From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Kara Subject: Re: [PATCH 6/7] ext4: endless truncate due to nonlocked dio readers V2 Date: Thu, 13 Sep 2012 16:34:55 +0200 Message-ID: <20120913143455.GB4328@quack.suse.cz> References: <1347211634-11509-1-git-send-email-dmonakhov@openvz.org> <1347211634-11509-7-git-send-email-dmonakhov@openvz.org> <20120913104136.GB11330@gmail.com> <20120913120736.GA4328@quack.suse.cz> <20120913125726.GA32155@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Jan Kara , Dmitry Monakhov , linux-ext4@vger.kernel.org, tytso@mit.edu, wenqing.lz@taobao.com To: Zheng Liu Return-path: Received: from cantor2.suse.de ([195.135.220.15]:47999 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751056Ab2IMOfD (ORCPT ); Thu, 13 Sep 2012 10:35:03 -0400 Content-Disposition: inline In-Reply-To: <20120913125726.GA32155@gmail.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Thu 13-09-12 20:57:26, Zheng Liu wrote: > On Thu, Sep 13, 2012 at 02:07:36PM +0200, Jan Kara wrote: > > Hello, > > > > On Thu 13-09-12 18:41:36, Zheng Liu wrote: > > > Could you please provide more detailed workload to convince me? I > > > am thinking about whether we really need to disable dioread_nolock > > > feature in here. In our benchmarks, we don't see this problem. > > I just did: > > > > # Create file > > dd if=/dev/zero of=/mnt/file bs=1M count=30 > > sync > > # Start 10 DIO dio readers in parallel reading the file in a loop > > for (( i = 0; i < 10; i++ )); do > > while true; do > > dd if=/mnt/file bs=4k iflag=direct of=/dev/null > > done & > > done > > sleep 1 > > > > # Try to truncate the file - never finishes. > > truncate -s 16 /mnt/file > > > > It is pretty easy to hit this. Besides being a DOS attack vector (but I > > won't be too concerned about this - there are plenty of ways how local > > process can screw you) I can easily imagine some application to get bitten > > by this. > > Hi Jan, > > Thanks for your explanation, but in my desktop I cannot reproduce this > problem. The size of `file' is 16. Am I missing something? Hum, on my test machine with 3.6-rc1 it does not... Maybe for your desktop you need a larger sleep before running truncate so that readers have time to start up? Also I suppose you have ext4 mounted with dioread_nolock mount option? Honza -- Jan Kara SUSE Labs, CR