From mboxrd@z Thu Jan 1 00:00:00 1970 From: Theodore Ts'o Subject: Re: [PATCH v2 3/5] ext4: fix wrong m_len value after unwritten extent conversion Date: Tue, 12 Mar 2013 21:57:41 -0400 Message-ID: <20130313015741.GA16919@thunk.org> References: <1362579435-6333-1-git-send-email-wenqing.lz@taobao.com> <1362579435-6333-4-git-send-email-wenqing.lz@taobao.com> <20130311010718.GC10090@thunk.org> <20130311054707.GA3867@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii To: linux-ext4@vger.kernel.org, Zheng Liu , Dmitry Monakhov Return-path: Received: from li9-11.members.linode.com ([67.18.176.11]:52993 "EHLO imap.thunk.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932439Ab3CMB5s (ORCPT ); Tue, 12 Mar 2013 21:57:48 -0400 Content-Disposition: inline In-Reply-To: <20130311054707.GA3867@gmail.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Mon, Mar 11, 2013 at 01:47:07PM +0800, Zheng Liu wrote: > On Sun, Mar 10, 2013 at 09:07:18PM -0400, Theodore Ts'o wrote: > > On Wed, Mar 06, 2013 at 10:17:13PM +0800, Zheng Liu wrote: > > > From: Zheng Liu > > > > > > We always assume that the return value of ext4_ext_map_blocks is equal > > > to map->m_len > > > > Note that in general, this is _never_ safe to assume. There are a > > number of times when the number of blocks mapped is less than what the > > caller originally requested, both when allocating blocks (and there > > isn't the requestd number of contiguous blocks available), and when > > EXT4_GET_BLOCKS_CREATE is not set. > > Yes, When EXT4_GET_BLOCKS_CREATE is not set, it could be 0 because there > is no block mapping, and we don't create it. Meanwhile when we want to > allocate some blocks, it could be less than the number of block we > requested. But IMHO at least when we try to allocate some blocks, m_len > should be changed according to the number of allocated blocks in order > to make them equal if the number of allocated blocks is less than the > number of blocks we requested. Namely, when the return value (retval) > is greater than 0, this assumption will be right. Because we will use > m_len value after map_blocks function returns. We need to let upper > level know it, such as write_begin, DIO, etc... Am I miss something? No, you didn't miss anything. I just wanted to say point out that any assumption that ext4_ext_map_blocks() is equial to map->m_len was always wrong, and not something that recently changed. I updated the commit description lightly to make this clear. Regards, - Ted