From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx2.suse.de ([195.135.220.15]:53430 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S2998769AbdDZK4E (ORCPT ); Wed, 26 Apr 2017 06:56:04 -0400 Date: Wed, 26 Apr 2017 12:55:56 +0200 From: Jan Kara Subject: Re: Reviewing determinism of xfs_reclaim_inodes_ag() Message-ID: <20170426105556.GA24825@quack2.suse.cz> References: <20170425082503.GN28800@wotan.suse.de> <20170426000426.GK12369@dastard> <20170426091206.GA28800@wotan.suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170426091206.GA28800@wotan.suse.de> Sender: linux-xfs-owner@vger.kernel.org List-ID: List-Id: xfs To: "Luis R. Rodriguez" Cc: Dave Chinner , linux-xfs@vger.kernel.org, Nikolay Borisov , Jan Kara , Michal Hocko On Wed 26-04-17 11:12:06, Luis R. Rodriguez wrote: > On Wed, Apr 26, 2017 at 10:04:26AM +1000, Dave Chinner wrote: > > On Tue, Apr 25, 2017 at 10:25:03AM +0200, Luis R. Rodriguez wrote: > > > I checked with Jan Kara and he believes the current code is correct but that > > > its the comment that that may be misleading. As per Jan the race is between > > > getting an inode reclaimed and grabbing it. Ie, XFS frees the inodes by RCU. > > > However it doesn't actually *reuse* the inode until RCU period passes > > > (unlike inodes allocated from slab with SLAB_RCU can be). So it can happen > > > > ..... I initially tried using SLAB_DESTROY_BY_RCU which meant the > > RCU grace period did not prevent reallocation of inodes that had > > been freed. Hence this check was (once) necessary to prevent the > > reclaim index going whacky on a reallocated inode. > > Alright this helps, but why does *having* the RCU grace period prevent > such type of race ? I can see it helping but removing completely such > a race as a possibility ? Well, if the inode is freed only after RCU period expires and we are doing xfs_reclaim_inode_grab() under rcu_read_lock - which we are - then this surely prevents us from seeing inode reallocated. What are you missing? > Also, just so I understand I am following, this then implicates our > reclaim rate is directly linked to the RCU grace period ? Yes, as for any RCU-freed object... Honza -- Jan Kara SUSE Labs, CR