From: Jan Kara Subject: Re: Journal under-reservation bug on first >2G file Date: Thu, 2 Oct 2014 13:26:45 +0200 Message-ID: <20141002112645.GD19748@quack.suse.cz> References: <542B1C38.9010409@redhat.com> <542B1EFC.4050500@redhat.com> <20141001115320.GA2903@thunk.org> <542C1314.3030603@redhat.com> <20141001195954.GD2903@thunk.org> <542C65FD.5040405@redhat.com> <20141001224325.GE2903@thunk.org> <542CE755.9040503@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Theodore Ts'o , Andreas Dilger , ext4 development To: Eric Sandeen Return-path: Received: from cantor2.suse.de ([195.135.220.15]:60246 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750987AbaJBL0s (ORCPT ); Thu, 2 Oct 2014 07:26:48 -0400 Content-Disposition: inline In-Reply-To: <542CE755.9040503@redhat.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Thu 02-10-14 00:49:09, Eric Sandeen wrote: > On 10/1/14 5:43 PM, Theodore Ts'o wrote: > > On Wed, Oct 01, 2014 at 03:37:17PM -0500, Eric Sandeen wrote: > >> > >> Ok. I guess this is only an issue for ext4 - well, at least this specific > >> issue. Delalloc makes it much different than ext2 & ext3, which reserve quite a > >> lot more. Whether there's a corner case over there which breaks, I dunno... > >> > >> So it seems like the simplest test is simply: Are we RW mounted with delalloc? > >> And if so, update the feature. Seems simpler than mucking with "which features > >> are unique to ext4" > > > > I'd do "are we RW mounted with the extents feature". That way we > > don't need to worry about someone accidentally mounting a partition > > meant for Hurd using ext4, which would imply delalloc, and then > > causing Hurd to no longer be able to deal with the file system. That > > *shouldn't* happen, but if someone accidentally mounts the file system > > with -t ext4, but it seems safer to gate it on the existence of the > > extents feature. > > Problem is, we can hit the same problem with an ext3 filesystem (no > extents) mounted with -t ext4 (enabling delalloc). > > Ugh. Can't we just bump the da write reservation to 2 and be done with it? ;) > (AFAICT the non-delalloc reservations can be wildly overestimated). > > Or maybe ext4_journal_extend() when we try to update the superblock? > It could fail, but it wouldn't be catastrophic if it did, fsck would find > that the feature is missing... A couple of notes: 1) Using 2 would be fine. Journal code is clever enough and it returns unused handle credits to the transaction so using 2 instead of 1 limits only the number of handles in ext4_da_write_begin() running in parallel. So I'd frankly just bump the number to 2 (with a comment!) and be done with it. 2) If we want to optimize a bit, we can check whether the write is going to extend beyond 2G and first set the feature in a separate transaction. Honza -- Jan Kara SUSE Labs, CR