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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3307BC433F5 for ; Thu, 10 Feb 2022 19:13:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1343787AbiBJTNK (ORCPT ); Thu, 10 Feb 2022 14:13:10 -0500 Received: from mxb-00190b01.gslb.pphosted.com ([23.128.96.19]:49320 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1343727AbiBJTM7 (ORCPT ); Thu, 10 Feb 2022 14:12:59 -0500 Received: from smtp-out2.suse.de (smtp-out2.suse.de [195.135.220.29]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id F2550101C; Thu, 10 Feb 2022 11:12:59 -0800 (PST) Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out2.suse.de (Postfix) with ESMTP id 845F71F399; Thu, 10 Feb 2022 19:12:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1644520378; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=nKOAp2jXT8jysY+hvn3O+c1AEmNJxMIGUyiP1y/9IdU=; b=FwHTiUN8CUXLrAWhQchp2Ec5RiMF4n/G+ucGZiv6NJd70nxmVMhzOzr1KFotknnihWXhi4 uilq+8BzbWoxJtOgmSRe6vN0541hOHUurfDNYl7TNI3qcXCYEVIBChfEfrw92CuKdnGfa0 ZfBEYOmAg0AfMu2jg0WmKJHPyMWnjM8= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1644520378; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=nKOAp2jXT8jysY+hvn3O+c1AEmNJxMIGUyiP1y/9IdU=; b=W1fNULM9xsFbn5L0d8YfNmBjjwwPwYUCx5Hp3ybYMKJsXIiUyooJazK/XT2RQE+yNVukpd SS0OKWeAR97D6wAA== Received: from quack3.suse.cz (unknown [10.163.28.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id 58EE4A3B83; Thu, 10 Feb 2022 19:12:58 +0000 (UTC) Received: by quack3.suse.cz (Postfix, from userid 1000) id 8F409A05BC; Thu, 10 Feb 2022 20:12:52 +0100 (CET) Date: Thu, 10 Feb 2022 20:12:52 +0100 From: Jan Kara To: "Paul E. McKenney" Cc: Jan Kara , Qian Cai , Theodore Ts'o , Jan Kara , Neeraj Upadhyay , Joel Fernandes , Boqun Feng , linux-ext4@vger.kernel.org, rcu@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH] jbd2: avoid __GFP_ZERO with SLAB_TYPESAFE_BY_RCU Message-ID: <20220210191252.3cbe2sy3jmqul2mh@quack3.lan> References: <20220209165742.5659-1-quic_qiancai@quicinc.com> <20220209181010.gfn66rvip56i54df@quack3.lan> <20220209201137.GY4285@paulmck-ThinkPad-P17-Gen-1> <20220210091648.w5wie3llqri5kfw3@quack3.lan> <20220210145713.GK4285@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220210145713.GK4285@paulmck-ThinkPad-P17-Gen-1> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 10-02-22 06:57:13, Paul E. McKenney wrote: > On Thu, Feb 10, 2022 at 10:16:48AM +0100, Jan Kara wrote: > > On Wed 09-02-22 12:11:37, Paul E. McKenney wrote: > > > On Wed, Feb 09, 2022 at 07:10:10PM +0100, Jan Kara wrote: > > > > On Wed 09-02-22 11:57:42, Qian Cai wrote: > > > > > Since the linux-next commit 120aa5e57479 (mm: Check for > > > > > SLAB_TYPESAFE_BY_RCU and __GFP_ZERO slab allocation), we will get a > > > > > boot warning. Avoid it by calling synchronize_rcu() before the zeroing. > > > > > > > > > > Signed-off-by: Qian Cai > > > > > > > > No, the performance impact of this would be just horrible. Can you > > > > ellaborate a bit why SLAB_TYPESAFE_BY_RCU + __GFP_ZERO is a problem and why > > > > synchronize_rcu() would be needed here before the memset() please? I mean > > > > how is zeroing here any different from the memory just being used? > > > > > > Suppose a reader picks up a pointer to a memory block, then that memory > > > is freed. No problem, given that this is a SLAB_TYPESAFE_BY_RCU slab, > > > so the memory won't be freed while the reader is accessing it. But while > > > the reader is in the process of validating the block, it is zeroed. > > > > > > How does the validation step handle this in all cases? > > > > > > If you have a way of handling this, I will of course drop the patch. > > > And learn something new, which is always a good thing. ;-) > > > > So I maybe missed something when implementing the usage of journal_heads > > under RCU but let's have a look. An example of RCU user of journal heads > > is fs/jbd2/transaction.c:jbd2_write_access_granted(). It does: > > > > rcu_read_lock(); > > > > // This part fetches journal_head from buffer_head - not related to > > // our slab RCU discussion > > > > if (!buffer_jbd(bh)) > > goto out; > > /* This should be bh2jh() but that doesn't work with inline functions */ > > jh = READ_ONCE(bh->b_private); > > if (!jh) > > goto out; > > > > // The validation comes here > > > > /* For undo access buffer must have data copied */ > > if (undo && !jh->b_committed_data) > > goto out; > > OK, so if *jh was freed and re-zallocated in the meantime, this test > should fail. One concern would be if the zeroing was not at least eight > bytes at a time, maybe due to overly eager use of fancy SIMD hardware. > Though perhaps you also do something about ->b_committed_data on > the free path, the commit-done path, or whatever? (I do see a > "jh->b_committed_data = NULL" on what might well be the commit-done path.) > > > if (READ_ONCE(jh->b_transaction) != handle->h_transaction && > > READ_ONCE(jh->b_next_transaction) != handle->h_transaction) > > goto out; > > And same with these guys. Yes, on commit-done path we zero out jh->b_transaction (or set jh->b_transaction = jh->b_next_transaction; jh->b_next_transaction = NULL). So these fields are actually guaranteed to be zero on free. > > // Then some more checks unrelated to the slab itself. > > > > /* > > * There are two reasons for the barrier here: > > * 1) Make sure to fetch b_bh after we did previous checks so that we > > * detect when jh went through free, realloc, attach to transaction > > * while we were checking. Paired with implicit barrier in that path. > > * 2) So that access to bh done after jbd2_write_access_granted() > > * doesn't get reordered and see inconsistent state of concurrent > > * do_get_write_access(). > > */ > > smp_mb(); > > if (unlikely(jh->b_bh != bh)) > > goto out; > > > > // If all passed > > > > rcu_read_unlock(); > > return true; > > > > So if we are going to return true from the function, we know that 'jh' was > > attached to handle->h_transaction at some point. And when 'jh' was attached > > to handle->h_transaction, the transaction was holding reference to the 'jh' > > and our 'handle' holds reference to the transaction so 'jh' could not be > > freed since that moment. I.e., we are sure our reference to the handle keeps > > 'jh' alive and we can safely use it. > > > > I don't see how any amount of scribbling over 'jh' could break this > > validation. But maybe it is just a lack of my imagination :). > > Regardless of whether you are suffering a lack of imagination, you > have clearly demonstrated that it is possible to correctly use the > SLAB_TYPESAFE_BY_RCU flag in conjunction with kmem_cache_alloc(), thus > demonstrating that I was suffering from a lack of imagination. ;-) > > I have therefore reverted my commit. Please accept my apologies for > the hassle! No problem. Thanks for reverting the patch. I can imagine that jbd2's use of SLAB_TYPESAFE_BY_RCU is an unusual one... Honza -- Jan Kara SUSE Labs, CR