From: Ted Ts'o Subject: Re: [PATCH] ext4: s_dirtyclusters_counter should tranform to unit of cluster before assigning to "dirty_clusters" in ext4_has_free_clusters() Date: Thu, 9 Feb 2012 21:55:26 -0500 Message-ID: <20120210025526.GB21496@thunk.org> References: <1328066270-4461-1-git-send-email-hao.bigrat@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-ext4@vger.kernel.org To: Robin Dong Return-path: Received: from li9-11.members.linode.com ([67.18.176.11]:48280 "EHLO test.thunk.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758083Ab2BJCz3 (ORCPT ); Thu, 9 Feb 2012 21:55:29 -0500 Content-Disposition: inline In-Reply-To: Sender: linux-ext4-owner@vger.kernel.org List-ID: On Thu, Feb 02, 2012 at 03:48:49PM +0800, Robin Dong wrote: > > The reason is ext4_has_free_clusters reporting wrong > result. Actually, the unit of argument "dirty_clusters" is block, so > we should tranform the sbi->s_dirtyclusters_counter to block , just > like "free_clusters". I've been looking at the the delayed allocation reservation code, and I think it's a lot more confused that this. Some places we are playing with fields as if they are clusters, and some times as if they are blocks, and I've definitely found places where we're not handling quota correctly with bigalloc (ext4_map_blocks is calling ext4_da_update_reserve_space() with a value which is clearly in units of blocks, but we are treating that vale as if it is clusters, etc.) So I think the problem is a lot larger than what you pointed out, and we shouldn't fix it just by throwing in an EXT4_C2B call. If s_dirtyclusters_counter should be denominated in blocks, then we need to rename it. And there there's this comment which made me wince: * Note that in case of bigalloc, i_reserved_meta_blocks, * i_reserved_data_blocks, etc. refer to number of clusters. Argh, no. If something is denominated in clusters, then it should be appropriately named as such. Actually, I think the problem is a lot bigger than this, and it's a conceptual one. When we try writing to a block which does not have a mapping, and bigalloc is enabled, there are three cases: 1) This is the first time the cluster has ever been written to; hence, even though we are dirtying a block, we need to reserve room for the entire cluster. 2) While the block has not been mapped (and so the data on disk is uninitialized) the cluster has indeed been allocated, so we don't need to reserve any additional room for the block. 3) One or more blocks in the cluster have already been written to via delayed allocation, but the cluster itself has not been selected (allocated) yet. But since the space for the entire cluster was reserved when the first block in the cluster was dirtied (case #1), we don't need to reserve any more space. We aren't handling these cases correctly today, since we don't have this logic here. These cases don't matter if you're only using fallocate() and direct I/O, but if you are using buffered writes and delayed allocation, they obviously matter a huge amount. And I don't think we're handling it correctly. Argh... - Ted