, we still use block_write_begin() to prepare
page for writing. This function can occasionally mark buffer dirty
which violates journalling assumptions - when a buffer is part of
a transaction, it should be dirty and a buffer can be already part
of a forget list of some transaction when block_write_begin()
gets called. This violation of journalling assumptions then results
in "JBD: Spotted dirty metadata buffer..." warnings.

In fact, temporary dirtying the buffer while the page is still locked
does not really cause problems to the journalling because we won't write
the buffer until the page gets unlocked. So we just have to make sure
to clear dirty bits before unlocking the page.
Signed-off-by: Jan Kara <jack@suse.cz>

fs/ext4/inode.c

@@ -1527,9 +1527,25 @@ static int walk_page_buffers(handle_t *handle,
 static int do_journal_get_write_access(handle_t *handle,
 				       struct buffer_head *bh)
 {
+	int dirty = buffer_dirty(bh);
+	int ret;
+
 	if (!buffer_mapped(bh) || buffer_freed(bh))
 		return 0;
-	return ext4_journal_get_write_access(handle, bh);
+	/*
+	 * __block_prepare_write() could have dirtied some buffers. Clean
+	 * the dirty bit as jbd2_journal_get_write_access() could complain
+	 * otherwise about fs integrity issues. Setting of the dirty bit
+	 * by __block_prepare_write() isn't a real problem here as we clear
+	 * the bit before releasing a page lock and thus writeback cannot
+	 * ever write the buffer.
+	 */
+	if (dirty)
+		clear_buffer_dirty(bh);
+	ret = ext4_journal_get_write_access(handle, bh);
+	if (!ret && dirty)
+		ret = ext4_handle_dirty_metadata(handle, NULL, bh);
+	return ret;
 }
 
 /*