From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754792AbeBGQ6H (ORCPT ); Wed, 7 Feb 2018 11:58:07 -0500 Received: from bombadil.infradead.org ([65.50.211.133]:39011 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754715AbeBGQ6F (ORCPT ); Wed, 7 Feb 2018 11:58:05 -0500 Date: Wed, 7 Feb 2018 17:58:02 +0100 From: Peter Zijlstra To: Joel Fernandes Cc: LKML , Michal Hocko , Minchan Kim , "open list:MEMORY MANAGEMENT" Subject: Re: [PATCH RFC] ashmem: Fix lockdep RECLAIM_FS false positive Message-ID: <20180207165802.GC25219@hirez.programming.kicks-ass.net> References: <20180206004903.224390-1-joelaf@google.com> <20180207080740.GH2269@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 07, 2018 at 08:09:36AM -0800, Joel Fernandes wrote: > Hi Peter, > > On Wed, Feb 7, 2018 at 12:07 AM, Peter Zijlstra wrote: > > On Mon, Feb 05, 2018 at 04:49:03PM -0800, Joel Fernandes wrote: > > > >> [ 2115.359650] -(1)[106:kswapd0]================================= > >> [ 2115.359665] -(1)[106:kswapd0][ INFO: inconsistent lock state ] > >> [ 2115.359684] -(1)[106:kswapd0]4.9.60+ #2 Tainted: G W O > >> [ 2115.359699] -(1)[106:kswapd0]--------------------------------- > >> [ 2115.359715] -(1)[106:kswapd0]inconsistent {RECLAIM_FS-ON-W} -> > >> {IN-RECLAIM_FS-W} usage. > > > > Please don't wrap log output, this is unreadable :/ > > Sorry about that, here's the unwrapped output, I'll fix the commit > message in next rev: https://pastebin.com/e0BNGkaN So if you trim that leading garbage: "[ 2115.359650] -(1)[106:kswapd0]" you instantly have half you screen back. > > Also, the output is from an ancient kernel and doesn't match the current > > code. > > Right, however the driver hasn't changed and I don't see immediately > how lockdep handles this differently upstream, so I thought of fixing > it upstream. Well, the annotation got a complete rewrite. Granted, it _should_ be similar, but the output will be different. > The bail out happens when GFP_FS is *not* set. Argh, reading is hard. > Lockdep reports this issue when GFP_FS is infact set, and we enter > this path and acquire the lock. So lockdep seems to be doing the right > thing however by design it is reporting a false-positive. So I'm not seeing how its a false positive. fs/inode.c sets a different lock class per filesystem type. So recursing on an i_mutex within a filesystem does sound dodgy. > The real issue is that the lock being acquired is of the same lock > class and a different lock instance is acquired under GFP_FS that > happens to be of the same class. > > So the issue seems to me to be: > Process A kswapd > --------- ------ > acquire i_mutex Enter RECLAIM_FS > > Enter RECLAIM_FS acquire different i_mutex That's not a false positive, that's a 2 process way of writing i_mutex recursion. What are the rules of acquiring two i_mutexes within a filesystem? > Neil tried to fix this sometime back: > https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg623909.html > but it was kind of NAK'ed. So that got nacked because Neil tried to fix it in the vfs core. Also not entirely sure that's the same problem.