From mboxrd@z Thu Jan 1 00:00:00 1970 From: Theodore Tso Subject: Re: on disk format: value of bg_inode_table_hi? Date: Thu, 29 Jan 2009 23:19:40 -0500 Message-ID: <20090130041940.GB23220@mit.edu> References: <20090129051221.GH8990@mit.edu> <419866.84825.qm@web23605.mail.ird.yahoo.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Andreas Dilger , linux-ext4@vger.kernel.org To: Etienne Lorrain Return-path: Received: from thunk.org ([69.25.196.29]:59656 "EHLO thunker.thunk.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752759AbZA3ETq (ORCPT ); Thu, 29 Jan 2009 23:19:46 -0500 Content-Disposition: inline In-Reply-To: <419866.84825.qm@web23605.mail.ird.yahoo.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Thu, Jan 29, 2009 at 11:47:29AM +0000, Etienne Lorrain wrote: > - If I only access the ext4 filesystem readonly, I do not > have any difference considering the flags of the superblock: > EXT4_FEATURE_INCOMPAT_META_BG, EXT4_FEATURE_INCOMPAT_MMP, > EXT4_FEATURE_INCOMPAT_FLEX_BG. meta_bg is an INCOMPAT feature because kernels that don't understand this option won't be able to find the inode table blocks, and would crash and burn. flex_bg is an INCOMPAT feature because older kernels do an explicit sanity check to make sure a block group's metadata is in the block group. This one could have been ro_incompat, I suppose, since the worst that would happen with an ro mount on those older kernels is they would cause an error when the filesystem is mounted. On the other hand, if the filesystem was also marked "panic and reboot on error", some users might consider it unfriendly that mounting such a filesystem on an older kernel would cause an immediate reboot. Given that there are some security extremists who consider the possibility of filesystem images that cause a reboot as a "security bug" that worthy of assignment of a CVE and urgent bug reports forcing other developers to drop everything and address said "security bug", it's probably best that flex_bg is an INCOMPAT feature. :-) - Ted