From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 95767C3A59F for ; Mon, 26 Aug 2019 12:21:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6D5422184D for ; Mon, 26 Aug 2019 12:21:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1566822074; bh=Ch6Xj7G1lAJ86IAn5C4lLsB1o+ZvZrAx4p5e3cYLhjM=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=GTzbpI+uaJBUZzn0TPam5Z5VVoPi6tmDKh2SJMGKQqhJYZebiUK/8D49FwWXqt2FS FZpfPLWCQBvosgGJKNKeDXXhDrjKkfgxpvC6oZ0LB3B0vLmkI1jOMoO3RAD68/Yxqz 26ksTgnYZmLYBFqYWhKArUcMXuPwJALgUE2nrydw= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731974AbfHZMVN (ORCPT ); Mon, 26 Aug 2019 08:21:13 -0400 Received: from mx2.suse.de ([195.135.220.15]:48486 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727234AbfHZMVM (ORCPT ); Mon, 26 Aug 2019 08:21:12 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 3982BAF03; Mon, 26 Aug 2019 12:21:11 +0000 (UTC) Date: Mon, 26 Aug 2019 14:21:10 +0200 From: Michal Hocko To: Vlastimil Babka Cc: Dave Chinner , Peter Zijlstra , Christoph Hellwig , linux-xfs@vger.kernel.org, Ingo Molnar , Will Deacon , linux-kernel@vger.kernel.org, linux-mm@kvack.org, penguin-kernel@I-love.SAKURA.ne.jp Subject: Re: [PATCH 2/3] xfs: add kmem_alloc_io() Message-ID: <20190826122110.GB7659@dhcp22.suse.cz> References: <20190822003131.GR1119@dread.disaster.area> <20190822075948.GA31346@infradead.org> <20190822085130.GI2349@hirez.programming.kicks-ass.net> <20190822091057.GK2386@hirez.programming.kicks-ass.net> <20190822101441.GY1119@dread.disaster.area> <20190822120725.GA1119@dread.disaster.area> <20190822131739.GB1119@dread.disaster.area> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 22-08-19 16:26:42, Vlastimil Babka wrote: > On 8/22/19 3:17 PM, Dave Chinner wrote: > > On Thu, Aug 22, 2019 at 02:19:04PM +0200, Vlastimil Babka wrote: > >> On 8/22/19 2:07 PM, Dave Chinner wrote: > >> > On Thu, Aug 22, 2019 at 01:14:30PM +0200, Vlastimil Babka wrote: > >> > > >> > No, the problem is this (using kmalloc as a general term for > >> > allocation, whether it be kmalloc, kmem_cache_alloc, alloc_page, etc) > >> > > >> > some random kernel code > >> > kmalloc(GFP_KERNEL) > >> > reclaim > >> > PF_MEMALLOC > >> > shrink_slab > >> > xfs_inode_shrink > >> > XFS_ILOCK > >> > xfs_buf_allocate_memory() > >> > kmalloc(GFP_KERNEL) > >> > > >> > And so locks on inodes in reclaim are seen below reclaim. Then > >> > somewhere else we have: > >> > > >> > some high level read-only xfs code like readdir > >> > XFS_ILOCK > >> > xfs_buf_allocate_memory() > >> > kmalloc(GFP_KERNEL) > >> > reclaim > >> > > >> > And this one throws false positive lockdep warnings because we > >> > called into reclaim with XFS_ILOCK held and GFP_KERNEL alloc > >> > >> OK, and what exactly makes this positive a false one? Why can't it continue like > >> the first example where reclaim leads to another XFS_ILOCK, thus deadlock? > > > > Because above reclaim we only have operations being done on > > referenced inodes, and below reclaim we only have unreferenced > > inodes. We never lock the same inode both above and below reclaim > > at the same time. > > > > IOWs, an operation above reclaim cannot see, access or lock > > unreferenced inodes, except in inode write clustering, and that uses > > trylocks so cannot deadlock with reclaim. > > > > An operation below reclaim cannot see, access or lock referenced > > inodes except during inode write clustering, and that uses trylocks > > so cannot deadlock with code above reclaim. > > Thanks for elaborating. Perhaps lockdep experts (not me) would know how to > express that. If not possible, then replacing GFP_NOFS with __GFP_NOLOCKDEP > should indeed suppress the warning, while allowing FS reclaim. This was certainly my hope to happen when introducing __GFP_NOLOCKDEP. I couldn't have done the second step because that requires a deep understanding of the code in question which is beyond my capacity. It seems we still haven't found a brave soul to start converting GFP_NOFS to __GFP_NOLOCKDEP. And it would be really appreciated. Thanks. -- Michal Hocko SUSE Labs